MicroCT illuminates the unique morphology of Shiinoidae (Copepoda: Cyclopoida), an unusual group of fish parasites

The copepod family Shiinoidae Cressey, 1975 currently comprises nine species of teleost parasites with unusual morphology and a unique attachment mechanism. Female shiinoids possess greatly enlarged antennae that oppose a rostrum, an elongate outgrowth of cuticle that originates between the antennules. The antennae form a moveable clasp against the rostrum which they use to attach to their host. In this study, we use micro-computed tomography (microCT) to examine specimens of Shiinoa inauris Cressey, 1975 in situ attached to host tissue in order to characterize the functional morphology and specific muscles involved in this novel mode of attachment and to resolve uncertainty regarding the segmental composition of the regions of the body. We review the host and locality data for all reports of shiinoids, revise the generic diagnoses for both constituent genera Shiinoa Kabata, 1968 and Parashiinoa West, 1986, transfer Shiinoa rostrata Balaraman, Prabha & Pillai, 1984 to Parashiinoa as Parashiinoa rostrata (Balaraman, Prabha & Pillai, 1984) n. comb., and present keys to the females and males of both genera.

Multigenerational effects of arsenate on development and reproduction in marine copepod Tigriopus japonicus

Arsenic (As) is a persistent toxic substance, however, its toxicity to marine zooplankton remains unclear. In this study, copepods were exposed to a series of dissolved arsenate (As(V)) for four generations (F0-F3) and subsequently depurated in clean seawater for two generations (F4-F5) to assess multigenerational toxicity of As(V). As(V) exposure prolonged copepod development. The development time were 1.9, 2.4, and 3.4 days longer than the control in F0 when exposed to 50, 100, and 500 μg/L As(V), respectively, and the toxicity increased with generations. Moreover, As(V) reduced the reproductive capacity of copepods, and this effect become more severe during generation succession. The 10-day fecundities were reduced from 80 to 85 eggs per female in the control to 42 eggs per female, the lowest level, in 500 μg/L As(V) exposure group in F3. Nevertheless, the fecundity was recovered to the control level in the offspring of the 50 and 100 μg/L As(V) exposed groups (F4), suggesting it was an acclimation effect of copepods during As(V) exposure. In addition, the survival rate, development time, and reproductive parameters were significantly correlated with the As accumulation in copepods. Overall, As(V) exposure caused As bioaccumulation which negatively affected copepods' survival, development, and reproductive traits, and this toxic effect was amplified with generations and concentrations. Therefore, the multigenerational toxicity of As should be considered in the environmental risk assessments.

Beneficial microbes to suppress Vibrio and improve the culture performance of copepod Tigriopus japonicus Mori

The use of beneficial microbes, i.e., probiotics, to reduce pathogens and promote the performance of the target species is an important management strategy in mariculture. This study aimed to investigate the potential of four microbes, Debaryomyces hansenii, Ruegeria mobilis, Lactobacillus plantarum, and Bacillus subtilis, to suppress Vibrio and increase survival, population growth and digestive enzyme activity (protease, lipase, and amylase) in the harpacticoid copepod, Tigriopus japonicus. Copepod, T. japonicus stock culture with an initial mean density of 50 individual/mL (25 adult male and 25 adult female) was distributed into five treatments (i.e., four experimental and a control, each with four replicates; repeated twice) using 20 beakers (100 mL capacity each). The copepods were fed a mixture of the dinoflagellate Alexandrium tamarense and the diatom Phyaeodactylum tricornutum (3 × 104 cells/mL-1). Each microbe's concentration was adjusted at 108 CFU/mL-1 and applied to the culture condition. D. hansenii, L. plantarum, and B. subtilis all improved the copepods' survival and population growth, likely by including a higher lipase activity (P < 0.05). In contrast, using R. mobilis did not improve the copepod's culture performance compared to control. B. subtilis was the most effective in decreasing the copepod's external and internal Vibrio loading. The probiotic concentrations in the copepod decreased within days during starvation, suggesting that routine re-application of the probiotics would be needed to sustain the microbial populations and the benefits they provide. Our results demonstrated that D. hansenii and B. subtilis are promising probiotics for mass copepod culture as live food for mariculture purposes.

Spatial and temporal variability in tropical off-reef zooplankton across broad spatial and temporal scales

Productivity of oligotrophic coral reefs is largely dependent on the constant influx of zooplankton. However, our understanding of how zooplankton communities in tropical reef-associated regions vary over large spatial and temporal scales is limited. Using the Australian continuous plankton recorder dataset, we explored if, and to what extent, the off-reef zooplankton community along the Queensland shelf (including most of the Great Barrier Reef lagoon) varied with latitude, month, and diel time. The zooplankton community was consistently dominated by copepods (∼60%) which, with appendicularians, chaetognaths, non-copepod crustaceans, and thaliaceans, comprised ∼98% of the zooplankton. However, the abundance of these taxonomic groups did not vary predictably across latitude, month, or diel time, with these gradients only explaining 5% of community variation. At the scales sampled herein the composition of zooplankton was highly predictable in terms of broad taxonomic groups but variation in the relative abundance of these groups was not predictable.

Parasitism does not reduce thermal limits in the intermediate host of a bopyrid isopod

Parasitism has strong effects on community dynamics. Given the detrimental effects parasites have on host health, infection or infestation might be expected to reduce upper thermal limits, increasing the vulnerability of host species to future climate change. Copepods are integral components of aquatic food webs and biogeochemical cycles. They also serve as intermediate hosts in the life cycle of parasitic isopods in the family Bopyridae. As both copepods and isopod parasites play important roles in aquatic communities, it is important to understand how the interaction between parasite and host affects thermal limits in order to better predict how community dynamics may change in a warming climate. Here we examined the effect of infestation by larvae of a bopyrid isopod on the cosmopolitan copepod Acartia tonsa to test the hypothesis that infestation reduces thermal limits. To aid with this work, we developed an affordable, highly portable system for measuring critical thermal maxima of small ectotherms. We also used meta-analysis to summarize the effects of parasitism on critical thermal maxima in a wider range of taxa to help contextualize our findings. Contrary to both our hypothesis and the results of previous studies, we observed no reduction of thermal limits by parasitism in A. tonsa. These results suggest that life history of the host and parasite may interact to determine how parasite infestation affects environmental sensitivity.

Climate forcing on estuarine zooplanktonic production

Estuaries are among the most valuable aquatic systems in the world and resolving how there are impacted by climate change is fundamental to their management under global change scenarios. In this study, a ten-year time series (2003-2013) of zooplankton in an estuarine area (Mondego estuary, Portugal) is used to determine the impact of climate variability on estuarine zooplanktonic secondary production. For that, a trend analysis of seasonal zooplankton production was applied and their link with large-scale, regional, and local environment was tested by Distance-based multivariate multiple regression (DistLM). The annual integrated production of zooplankton varied between 34.27 mg C m-3 (2003) and 179.804 mg C m-3 (2013). Results showed that estuarine and marine zooplanktonic production increased in the estuary, mostly during summer/autumn and spring/summer, respectively. Local and regional environmental forcing drove copepod production in the estuary, with large-scale regime shifts affecting both directly and indirectly.

Without a pinch of salt: effect of low salinity on eggs and nauplii of the salmon louse (Lepeophtheirus salmonis)

The salmon louse is an economically important parasite on Atlantic salmon and poses a major threat to aquaculture. Several treatment methods have lost their effect due to resistance development in the lice. A rather new method for combatting sea lice is freshwater treatment where the various life stages of lice are differently affected by this treatment. In this study, we analyzed the effect of freshwater on the egg strings. A 3-h treatment with freshwater had a detrimental effect on the egg strings. First, the water penetrated the string, widening it, then entering the eggs and enlarging them. Finally, the ordered structure of the egg strings collapsed, and no alive animals hatched. Shorter treatments had a lower effectivity, and treatments with brackish water also showed milder effects. The egg strings were found to have a protective effect against low salinities, as hatched nauplii died rapidly under conditions that embryos survived. We also found that embryos react to low salinity on a molecular level by changing gene expression of several genes, when incubated in brackish water. Additionally, the hatching of embryos treated with brackish water was delayed in comparison to seawater controls.

