Grass Shrimp (Palaemonetes pugio) as a Trophic Link for Methylmercury Accumulation in Urban Salt Marshes

Grass shrimp (Palaemonetes pugio) represent a potential link in the transfer of methylmercury (MeHg) from salt marsh sediments to transient young-of-the-year (YOY) fish. Across six salt marshes subject to varying degrees of Hg contamination, MeHg concentration in grass shrimp was significantly correlated with MeHg in sediment (p < 0.05, R² = 0.81). Bioenergetic models show that grass shrimp alone account for 12-90% of MeHg observed in YOY striped bass and 6-22% of MeHg in YOY summer flounder. Direct accumulation of MeHg from grass shrimp to YOY fish increased with MeHg levels in grass shrimp and sediment. However, in the most contaminated salt marshes with the highest levels of MeHg in grass shrimp and sediment, indirect accumulation of MeHg from grass shrimp by YOY summer flounder, whose diet is dominated by benthic forage fish (mummichog), is predicted to plateau because higher concentrations of MeHg in grass shrimp are offset by a lower proportion of grass shrimp in the mummichog diet. Our results demonstrate that grass shrimp are an important trophic link in the bioaccumulation of MeHg in salt marsh food webs and that MeHg accumulation in YOY fish varies with both the concentration of MeHg in salt marsh sediments and benthic food web structure.

The function of glucose metabolism in embryonic diapause of annual killifish

Annual killifish could survive as diapaused embryos buried in soil during dry seasons. When the embryos in diapause III were incubated in water, the larvae could be hatched quickly. However, the mechanism of diapause and hatching of annual killifish was ambiguous. In the present study, Nothobranchius guentheri were used as the model to clarify the physiological mechanism of diapause and hatching of annual killifish. The results indicated that incubation with water could significantly enhance the heart rate and blood circulation of embryos. To clarify the molecular mechanism, the transcriptomic analysis was used to compare the embryos in diapause I, diapause III, and hatching period. The results showed that DNA replication-related genes, cell division cycle 45 and proliferating cell nuclear antigen were more highly expressed in diapause I compared to diapause III. In addition, the transcript levels of glucagon, glucokinase and phosphofructokinase were more abundantly detected in hatching period compared to diapause III, but insulin receptor and insulin-like growth factor-binding protein were lower. These results indicated glucose metabolism might play an important role in diapause and hatching of killifish. To further confirm this result, several reagents involved in glucose metabolism were used to incubate embryos in diapause III. The results displayed that glucose and glucagon could both shorten the hatching time of embryos. In contrast, 2-deoxy-d-glucose, metformin, and insulin could prolong the hatching time and reduce the hatching rate. The results further confirmed that glucose metabolism played an important role in the diapause and hatching of annual killifish.

Retention of larval skin traits in adult amphibious killifishes: a cross-species investigation

The gills are the primary site of exchange in fishes. However, during early life-stages or in amphibious fishes, ionoregulation and gas-exchange may be primarily cutaneous. Given the similarities between larval and amphibious fishes, we hypothesized that cutaneous larval traits are continuously expressed in amphibious fishes across all life-stages to enable the skin to be a major site of exchange on land. Alternatively, we hypothesized that cutaneous larval traits disappear in juvenile stages and are re-expressed in amphibious species in later life-stages. We surveyed six species spanning a range of amphibiousness and characterized cutaneous ionocytes and neuroepithelial cells (NECs) as representative larval skin traits at up to five stages of development. We found that skin ionocyte density remained lower and constant in exclusively water-breathing, relative to amphibious species across development, whereas in amphibious species ionocyte density generally increased. Additionally, adults of the most amphibious species had the highest cutaneous ionocyte densities. Surprisingly, cutaneous NECs were only identified in the skin of one amphibious species (Kryptolebias marmoratus), suggesting that cutaneous NECs are not a ubiquitous larval or amphibious skin trait, at least among the species we studied. Our data broadly supports the continuous-expression hypothesis, as three of four amphibious experimental species expressed cutaneous ionocytes in all examined life-stages. Further, the increasing density of cutaneous ionocytes across development in amphibious species probably facilitates the prolonged occupation of terrestrial habitats.

In vitro-in vivo biotransformation and phase I metabolite profiling of benzo[a]pyrene in Gulf killifish (Fundulus grandis) populations with different exposure histories

Chronic exposure to pollution may lead populations to display evolutionary adaptations associated with cellular and physiological mechanisms of defense against xenobiotics. This could result in differences in the way individuals of the same species, but inhabiting different areas, cope with chemical exposure. In the present study, we explore two Gulf killifish (Fundulus grandis) populations with different exposure histories for potential differences in the biotransformation of benzo[a]pyrene (BaP), and conduct a comparative evaluation of in vitro and in vivo approaches to describe the applicability of new approach methodologies (NAMs) for biotransformation assessments. Pollution-adapted and non-adapted F. grandis were subjected to intraperitoneal (IP) injections of BaP in time-course exposures, prior to measurements of CYP biotransformation activity, BaP liver concentrations, and the identification and quantification of phase I metabolites. Additionally, substrate depletion bioassays using liver S9 fractions were employed for measurements of intrinsic hepatic clearance and to evaluate the production of metabolites in vitro. Pollution-adapted F. grandis presented significantly lower CYP1A activity and intrinsic clearance rates that were 3 to 4 times lower than non-adapted fish. The metabolite profiling of BaP showed the presence of 1‑hydroxy-benzo[a]pyrene in both the in vitro and in vivo approaches but with no significant population differences. Contrarily, 9‑hydroxy-benzo[a]pyrene and benzo[a]pyrene-4,5-dihydrodiol, only identified through the in vivo approach, presented higher concentrations in the bile of pollution-adapted fish relative to non-adapted individuals. These observations further the understanding of the evolutionary adaptation of F. grandis inhabiting heavily polluted environments in the Houston Ship Channel, TX, USA, and highlight the need to consider the evolutionary history of populations of interest during the implementation of NAMs.

Reduced biotransformation of polycyclic aromatic hydrocarbons (PAHs) in pollution-adapted Gulf killifish (Fundulus grandis)

Anthropogenic pollution represents a significant source of selection, potentially leading to the emergence of evolutionary adaptations in chronically exposed organisms. A recent example of this scenario corresponds to Gulf killifish (Fundulus grandis) populations inhabiting the Houston Ship Channel (HSC), Texas, USA, which have been documented to have adapted to this heavily contaminated environment. Although not fully elucidated, one particularly important aspect of their adaptation involves the reduced inducibility of the aryl hydrocarbon receptor (AhR) and, potentially, the alteration of major biotransformation pathways. In the present study, we employed a modified Organization for Economic Cooperation and Development (OECD) 319-B test guideline to explore population and sex-related differences in the hepatic biotransformation of six polycyclic aromatic hydrocarbons (PAHs) in F. grandis populations with different exposure histories. Pollution-adapted F. grandis showed significantly lower hepatic clearance of PAHs than non-adapted fish, especially for high molecular weight PAHs (chrysene, benzo[k]fluoranthene, and benzo[a]pyrene), with pollution-adapted females presenting the lowest clearance. The characterization of different phase I biotransformation enzymes revealed that the basal activity of CYP1A, fundamental in the biotransformation of PAHs, was significantly lower in pollution-adapted fish, especially in females, which showed the lowest activity. Contrarily