Transcriptomic and targeted immune transcript analyses confirm localized skin immune responses in Atlantic salmon towards the salmon louse

Atlantic salmon (Salmo salar) are highly susceptible to infestations with the ectoparasite Lepeophtheirus salmonis, the salmon louse. Infestations elicit an immune response in the fish, but the response does not lead to parasite clearance, nor does it protect against subsequent infestations. It is, however, not known why the immune response is not adequate, possibly because the local response directly underneath the louse has been poorly evaluated. The present study describes the transcriptomic response by RNA sequencing of skin at the site of copepodid attachment. Analysing differentially expressed genes, 2864 were higher and 1357 were lower expressed at the louse attachment site compared to uninfested sites in the louse infested fish, while gene expression at uninfested sites were similar to uninfested control fish. The transcriptional patterns of selected immune genes were further detailed in three skin compartments/types: Whole skin, scales only and fin tissue. The elevation of pro-inflammatory cytokines and immune cell marker transcripts observed in whole skin and scale samples were not induced in fin, and a higher cytokine transcript level in scale samples suggest it can be used as a nonlethal sampling method to enhance selective breeding trials. Furthermore, the immune response was followed in both skin and anterior kidney as the infestation developed. Here, newly moulted preadult 1 stage lice induced a higher immune response than chalimi and adult lice. Overall, infestation with salmon louse induce a modest but early immune response with an elevation of mainly innate immune transcripts, with the response primarily localized to the site of attachment.

Mining Lepeophtheirus salmonis RNA-Seq data for qPCR reference genes and their application in Caligus elongatus

Lepeophtheirus salmonis and Caligus elongatus are two parasitic copepod species posing a significant threat to salmonid aquaculture. Consequently, several gene expression studies are executed each year to gain new knowledge and treatment strategies. Though, to enable accurate gene expression measurements by quantitative real time PCR, stable reference genes are needed. Previous studies have mainly focused on a few genes selected based on their function as housekeeping genes, as these are often stably expressed in various cells and tissues. In the present study, however, RNA-sequencing data from 127 L. salmonis samples from different life stages and diverse environmental conditions were used to identify new candidate reference genes displaying low variation. From this, six genes were selected, and the stability validated by qPCR on samples from different life stages. Since neither a genome nor comprehensive RNA sequencing data are available for C. elongatus, homologous genes to those identified for L. salmonis were identified within a C. elongatus transcriptome assembly and validated by qPCR in different life stages. Overall, the genes eukaryotic translation initiation factor 1A (EIF1A) and serine/threonine-protein phosphatase 1 (PP1) displayed the highest stability in L. salmonis, while the combination of PP1 and ribosomal protein S13 (RPS13) was found to have the highest stability in C. elongatus. These genes are well-suited reference genes for qPCR applications which allow for accurate normalization of target genes.

Transcriptional and toxic responses to saxitoxin exposure in the marine copepod Tigriopus japonicus

Saxitoxin (STX) is a highly toxic marine neurotoxin produced by phytoplankton and a growing threat to ecosystems worldwide due to the spread of toxic algae. Although STX is an established sodium channel blocker, the overall profile of transcriptional levels in STX-exposed organisms has yet to be described. Here, we describe a toxicity assay and transcriptome analysis of the copepod Tigriopus japonicus exposed to STX. The half-maximal lethal concentration of STX was 12.35 μM, and a rapid mortality slope was evident at concentrations between 12 and 13 μM. STX induced changes in swimming behavior among the copepods after 10 min of exposure. In transcriptome analysis, gene ontology revealed that the genes involved in nervous system and gene expression were highly enriched. In addition, the congenital neurological disorder and nuclear factor erythroid 2-related factor 2-mediated oxidative stress pathways were identified to be the most significant in network analysis and toxicity pathway analysis, respectively. This study provides valuable information about the effects of STX and related transcriptional responses in T. japonicus.

A ferritin gene in the marine copepod Acartia tonsa as a highly sensitive biomonitor for nano-contamination

Toxicology is not only for eco-risk assessments, but also for the real-time environmental monitoring based on the quick response of specific biomarkers. Ferritin gene (ftn) is a potential biomarker involving in crucial protective responses in biota. However, little information is available concerning the ftn in marine copepod Acartia tonsa (A. tonsa), a model organism widely applied in toxicology assessments. Our study for the first time identified and characterized the ftn in A. tonsa, along with its time-dependent transcriptional response to the reproductive toxicity of two newly emerged nanomaterials. The full-length cDNA of ftn contains a 114-bp 5'-untranslated region (UTR), a 236-bp 3'-untranslated region, and a 510-bp open reading frame which encodes an 18.51 kDa polypeptide composed of 169 amino acids. The ftn sequence has an iron binding signature and a potential phosphorylation site, which is closely-related to the ftn of Calanus sinicus and Pseudodiaptomus annandalei genes at the phylogenetical level. The ftn showed a quick and highly sensitive response to nanomaterial exposures, even at no observed effect concentrations. In detail, after exposure to nickel nanomaterials (up to 17.0 mg/L), the ftn was significantly upregulated immediately at 0.5 h and peaked at 9.5-fold in adults within 48 h, along with a significant reduction of egg hatching rate. When exposed to CdSe/ZnS quantum dots (up to 135 mg/L), no significant change in egg productions or hatching rates was observed, while the expression of ftn still significantly increased to over 3.0-fold in the initial 48 h. After that, the upregulation of ftn induced by CdSe/ZnS quantum dots or nickel nanoparticles both gradually returned back within 96 h. These findings demonstrate the highly sensitive response of this new cloned ftn to nanomaterial exposures, and highlight the suitability of ftn in A. tonsa as a promising biomonitor for nano-contamination in marine environments.

RNA-seq transcriptome analysis and identification of the theromacin antimicrobial peptide of the copepod Apocyclops royi

Copepods, including Apocyclops royi, are small aquatic crustaceans and one of the important foods for fish and shellfish larvae. However, studies of the host-pathogen interactions and understanding of infectious disease in copepods are still very limited, yet they are likely to be a significant factor in the sustainable development of copepod aquaculture. In the present study, we performed de novo RNA sequence analysis of A. royi-TH (a Thai isolate of A. royi), which yielded 4.80 Gb bases of clean data and a total of 29,786 unigenes. Annotation was then performed by comparison against seven functional databases, yielding 17,617 (NR: 59.15%), 2,969 (NT: 9.97%), 15,023 (SwissProt: 50.44%), 14,543 (KOG: 48.82%), 15,077 (KEGG: 50.62%), 6,763(GO: 22.71%), and 15,841 (InterPro: 53.18%) unigenes. In comparison to the components of the shrimp Toll pathway, LGBP, Spätzle, Toll receptors, MyD88, Pelle, TRAF6, Dorsal, and Cactus homologs were successfully identified in A. royi-TH. Additionally, a novel antimicrobial peptide (Theromacin-like) was characterized in A. royi (ArTM-like). The ArTM-like ORF was 279 bp and predicted to encode for 92 amino acid residues, with a mature peptide of 75 amino acids and a molecular mass of 8.56 kDa. The genomic organization of the ArTM-like gene consisted of three exons and two introns. Expression analysis indicated that ArTM-like mRNA was abundantly expressed in copepodid and adult stages as an immune responsive gene after infection with the pathogenic Vibrio parahaemolyticus-(AHPND)-causing strain. Altogether, the knowledge obtained in this study will provide a basis for future functional studies of the molecular mechanisms in copepod immunity that may eventually be applied for disease prevention in copepod aquaculture.

Beyond the fish-Daphnia paradigm: testing the potential for pygmy backswimmers (Neoplea striola) to cause trophic cascades in subtropical ponds

Trophic cascades, or indirect effects of predators on non-adjacent lower trophic levels, are a classic phenomenon in ecology, and are thought to be strongest in aquatic ecosystems. Most research on freshwater trophic cascades focused on temperate lakes, where fish are present and where Daphnia frequently dominate the zooplankton community. These studies identified that Daphnia often play a key role in facilitating trophic cascades by linking fish to algae with strong food web interactions. However, Daphnia are rare or absent in most tropical and subtropical lowland freshwaters, and fish are absent from small and temporary water bodies, where invertebrates fill the role of top predator. While invertebrate predators are ubiquitous in freshwater systems, most have received little attention in food web research. Therefore, we aimed to test whether trophic cascades are possible in small warmwater ponds where Daphnia are absent and small invertebrates are the top predators. We collected naturally occurring plankton communities from small fishless water bodies in central Texas and propagated them in replicate pond mesocosms. We removed zooplankton from some mesocosms, left the plankton community intact in others, and added one of two densities of the predaceous insect Neoplea striola to others. Following an incubation period, we then compared biomasses of plankton groups to assess food web effects between the trophic levels, including whether Neoplea caused a trophic cascade by reducing zooplankton. The zooplankton community became dominated by copepods which prefer large phytoplankton and exhibit a fast escape response. Perhaps due to these qualities of the copepods and perhaps due to other reasons such as high turbidity impairing predation, no evidence for food web effects were found other than somewhat weak evidence for zooplankton reducing large phytoplankton. More research is needed to understand the behavior and ecology of Neoplea, but trophic cascades may generally be weak or absent in fishless low latitude lowland water bodies where Daphnia are rare.

Evaluation of ecotoxicological effects associated with coastal sediments of the Yellow Sea large marine ecosystem using the marine copepod Tigriopus japonicus

A copepod bioassay with Tigriopus japonicus was applied to evaluate the relative ecotoxicity of sediments in the Yellow and Bohai seas, and contributions of individual PAHs to copepod toxicity were evaluated. Mean toxicity was greatest in the Yellow Sea of China, followed by the Bohai Sea and Yellow Sea of Korea. Elevated concentrations of sedimentary PAHs, alkylphenols, and styrene oligomers back-supported the significant toxicities observed in bioassay. Copepod toxicity in relation to PAHs indicated the greatest contribution by indeno[1,2,3-c,d]pyrene. However, lacked contribution by PAHs, viz., 2.4 and 3.0 % for the total immobilization and mortality, respectively, indicated a large proportion of unknown toxicants being widely distributed along the Yellow Sea Large Marine Ecosystem (YSLME) coastline. Overall, the present study provides useful baseline information for evaluating the potential sedimentary toxicants, with emphasizing further investigation to identify the unknown toxicants at an LME scale, and elsewhere.

Evidence of 2,4-diaminobutyric acid (DAB) production as a defense mechanism in diatom Thalassiosira pseudonana

The neurotoxic secondary metabolite β-N-methylamino-L-alanine (BMAA) and its structural isomer 2,4-diaminobutyric acid (DAB) are known to be produced by various phytoplankton groups. Despite the worldwide spread of these toxin producers, no obvious role and function of BMAA and DAB in diatoms have been identified. Here, we investigated the effects of biotic factors, i.e., predators and competitors, as possible causes of BMAA and/or DAB regulation in the two diatom species Phaeodactylum tricornutum and Thalassiosira pseudonana. DAB was specifically regulated in T. pseudonana by the presence of predators and competitors. The effects of DAB on both diatoms as competitors and on the copepod Tigriopus sp. as predator at individual and at population levels were examined. The toxic effects of DAB on the growth of T. pseudonana and the population of Tigriopus sp. were significant. The effect of DAB as a defensive secondary metabolite is assumed to be environmentally relevant depending on the number of the copepods. The results show a potential function of DAB that can play an important role in defense mechanisms of T. pseudonana.

Microplastic ingestion in zooplankton from the Fram Strait in the Arctic

Some of the highest microplastic concentrations in marine environments have been reported from the Fram Strait in the Arctic. This region supports a diverse ecosystem dependent on high concentrations of zooplankton at the base of the food web. Zooplankton samples were collected during research cruises using Bongo and MOCNESS nets in the boreal summers of 2018 and 2019. Using FTIR scanning spectroscopy in combination with an automated polymer identification approach, we show that all five species of Arctic zooplankton investigated had ingested microplastics. Amphipod species, found in surface waters or closely associated with sea ice, had ingested significantly more microplastic per individual (Themisto libellula: 1.8, Themisto abyssorrum: 1, Apherusa glacialis: 1) than copepod species (Calanus hyperboreus: 0.21, Calanus glacialis/finmarchicus: 0.01). The majority of microplastics ingested were below 50 μm in size, all were fragments and several different polymer types were present. We quantified microplastics in water samples collected at six of the same stations as the Calanus using an underway sampling system (inlet at 6.5 m water depth). Fragments of several polymer types and anthropogenic cellulosic fibres were present, with an average concentration of 7 microplastic particles (MP) L^-1 (0-18.5 MP L^-1). In comparison to the water samples, those microplastics found ingested by zooplankton were significantly smaller, highlighting that the smaller-sized microplastics were being selected for by the zooplankton. High levels of microplastic ingestion in zooplankton have been associated with negative effects on growth, development, and fecundity. As Arctic zooplankton only have a short window of biological productivity, any negative effect could have broad consequences. As global plastic consumption continues to increase and climate change continues to reduce sea ice cover, releasing ice-bound microplastics and leaving ice free areas open to exploitation, the Arctic could be exposed to further plastic pollution which could place additional strain on this fragile ecosystem.

Effects of Microplastic on the Population Dynamics of a Marine Copepod: Insights from a Laboratory Experiment and a Mechanistic Model

Microplastic is ubiquitously and persistently present in the marine environment, but knowledge of its population-level effects is limited. In the present study, to quantify the potential theoretical population effect of microplastic, a two-step approach was followed. First, the impact of microplastic (polyethylene, 0.995 g cm^-3 , diameter 10-45 µm) on the filtration rate of the pelagic copepod Temora longicornis was investigated under laboratory conditions. It was found that the filtration rate decreased at increasing microplastic concentrations and followed a concentration-response relationship but that at microplastic concentrations <100 particles L^-1 the filtration rate was not affected. From the concentration-response relationship between the microplastic concentrations and the individual filtration rate a median effect concentration of the individual filtration rate (48 h) of 1956 ± 311 particles L^-1 was found. In a second step, the dynamics of a T. longicornis population were simulated for realistic environmental conditions, and the effects of microplastics on the population density equilibrium were assessed. The empirical filtration rate data were incorporated in an individual-based model implementation of the dynamic energy budget theory to deduct potential theoretical population-level effects. The yearly averaged concentration at which the population equilibrium density would decrease by 50% was 593 ± 376 particles L^-1 . The theoretical effect concentrations at the population level were 4-fold lower than effect concentrations at the individual level. However, the theoretical effect concentrations at the population level remain 3-5 orders of magnitude higher than ambient microplastic concentrations. Because the present experiment was short-term laboratory-based and the results were only indirectly validated with field data, the in situ implications of microplastic pollution for the dynamics of zooplankton field populations remain to be further investigated. Environ Toxicol Chem 2022;41:1663-1674. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Influence of salinity on the meiofaunal distribution in a hypersaline lake along the southeast coast of India

The present study highlights that seasonal salinity variability plays a significant role in meiobenthic distribution with special reference to nematode assemblages. Sediment and water samples were collected from Pulicat Lake, a hypersaline lake along the southeast coast of India during two seasons (Southwest Monsoon (SWM) and Northeast Monsoon (NEM)). Based on the salinity distribution, the lake is categorized into four regions, viz., southern inlet, central region, middle inlet, and northern inlet. Meiobenthic abundance was higher during SWM (226-12,206 Ind/10 cm²) than in NEM (640-10,424 Ind/10 cm²). The meiofaunal abundance was high in the central region during both the seasons, followed by the southern, northern, and middle inlet. The nematode was the dominant meiobenthic group, followed by copepod, polychaete, and foraminifera. Due to high organic matter, the central region was dominated by deposit feeding nematode species like Halalaimus longicaudatus and Terschellingia longicaudata. The southern and northern regions were dominated by free-living nematodes Rhabditis olitoria, Mesorhabditis capitata, Mononochus bastian, Paramononchus sp., Piranchulus sp., and Diploscapter cylindricus. Oncholaimus sp., a hypersaline indicator species, was reported from the middle inlet location. Statistical analysis suggests salinity as a critical parameter for the distribution and diversity of nematodes.

CO2-driven seawater acidification increases cadmium toxicity in a marine copepod

Here, we examined the 48-h acute toxicity of cadmium (Cd) in the marine copepod Tigriopus japonicus under two pCO₂ concentrations (400 and 1000 μatm). Subsequently, T. japonicus was interactively exposed to different pCO₂ (400, 1000 μatm) and Cd (control, 500 μg/L) treatments for 48 h. After exposure, biochemical and physiological responses were analyzed for the copepods. The results showed that the 48-h LC₅₀ values of Cd were calculated as 12.03 mg/L and 9.08 mg/L in T. japonicus, respectively, under 400 and 1000 μatm pCO₂ conditions. Cd exposure significantly promoted Cd exclusion/glycolysis, detoxification/stress response, and oxidative stress/apoptosis while it depressed that of antioxidant capacity. Intriguingly, CO₂-driven acidification enhanced Cd bioaccumulation and its toxicity in T. japonicus. Overall, our study provides a mechanistic understanding about the interaction between seawater acidification and Cd pollution in marine copepods.

Differentiating toxic and nontoxic congeneric harmful algae using the non-polar metabolome

Recognition and rejection of chemically defended prey is critical to maximizing fitness for predators. Paralytic shellfish toxins (PSTs) which strongly inhibit voltage-gated sodium channels in diverse animal taxa are produced by several species of the bloom-forming algal genus Alexandrium where they appear to function as chemical defenses against grazing copepods. Despite PSTs being produced and localized within phytoplankton cells, some copepods distinguish toxic from non-toxic prey, selectively ingesting less toxic cells, in ways that suggest cell surface recognition perhaps associated with non-polar metabolites. In this study LC/MS and NMR-based metabolomics revealed that the non-polar metabolomes of two toxic species (Alexandrium catenella and Alexandrium pacificum) vary considerably from their non-toxic congener Alexandrium tamarense despite all three being very closely related. Toxic and non-toxic Alexandrium spp. were distinguished from each other by metabolites belonging to seven lipid classes. Of these, 17 specific metabolites were significantly more abundant in both toxic A. catenella and A. pacificum compared to non-toxic A. tamarense suggesting that just a small portion of the observed metabolic variability is associated with toxicity. Future experiments aimed at deciphering chemoreception mechanisms of copepod perception of Alexandrium toxicity should consider these metabolites, and the broader lipid classes phosphatidylcholines and sterols, as potential candidate cues.

Differential response of distinct copepod life history types to spring environmental forcing in Rivers Inlet, British Columbia, Canada

The temporal dynamics of five copepod species common to coastal waters of the Pacific Northwest were examined in relation to variability in spring temperature and phytoplankton dynamics in 2008, 2009, and 2010 in Rivers Inlet, British Columbia, Canada. The five species were differentiated by life history strategies. Acartia longiremis, Metridia pacifica, and Paraeuchaeta elongata remained active over most of the year. By contrast, the reproductive effort of Eucalanus bungii and Calanus marshallae was concentrated over the spring period and they spent most of the year in diapause as C5 copepodites. A delay in the timing of the spring bloom was associated with a shift in the phenology of all species. However, following the delay in spring bloom timing, recruitment to the G₁ cohort was reduced only for E. bungii and C. marshallae. Recruitment successes of E. bungii and C. marshallae was also drastically reduced in 2010, an El Niño year, when spring temperatures were highest. Reasons for the observed differential response to spring environmental forcing, and its effect on upper trophic levels, are discussed.

Physiological and behavioral responses of the copepod Temora turbinata to hypoxia

Hypoxia is intensified in tropical and subtropical estuarine and coastal waters and brings about lethal and sublethal effects to marine copepods. The physiological and behavioral responses of the subtropical calanoid copepod Temora turbinata were tested after short-term exposure to hypoxia. The LD₅₀ values were 3.02 ± 0.21, 2.00 ± 0.35, and 3.11 ± 0.31 mg L^-1 for nauplii (II-III), copepodites (II-III), and female adults, respectively. With a decrease in the ambient dissolved oxygen (DO) level from 8 to 0.5 mg L^-1, the ingestion rates decreased significantly at all life stages, as did oxygen consumption in female adults. In an artificial stratification column with a DO gradient, female adults exhibited an obvious avoidance response to the hypoxic bottom layer. Our study provides preliminary evidence for high hypoxia sensitivity in T. turbinata and implies that the DO level may be the main factor controlling the distribution of this species in tropical and subtropical coastal and estuarine waters.

A synthesis tree of the Copepoda: integrating phylogenetic and taxonomic data reveals multiple origins of parasitism

The Copepoda is a clade of pancrustaceans containing 14,485 species that are extremely varied in their morphology and lifestyle. Not only do copepods dominate marine plankton and sediment communities and make up a sizeable component of the freshwater plankton, but over 6,000 species are symbiotically associated with every major phylum of marine metazoans, mostly as parasites. Unfortunately, our understanding of copepod evolutionary relationships is relatively limited in part because of their extremely divergent morphology, sparse taxon sampling in molecular phylogenetic analyses, a reliance on only a handful of molecular markers, and little taxonomic overlap between phylogenetic studies. Here, a synthesis tree method is used to integrate published phylogenies into a more comprehensive tree of copepods by leveraging phylogenetic and taxonomic data. A literature review in this study finds fewer than 500 species of copepods have been sampled in molecular phylogenetic studies. Using the Open Tree of Life platform, those taxa that have been sampled in previous phylogenetic studies are grafted together and combined with the underlying copepod taxonomic hierarchy from the Open Tree of Life Taxonomy to make a synthesis phylogeny of all copepod species. Taxon sampling with respect to molecular phylogenetic analyses is reviewed for all orders of copepods and shows only 3% of copepod species have been sampled in phylogenetic studies. The resulting synthesis phylogeny reveals copepods have transitioned to a parasitic lifestyle on at least 14 occasions. We examine the underlying phylogenetic, taxonomic, and natural history data supporting these transitions to parasitism; review the species diversity of each parasitic clade; and identify key areas for further phylogenetic investigation.

Occurrence, characterization, partition, and toxicity of cigarette butts in a highly urbanized coastal area

Cigarette butts (CBs) comprise one of the most relevant categories of marine litter worldwide. This study was structured as a case study simultaneously assessing (i) the occurrence of CBs in a highly urbanized coastal area, (ii) their partitioning in two environmental compartments (water column and sediments) by laboratory settling tests, and (iii) the toxicity produced by the CBs leachates (measured as copepod reproduction). Marine litter was sampled in beaches of the city of Santos (SW Brazil) and CBs were collected for analysis. The characterization showed that CBs were one of the most prevalent items (51.5% in summer and 34.4% in winter) and 22 cigarettes brands were identified at different stages of decomposition. Laboratory settling tests showed that CBs remain in the water column between 3 and 20 days, subsequently reaching the sediments. The toxicity results indicated that CBs the leachates extracted from a small amount of CBs was sufficient to affect copepod reproduction (0.1 and 0.01 CBs L^-1). Furthermore, the approaches adopted by the present study can serve as a preliminary assessment to estimate local impacts resulting from inadequate disposal of cigarette butts in coastal areas around the world.

Copepod consumption by amphibians and fish with implications for transmission of Dracunculus species

Parasitic nematodes in the genus Dracunculus have a complex life cycle that requires more than one host species in both aquatic and terrestrial habitats. The most well-studied species, Dracunculus medinensis, is the causative agent of human Guinea worm disease (dracunculiasis). There are several other Dracunculus species that infect non-human animals, primarily wildlife (reptiles and mammals). The classic route of D. medinensis transmission to humans is through the ingestion of water containing the intermediate host, a cyclopoid copepod, infected with third-stage larvae (L3s). However, many animal hosts (e.g., terrestrial snakes, dogs) of other Dracunculus sp. appear unlikely to ingest a large number of copepods while drinking. Therefore, alternative routes of infection (e.g., paratenic or transport hosts) may facilitate Dracunculus transmission to these species. To better understand the role of paratenic and transport hosts in Dracunculus transmission to animal definitive hosts, we compared copepod ingestion rates for aquatic species (fish, frogs [tadpoles and adults], and newts) which may serve as paratenic or transport hosts. We hypothesized that fish would consume more copepods than amphibians. Our findings confirm that African clawed frogs (Xenopus laevis) and fish consume copepods, but that fish ingest, on average, significantly higher numbers (68% [34/50]) than adult African clawed frogs (36% [18/50]) during a 24-h time period. Our results suggest that amphibians and fish may play a role in the transmission of Dracunculus to definitive hosts. Still, additional research is required to determine whether, in the wild, fish or frogs are serving as paratenic or transport hosts. If so, they may facilitate Dracunculus transmission. However, if these animals simply act as dead-end hosts or as means of copepod population control, they may decrease Dracunculus transmission.

•

Copepod ingestion during 24 h was assessed for fish and amphibians.

•

Significant numbers of copepods were consumed by fish and adult Xenopus.

•

Tadpoles and newts did not consume large numbers of copepods during this time.

•

Fish and amphibians may facilitate Dracunculus transmission.

•

Further studies may elucidate how copepod ingestion impact parasite transmission.

Post-diapause transcriptomic restarts: insight from a high-latitude copepod

Background

Diapause is a seasonal dormancy that allows organisms to survive unfavorable conditions and optimizes the timing of reproduction and growth. Emergence from diapause reverses the state of arrested development and metabolic suppression returning the organism to an active state. The physiological mechanisms that regulate the transition from diapause to post-diapause are still unknown. In this study, this transition has been characterized for the sub-arctic calanoid copepod Neocalanus flemingeri, a key crustacean zooplankter that supports the highly productive North Pacific fisheries. Transcriptional profiling of females, determined over a two-week time series starting with diapausing females collected from > 400 m depth, characterized the molecular mechanisms that regulate the post-diapause trajectory.

Results

A complex set of transitions in relative gene expression defined the transcriptomic changes from diapause to post-diapause. Despite low temperatures (5–6 °C), the switch from a “diapause” to a “post-diapause” transcriptional profile occurred within 12 h of the termination stimulus. Transcriptional changes signaling the end of diapause were activated within one-hour post collection and included the up-regulation of genes involved in the 20E cascade pathway, the TCA cycle and RNA metabolism in combination with the down-regulation of genes associated with chromatin silencing. By 12 h, females exhibited a post-diapause phenotype characterized by the up-regulation of genes involved in cell division, cell differentiation and multiple developmental processes. By seven days post collection, the reproductive program was fully activated as indicated by up-regulation of genes involved in oogenesis and energy metabolism, processes that were enriched among the differentially expressed genes.

Conclusions

The analysis revealed a finely structured, precisely orchestrated sequence of transcriptional changes that led to rapid changes in the activation of biological processes paving the way to the successful completion of the reproductive program. Our findings lead to new hypotheses related to potentially universal mechanisms that terminate diapause before an organism can resume its developmental program.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07557-7.

Caligus madeirensis sp. nov. (Copepoda: Caligidae) Parasitic on Pompano, Trachinotus ovatus (Linnaeus, 1758), from Eastern Atlantic Waters, Surrounding the Madeira Archipelago, Portugal

INTRODUCTION

A new species of the genus Caligus is described based on specimens collected from pompano, Trachinotus ovatus (Linnaeus), caught in eastern Atlantic waters, near the Madeira archipelago.

METHODS

Pompano (n = 21) were purchased from the local fish market and examined for parasitic copepods. Morphological features of the copepods were examined and drawn using an Olympus BX51 equipped with a drawing tube. Key diagnostic characters were scanned using a confocal laser scanning microscope and a scanning electron microscope.

RESULTS

Caligus madeirensis sp. nov., can be distinguished from all congeners (270 spp.) by the combination of the following characters: (i) an accessory process on the 3 outer terminal spines located on the distal exopodal segment of leg 1, (ii) a mandible with 14 teeth, (iii) a sternal furca with parallel, spatulate tines, (iv) a maxilliped with 2 simple setae at base of a claw, (v) a leg 3 with second endopodal segment with 5 pinnate setae, (vi) a 2-segmented leg 4 exopod with I; III spine formula, (vii) a male antenna with 4 overlapping plates on the distal segment, (viii) a male maxilliped corpus bearing a prominent myxal process ornamented with dense corrugations along inner margin.

CONCLUSION

Together with Caligus madeirensis sp. nov., the Caligus fauna of Portugal consists of 13 species reported from 17 Portuguese marine fishes. Considering the large number of marine fish species (510 spp.) currently recorded from Portuguese waters it seems that only 3.3% of the Portuguese marine fishes have been found to be infested with species of the genus Caligus O.F. Müller, 1758, the result of which, however, might be due to limited sampling.

Polar and neutral lipid composition of the copepod Lernaeocera lusci and its host Merluccius merluccius in relationship with the parasite intensity

Parasitic copepod Lernaeocera lusci is a common mesoparasite of the hake Merluccius merluccius. Although widely distributed throughout the Mediterranean, little is known about this pathogen. The current study was designed to assess the impact of different L. lusci infection loads on lipid classes and their fatty acid (FA) composition in both parasite and the host organs (gills, liver, and muscle). Results showed a significant decrease in total lipid, neutral lipid (NL), and polar lipid (PL) contents in all analyzed host's organs in relationship with parasite intensity. Gills appeared to be the most impacted organ under the lowest parasite intensity (loss of 50% of NL and PL amounts). At the highest parasitic infection, a loss of about 80% of lipid moieties was recorded in all analyzed organs. Simultaneously, no significant differences were found for the parasite reflecting its ability to sustain an appropriate lipid amount required for its survival and development. Significant changes in the FA composition were recorded in both host and parasite. Particularly, we have noticed that for L. lusci, the intraspecific competition has resulted in an increased level of some essential FA such as C22:6n-3 (docosahexaenoic acid, DHA), C20:5n-3 (eicosapentaenoic acid, EPA), and C20:4n-6 (arachidonic acid, ARA). This probably reflects that in addition to a direct host FA diversion, L. Lusci can modulate its FA composition by increasing the activity of desaturation. Within the host, liver PL appeared to be the less impacted fraction which may mirror an adaptive strategy adopted by the host to preserve the structural and functional integrity of this vital organ.

Acartia tonsa Dana 1849 as a Model Organism: Considerations on Acclimation in Ecotoxicological Assays

The copepod Acartia tonsa was standardized as model organism in acute toxicity bioassays due to its key position in coastal food chains and high sensitivity. Once bioassays are performed according to a protocol their results may become tools for the protection of aquatic ecosystems. However, there are divergences in bioassays methods using A. tonsa. This study aimed to investigate: (i) the need for acclimation of A. tonsa collected from the environment for use in acute toxicological bioassays; and (ii) differences in sensitivity between copepods collected from the environment and laboratory-grown copepods. Laboratory-grown copepods are more sensitive to SDS than A. tonsa from the environment. The acclimation time of 30 h helped organisms to recover from stress of collection/handling and changing environment/conditions. Therefore, laboratory-grown copepods showed to be more sensitive than organisms from environment; and for ecotoxicological bioassays acclimating A. tonsa collected from the environment for 30 h can be adopted.

Roles of three putative salmon louse (Lepeophtheirus salmonis) prostaglandin E2 synthases in physiology and host-parasite interactions

BACKGROUND

The salmon louse (Lepeophtheirus salmonis) is a parasite of salmonid fish. Atlantic salmon (Salmo salar) exhibit only a limited and ineffective immune response when infested with this parasite. Prostaglandins (PGs) have many biological functions in both invertebrates and vertebrates, one of which is the regulation of immune responses. This has led to the suggestion that prostaglandin E₂ (PGE₂) is important in the salmon louse host-parasite interaction, although studies of a salmon louse prostaglandin E₂ synthase (PGES) 2 gene have not enabled conformation of this hypothesis. The aim of the present study was, therefore, to characterize two additional PGES-like genes.

METHODS

Lepeophtheirus salmonis microsomal glutathione S-transferase 1 like (LsMGST1L) and LsPGES3L were investigated by sequencing, phylogenetics, transcript localization and expression studies. Moreover, the function of these putative PGES genes in addition to the previously identified LsPGES2 gene was analyzed in double stranded (ds) RNA-mediated knockdown (KD) salmon louse.

RESULTS

Analysis of the three putative LsPGES genes showed a rather constitutive transcript level throughout development from nauplius to the adult stages, and in a range of tissues, with the highest levels in the ovaries or gut. DsRNA-mediated KD of these transcripts did not produce any characteristic changes in phenotype, and KD animals displayed a normal reproductive output. The ability of the parasite to infect or modulate the immune response of the host fish was also not affected by KD.

CONCLUSIONS

Salmon louse prostaglandins may play endogenous roles in the management of reproduction and oxidative stress and may be a product of salmon louse blood digestions.

Co-occurrence and niche overlap among gill parasites of the white mullet (Mugil curema Valenciennes, 1836) (Osteichthyes: Mugilidae) from the western Atlantic, Brazil

Different groups parasitize the gills of fishes, and the organisms can interact in positive, negative, or neutral ways, depending on parasitic abundance, environmental conditions, and the availability of resources. Here, we studied the distribution of ectoparasites on the gills of the mullet Mugil curema in the Cabedelo port region (Brazil), asking if (1) the gill parasites of M. curema fix on specific host gill arches and/or (2) co-occur in the same gill arches within a host individual; (3) if the abundance of one parasite taxon affects the abundance of another on the same gill arches; and (4) considering distinct major groups, if parasites present patterns of co-occurrence on hosts and gill arches different from the expected at random. We used generalized linear models, generalized linear mixed models, and models with zero inflation to define the microhabitats (arches) with the highest abundances of parasites and null models to analyze the co-occurrences of gill parasite groups. Three taxa of Monogenea (Ligophorus brasiliensis Abdallah, Azevedo & Luque, 2009, Ligophorus sp. 1 and Ligophorus sp. 2) and six species of Copepoda were recorded (Bomolochus nitidus Wilson, 1911; Caligus praetextus Bere, 1936; Ergasilus atafonensis Amado & Rocha, 1997; Ergasilus bahiensis Amado & Rocha, 1997; Ergasilus caraguatatubensis Amado & Rocha, 1997 and Ergasilus lizae Krøyer, 1863). We found that, in general, the gill parasites were more abundant/prevalent on specific arches (I and II) of the hosts, shared such preferences and co-occurred on the gill arches. The presence of one taxon influenced the occurrence of another only in the arches I and II; as a result, the infracommunities were characterized as interactive in the gills of this host species.

In situ observations and modelling revealed environmental factors favouring occurrence of Vibrio in microbiome of the pelagic Sargassum responsible for strandings

Historically, pelagic Sargassum were only found in the Sargasso Sea. Since 2011, blooms were regularly observed in warmer water, further south. Their developments in Central Atlantic are associated with mass strandings on the coasts, causing important damages and potentially dispersion of new bacteria. Microbiomes associated with pelagic Sargassum were analysed at large scale in Central Atlantic and near Caribbean Islands with a focus on pathogenic bacteria. Vibrio appeared widely distributed among pelagic Sargassum microbiome of our samples with higher occurrence than previously found in Mexico Gulf. Six out the 16 Vibrio-OTUs (Operational Taxonomic Unit), representing 81.2 ± 13.1% of the sequences, felt in cluster containing pathogens. Among the four different microbial profiles of pelagic Sargassum microbiome, Vibrio attained about 2% in two profiles whereas it peaked, in the two others, at 6.5 and 26.8% respectively, largely above the concentrations found in seawater surrounding raft (0.5%). In addition to sampling and measurements, we performed backward Lagrangian modelling of trajectories of rafts, and rebuilt the sampled rafts environmental history allowing us to estimate Sargassum growth rates along raft displacements. We found that Vibrio was favoured by high Sargassum growth rate and in situ ammonium and nitrite, modelled phosphate and nitrate concentrations, whereas zooplankters, benthic copepods, and calm wind (proxy of raft buoyancy near the sea surface) were less favourable for them. Relations between Vibrio and other main bacterial groups identified a competition with Alteromonas. According to forward Lagrangian tracking, part of rafts containing Vibrio could strand on the Caribbean coasts, however the strong decreases of modelled Sargassum growth rates along this displacement suggest unfavourable environment for Vibrio. For the conditions and areas observed, the sanitary risk seemed in consequence minor, but in other areas or conditions where high Sargassum growth rate occurred near coasts, it could be more important.

Effects of microplastics exposure on ingestion, fecundity, development, and dimethylsulfide production in Tigriopus japonicus (Harpacticoida, copepod)

The effects of microplastics pollution on the marine ecosystem have aroused attention. Copepod grazing stimulates dimethylsulfide (DMS) release from dimethylsulfoniopropionate (DMSP) in phytoplankton, but the effect of microplastics exposure on DMS and DMSP production during copepod feeding has not yet been revealed. Here, we investigated the effects of polyethylene (PE) and polyamide-nylon 6 (PA 6) microplastics on ecotoxicity and DMS/DMSP production in the copepod Tigriopus japonicus. The microplastics had detrimental effects on feeding, egestion, reproduction, survival, and DMS and DMSP production in T. japonicus and presented significant dose-response relationships. The 24 h-EC50 for ingestion rates (IRs) of female T. japonicus exposed to PE and PA 6 were 57.6 and 58.9 mg L^-1, respectively. In comparison, the body size of the copepods was not significantly affected by the microplastics during one generation of culture. Ingesting fluorescently labeled microplastics confirmed that microplastics were ingested by T. japonicus and adhered to the organs of the body surface. T. japonicus grazing promoted DMS release originating from degradation of DMSP in algal cells. Grazing-activated DMS production decreased because of reduced IR in the presence of microplastics. These results provide new insight into the biogeochemical cycle of sulfur during feeding in copepods exposed to microplastics.

Intergenerational effects of resuspended sediment and trace metal mixtures on life cycle traits of a pelagic copepod

Multiple stressors like metal toxicity, organic compounds and sediment pollution from the Seine estuary are raising concern and novel toxicological approaches are needed to better assess and monitor the risk. In the present study, the copepod Eurytemora affinis from the Seine, was exposed to two different sources of contaminants, which were resuspended polluted sediments and a mixture of trace metals (dissolved phase). The exposure continued for four generations (F0, F1, F2, F3) where F0 is a generation for acclimation to the exposure condition and F3 is a generation for decontamination followed without any exposure, to detect possible maternal carryover effects of pollutants (F0 - F2) and the role of recovery (in F3). Higher accumulation of metals resulted in higher mortalities at both exposure conditions, with particularly F1 being the most sensitive generation showing highest bioaccumulation of metals, highest mortality, and smallest population size. Copper accumulation was highest of all metals in mixture from both the resuspended sediment and the combined trace metal treatment. A significantly lower naupliar production was seen in copepods exposed to resuspended sediment compared to trace metal exposed copepods. However, the decontamination phase (F3) indicated that E. affinis pre-exposed to resuspended sediment had a higher ability to recover the total population size, increase naupliar production, and depurate accumulated Cu. The population exposed to a trace metal mixture showed lower recovery and lower ability to discharge accumulated toxic metals indicating its greater effect on our experimental model when compared to resuspended sediment.

Ingestion of microplastics by meiobenthic communities in small-scale microcosm experiments

Microplastics have been detected in many different environments. Nematodes are a rife meiofaunal taxon and occupy an important trophic position in benthic food webs. Laboratory-based ingestion experiments have demonstrated the susceptibility of single nematode species to microplastic uptake. However, the determinants of ingestion by meiofaunal assemblages, especially those of nematodes, have yet to be fully examined. We therefore conducted a microcosm study in which field-collected freshwater sediment was spiked with fluorescent polystyrene (PS) beads (1.0, 3.0 and 6.0 μm) in concentrations of 10³ and 10⁷ PS beads ml^-1 and the ingestion by the most dominant indigenous meiofaunal taxa (nematodes, rotifers, chironomids, copepods) was investigated after 2, 4 and 8 days using fluorescence microscopy. In additional small-scale microcosms, PS bead ingestion by nematode assemblages was quantified as a function of feeding type, exposure time (1-10 days), concentration (10³, 10⁵, 10⁷ PS beads ml^-1) and bead size (0.5, 1.0, 3.0, 6.0 μm). PS beads at 10⁷ beads ml^-1 were largely ingested by chironomids and copepods. Exposure time and concentration correlated positively with PS bead ingestion for all taxa. The most relevant size class for ingestion for the majority of meiofaunal taxa was PS beads of 1.0 μm. Nematode communities, especially deposit-feeding species, effectively ingested micropastics from sediment, as >30% of the exposed individuals and 56% of the species ingested 1.0-μm PS beads in <24 h. Ingestion rates were mainly influenced by PS bead size and nematode feeding type/habit, with the exception of a bead concentration of 10³ beads ml^-1, at which exposure time was also an important factor. Sediment particles reduced microplastic ingestion considerably for all investigated meiobenthic organisms. Our study demonstrates the ability of free-living nematodes communities to readily ingest PS beads of various sizes. If the feeding-type distribution is known, the potential exposure of nematode communities may be predicted.

Genetic differentiation underlies seasonal variation in thermal tolerance, body size, and plasticity in a short-lived copepod

Organisms experience variation in the thermal environment on several different temporal scales, with seasonality being particularly prominent in temperate regions. For organisms with short generation times, seasonal variation is experienced across, rather than within, generations. How this affects the seasonal evolution of thermal tolerance and phenotypic plasticity is understudied, but has direct implications for the thermal ecology of these organisms. Here we document intra-annual patterns of thermal tolerance in two species of Acartia copepods (Crustacea) from a highly seasonal estuary, showing strong variation across the annual temperature cycle. Common garden, split-brood experiments indicate that this seasonal variation in thermal tolerance, along with seasonal variation in body size and phenotypic plasticity, is likely affected by genetic polymorphism. Our results show that adaptation to seasonal variation is important to consider when predicting how populations may respond to ongoing climate change.

RNA-Seq and differential gene expression analysis in Temora stylifera copepod females with contrasting non-feeding nauplii survival rates: an environmental transcriptomics study

Background

Copepods are fundamental components of pelagic food webs, but reports on how molecular responses link to reproductive success in natural populations are still scarce. We present a de novo transcriptome assembly and differential expression (DE) analysis in Temora stylifera females collected in the Gulf of Naples, Mediterranean Sea, where this copepod dominates the zooplankton community. High-Throughput RNA-Sequencing and DE analysis were performed from adult females collected on consecutive weeks (May 23rd and 30th 2017), because opposite naupliar survival rates were observed. We aimed at detecting key genes that may have influenced copepod reproductive potential in natural populations and whose expression was potentially affected by phytoplankton-derived oxylipins, lipoxygenase-derived products strongly impacting copepod naupliar survival.

Results

On the two sampling dates, temperature, salinity, pH and oxygen remained stable, while variations in phytoplankton cell concentration, oxylipin concentration and oxylipin-per-diatom-cell production were observed. T. stylifera naupliar survival was 25% on May 23rd and 93% on May 30th. De novo assembly generated 268,665 transcripts (isoforms) and 120,749 unique ‘Trinity predicted genes’ (unigenes), of which 50% were functionally annotated. Out of the 331 transcript isoforms differentially expressed between the two sampling dates, 119 sequences were functionally annotated (58 up- and 61 down-regulated). Among predicted genes (unigenes), 144 sequences were differentially expressed and 31 (6 up-regulated and 25 down-regulated) were functionally annotated. Most of the significantly down-regulated unigenes and isoforms were A5 Putative Odorant Binding Protein (Obp). Other differentially expressed sequences (isoforms and unigenes) related to developmental metabolic processes, protein ubiquitination, response to stress, oxidation-reduction reactions and hydrolase activities. DE analysis was validated through Real Time-quantitative PCR of 9 unigenes and 3 isoforms.

Conclusions

Differential expression of sequences involved in signal detection and transduction, cell differentiation and development offered a functional interpretation to the maternally-mediated low naupliar survival rates observed in samples collected on May 23rd. Down-regulation of A5 Obp along with higher quantities of oxylipins-per-litre and oxylipins-per-diatom-cell observed on May 23rd could suggest oxylipin-mediated impairment of naupliar survival in natural populations of T. stylifera. Our results may help identify biomarker genes explaining variations in copepod reproductive responses at a molecular level.

Warmer temperature increases mercury toxicity in a marine copepod

Marine biota have been co-challenged with ocean warming and mercury (Hg) pollution over many generations because of human activities; however, the molecular mechanisms to explain their combined effects are not well understood. In this study, a marine planktonic copepod Pseudodiaptomus annandalei was acutely exposed to different temperature (22 and 25 °C) and Hg (0 and 118 μg/L) treatments in a 24-h cross-factored experiment. Hg accumulation and its subcellular fractions were determined in the copepods after exposure. The expression of the genes of superoxide dismutase (SOD), glutathione peroxidase (GPx), metallothionein1 (mt1), heat shock protein 70 (hsp70), hsp90, hexokinase (hk), and pyruvate kinase (pk) was also analyzed. Both the Hg treatment alone and the combined exposure of warmer temperature plus Hg pollution remarkably facilitated Hg bioaccumulation in the exposed copepods. Compared with the Hg treatment alone, the combined exposure increased total Hg accumulation and also the amount of Hg stored in the metal-sensitive fractions (MSF), suggesting elevated Hg toxicity in P. annandalei under a warmer environment, given that the MSF is directly related to metal toxicity. The warmer temperature significantly up-regulated the mRNA levels of mt1, hsp70, hsp90, and hk, indicating the copepods suffered from thermal stress. With exposure to Hg, the mRNA level of SOD increased strikingly but the transcript levels of hsp90, hk, and pk decreased significantly, indicating that Hg induced toxic events (e.g., oxidative damage and energy depletion). Particularly, in contrast to the Hg treatment alone, the combined exposure significantly down-regulated the mRNA levels of SOD and GPx but up-regulated the mRNA levels of mt1, hsp70, hsp90, hk, and pk. Collectively, the results of this study indicate that ocean warming will potentially boost Hg toxicity in the marine copepod P. annandalei, which is information that will increase the accuracy of the projections of marine ecosystem responses to the joint effects of climate change stressors and metal pollution on the future ocean.

Lack of dormancy to protect diversity: Decrease in diversity of active zooplankton community observed in lake with depauperate egg bank

The study of zooplankton communities in freshwater resources under anthropogenic pressures rarely includes the simultaneous assessment of dormant embryos in bottom sediments and active life-stages in the water column. A coastal lake with a history of coal-ash contamination and disruption by hurricanes provided an ideal opportunity to demonstrate the power of examining both dormant and active zooplankton. The primary objective of this study was to evaluate changes in structure of a multicellular zooplankton community that is under simultaneous pressure from anthropogenic pollution and hurricane-induced flooding. To evaluate change in community structure, the active zooplankton community in 2015 was compared to that observed in 1985. Shannon-Wiener and Simpson indices demonstrate that diversity of the active zooplankton community decreased during this 30-year span. In total, 31% of zooplankton species were lost, and new colonization accounts for 27% of species richness. Dominant species of all major taxonomic groupings changed. Because most zooplankton in freshwater lakes depend on dormant embryos to reestablish active populations after major disruptions, dormant embryos in the sediment "egg bank" were also quantified. Dormant cladoceran ephippia are present in bottom sediments, but dormant copepods and rotifers are missing. The existence of a dormant egg bank that is less diverse than the active community in a freshwater lake is unprecedented, and a depauperate "egg bank" would certainly impair community recovery after severe flooding from hurricanes. It is argued that a paradigm shift is needed in the ecological assessment of inland lakes in order to account for the critical role that dormant embryos (egg banks) play in freshwater zooplankton communities. Two challenges to achieving this are that 1. long-term monitoring is expensive and 2. data on dormant zooplankton are rarely available. This study provides an example of how to conduct such studies by leveraging historic data when long-term monitoring is not possible.

An improved genome assembly and annotation of the Antarctic copepod Tigriopus kingsejongensis and comparison of fatty acid metabolism between T. kingsejongensis and the temperate copepod T. japonicus

Copepods in the genus Tigriopus are widely distributed in the intertidal zone worldwide. To assess differences in fatty acid (FA) metabolism among congeneric species in this genus inhabiting polar and temperate environments, we analyzed and compared FA profiles of the Antarctic copepod Tigriopus kingsejongensis and the temperate copepod T. japonicus. Higher amounts of total FAs were found in the Antarctic copepod T. kingsejongensis than the temperate copepod T. japonicus under administration of the identical amount of Tetraselmis suecica. To determine the genomic basis for this, we identified fatty acid metabolism-related genes in an improved genome of T. kingsejongensis. The total length of the assembled genome was approximately 338 Mb with N50 = 1.473 Mb, 938 scaffolds, and a complete Benchmarking Universal Single-Copy Orthologs value of 95.8%. A total of 25,470 genes were annotated using newly established pipeline. We identified eight elongation of very long-chain fatty acid protein (Elovl) genes and nine fatty acid desaturase (Fad) genes in the genome of T. kingsejongensis. In addition, fatty acid profiling suggested that the duplicated Δ5/6 desaturase gene in T. kingsejongensis is likely to play an essential role in synthesis of different FAs in T. kingsejongensis to those in T. japonicus. However, further experimental research is required to validate our in silico findings. This study provides a better understanding of fatty acid metabolism in the Antarctic copepod T. kingsejongensis.

Effects of different routes of exposure to metals on bioaccumulation and population growth of the cyclopoid copepod Paracyclopina nana

We examined effects of the three metals cadmium (Cd), copper (Cu), and nickel (Ni) on two subpopulations of the cyclopoid copepod Paracyclopina nana. We sought to investigate the effects of metal exposure on population growth and structure of P. nana and to understand the parameters affecting the metal bioaccumulation in copepods. A first experiment tested the hypothesis of competition between these metals in a mixture using a P. nana mass culture in 10 L beakers with the sublethal concentrations (1/3 of LC50) as determined for E. affinis. A second experiment pursued the same with a P. nana population which was adapted to a higher Cu concentration for several generations (226.9 ± 15.9 μg g^-1 dw Cu in copepods) and using the proper sublethal concentrations for P. nana. After 96 h of exposure, results from the first experiment showed a decreasing population growth and instead of an increasing metal accumulation in copepods. Cd also appeared to be more accumulated when it was alone, confirming the hypothesis of metal competition in mixture. Results from the second experiment revealed less marked effects. When metal concentrations increased in the treatment it decreased in copepods, indicating depuration activity in the population already adapted to metal exposure. This paper is the first one investigating the parameters affecting the bioaccumulation capacity of P. nana in response to metals. It offers a better understanding of copepod responses to metal contamination in a complex aquatic environment.

The genome of the harpacticoid copepod Tigriopus japonicus: Potential for its use in marine molecular ecotoxicology

The copepod Tigriopus japonicus has been widely used as an experimental species in the field of ecotoxicology. We have sequenced and assembled the whole genome of T. japonicus with comparative analysis of gene families that represent detoxification phases in two additional public genomes of Tigriopus spp., namely, T. californicus and T. kingsejongensis. The total length of the T. japonicus assembled genome was 196.6 Mb with an N50 value of 10.65 Mb and consisted of 339 scaffolds and 25,143 annotated genes. The detoxification gene families encoding cytochrome P450s (CYP450s), glutathione S-transferases (GSTs), and ATP-binding cassette (ABC) proteins in Tigriopus spp. have shown species-dependent diversity in several gene sets, suggesting that these genes have undergone a species-specific expansion to increase their fitness to different marine habitats and environmental pressures. Our study will provide a better understanding of the detoxification system in Tigriopus spp. and will contribute to various areas of research, including ecotoxicology.

Sunscreens containing zinc oxide nanoparticles can trigger oxidative stress and toxicity to the marine copepod Tigriopus japonicus

The study, for the first time, evaluated the leaching rate of zinc oxide nanoparticles (nZnO) from human skins which were applied with three commercial sunscreens containing nZnO as an active ingredient. The leaching rate of nZnO varied greatly among the sunscreens, with a range of 8-72% (mean ± SD: 45% ± 33%). We further investigated their toxicities to the marine copepod Tigriopus japonicus. We found that 96-h median lethal concentrations of the three sunscreens to T. japonicus were > 5000, 230.6, and 43.0 mg chemical L^-1, respectively, equivalent to Zn²⁺ concentrations at >82.5, 3.2, and 1.2 mg Zn L^-1, respectively. Exposure to the individual sunscreens at environmentally realistic concentrations for 96 h led to up-regulation of antioxidant genes in T. japonicus, while they triggered the release of reactive oxygen species based on the results of in vivo assays. Evidently, these nZnO-included sunscreens can cause oxidative stress and hence pose risk to marine organisms.

Microhabitat preference, body size, and egg allocation in the gill parasite Naobranchia lizae (Copepoda)

The relationships between microhabitat preference, body size, and egg allocation were examined in the copepod Naobranchia lizae, which establishes on the gills of striped mullet Mugil cephalus. A total of 297 individual N. lizae (mean intensity = 5.0 ± 4.8 SD) were recovered from 60 infected hosts collected from the Charleston Harbor Estuarine System, South Carolina USA. For each mullet, we identified 16 microhabitats per gill arch, which yielded 128 microhabitats per host that could potentially be occupied. On average, only 5% of these microhabitats were occupied per host. The distribution pattern of the copepods on the gills revealed that microhabitat preferences occurred both among and within gill arches. For the microhabitats occupied, there was no effect of preference on body size, egg number, or egg size. Similarly, microhabitat sharing, which was more likely to occur at higher infection intensities, was not costly in terms of the copepod body size and egg allocation and there was no detectable trade-off between egg number and egg size. However, results also revealed that about half (48%) of the available microhabitats were never occupied by the copepods. We suggest that the occupancy of these potentially poor quality sites could carry fitness costs not realized in nature since numerous high quality sites are available per host. The findings are consistent with the interpretation that female N. lizae occupy a resource-rich habitat on the gill arches of striped mullet that provides conditions for optimal growth and reproduction.

Identification and Description of the Key Molecular Components of the Egg Strings of the Salmon Louse (Lepeophtheirus salmonis)

The salmon louse Lepeophtheirus salmonis is a parasite of Atlantic salmon and other salmonids. Every year, it causes high costs for the Norwegian aquaculture industry. While the morphology of the female genital tract has been described, knowledge of the molecular basis of reproduction is very limited. We identified nine genes which are expressed exclusively in the female cement gland, the organ responsible for cement production, which is used to hold the eggs together and keep them attached to their mother in egg strings. Six of these genes encode proteins with signal peptides and probably form the main component of the cement. Two other genes are peroxidases, which are probably important in the cement formation. The last gene is not similar to any known protein, but contains a transmembrane domain. A knockdown of all these genes leads to missing or deformed egg strings, preventing reproduction of the lice. The correct assemblage of the cement in the cement gland is essential for successful reproduction of salmon lice. Similar proteins seem to be present in other copepod species, as well.

Variation in developmental temperature alters adulthood plasticity of thermal tolerance in Tigriopus californicus

In response to environmental change, organisms rely on both genetic adaptation and phenotypic plasticity to adjust key traits that are necessary for survival and reproduction. Given the accelerating rate of climate change, plasticity may be particularly important. For organisms in warming aquatic habitats, upper thermal tolerance is likely to be a key trait, and many organisms express plasticity in this trait in response to developmental or adulthood temperatures. Although plasticity at one life stage may influence plasticity at another life stage, relatively little is known about this possibility for thermal tolerance. Here, we used locally adapted populations of the copepod Tigriopus californicus to investigate these potential effects in an intertidal ectotherm. We found that low latitude populations had greater critical thermal maxima (CT_max) than high latitude populations, and variation in developmental temperature altered CT_max plasticity in adults. After development at 25°C, CT_max was plastic in adults, whereas no adulthood plasticity in this trait was observed after development at 20°C. This pattern was identical across four populations, suggesting that local thermal adaptation has not shaped this effect among these populations. Differences in the capacities to maintain ATP synthesis rates and to induce heat shock proteins at high temperatures, two likely mechanisms of local adaptation in this species, were consistent with changes in CT_max owing to phenotypic plasticity, which suggests that there is likely mechanistic overlap between the effects of plasticity and adaptation. Together, these results indicate that developmental effects may have substantial impacts on upper thermal tolerance plasticity in adult ectotherms.

The planktonic stages of the salmon louse (Lepeophtheirus salmonis) are tolerant of end-of-century pCO2 concentrations

The copepod Lepeophtheirus salmonis is an obligate ectoparasite of salmonids. Salmon lice are major pests in salmon aquaculture and due to its economic impact Lepeophtheirus salmonis is one of the most well studied species of marine parasite. However, there is limited understanding of how increased concentration of pCO₂ associated with ocean acidification will impact host-parasite relationships. We investigated the effects of increased pCO₂ on growth and metabolic rates in the planktonic stages, rearing L. salmonis from eggs to 12 days post hatch copepodids under three treatment levels: Control (416 µatm), Mid (747 µatm), and High (942 µatm). The pCO₂ treatment had a significant effect on oxygen consumption rate with the High treatment animals exhibiting the greatest respiration. The treatments did not have a significant effect on the other biological endpoints measured (carbon, nitrogen, lipid volume, and fatty acid content). The results indicate that L. salmonis have mechanisms to compensate for increased concentration of pCO₂and that populations will be tolerant of projected future ocean acidification scenarios. The work reported here also describes catabolism during the lecithotrophic development of L. salmonis, information that is not currently available to parameterize models of dispersal and viability of the planktonic free-living stages.

Enhanced electrokinetic remediation of multi-contaminated dredged sediments and induced effect on their toxicity

Electrokinetic (EK) remediation is often developed for metal decontamination but shows limitations for polycyclic aromatic hydrocarbons (PAHs) and polychlorobiphenyls (PCBs) which are nonionic and involve low aqueous solubility. This paper reports many laboratory studies devoted to the investigations of EK efficiency on the mobility and the removal of metals, PAHs and PCBs from dredged sediments, using a mixture of chelating agent and surfactants. The results showed that increasing chelating agent concentration was favorable for both metal and PAH removal. Applying a periodic voltage gradient associated to a low concentration of additives provided the best removal of Zn, Cd and Pb and also the 16 priority PAHs. The tested fresh harbor sediment was highly resistant to metals and organics mobilization and transport because of an aged contamination, a high buffering capacity, a very low hydraulic permeability and a high organic matter content. However, experiments performed on a former sediment which was deposited many years ago provided better removal results, involving low organic matter and carbonates content. The efficiency of the EK process was also assessed by measuring the acute toxicity of the EK-treated sediment on the copepod Eurytemora affinis exposed to sediment elutriates.

Effects of atrazine on life parameters, oxidative stress, and ecdysteroid biosynthetic pathway in the marine copepod Tigriopus japonicus

Atrazine is a widely used pesticide which acts as an endocrine disruptor in various organisms. The aim of this study was to investigate adverse effects of atrazine on life parameters, oxidative stress, and ecdysteroid biosynthetic pathway in the marine copepod Tigriopus japonicus. In T. japonicus, no mortality was shown in response to atrazine up to 20 mg/L in acute toxicity assessment. In nauplii, retardation in the growth and prolonged molting and metamorphosis resulted under chronic exposure of atrazine at 20 mg/L. In addition, body sizes of T. japonicus nauplii were significantly decreased (P < 0.01 in length and P < 0.001 in width) in response to 20 mg/L of atrazine. Furthermore, atrazine induced oxidative stress by the generation of reactive oxygen species at all concentrations compared to the control in the nauplii. Also, significant increase in glutathione-S transferase activity was observed in adult T. japonicus at low concentration of atrazine. To understand effects of atrazine on ecdysteroid biosynthetic pathway-involved genes (e.g., neverland, CYP307E1, CYP306A1, CYP302A1, CYP3022A1 [CYP315A1], CYP314A1, and CYP18D1) were examined with mRNA expressions of ecdysone receptor (EcR) and ultraspiracle (USP) in response to 20 mg/L atrazine in nauplii and adults. In the nauplii, these genes were significantly downregulated (P < 0.05) in response to atrazine, compared to the control but not in the adult T. japonicus. These results suggest that atrazine can interfere in vivo life parameters by oxidative stress-induced retrogression and ecdysteroid biosynthetic pathway in this species.

Parasitic copepods infestation on commercially exploited fishes from Kayamkulam backwater, Kerala, India

Parasitic copepods infestation on three species of commercially important fishes belonging to the family Mugilidae (Liza parsia) and Cichlidae (Oreochromis mossambicus and Etroplus suratensis) from Kayamkulam backwater were analysed between February 2015 and January 2017, covering all the three seasons. A total of 2305 fishes were analysed for parasites. Out of these, 299 fishes were infested with different species of parasitic copepods. The analysis revealed the presence of ten species of parasites comprising of Ergasilus parvitergam, Ergasilus sieboldi, Ergasilus sp, Caligus epidemicus, caligus sp, Dermoergasilus hoi, Dermoergasilus sp, Nothobomolochus sp, Lernea sp and Lepiophtherius sp. The infestation dynamics of parasitic copepods in terms of prevalence, mean intensity and mean abundance were calculated to determine the degree of infestation on each species. A higher prevalence (40%), mean intensity (13.2) and mean abundance (5.3) were found in Liza parsia while Etroplus suratensis had the lesser prevalence. There is a significant difference in the prevalence (p < 0.05), mean intensity (p < 0.05) and mean abundance (p < 0.01) in the rate of infestation of Liza parsia among the stations during the year 2015-2016, where as Oreochromis mossambicus and Etroplus suratensis shows no significant difference in the rate of infestation. The rate of infestation with parasitic copepod was higher in 2015-2016 than in 2016-2017. A higher proportion of fishes were more infested in Choolatheruvu (Station 3) as compared to Ayiramthengu (Station 1) and Valiyazheekal (Station 2).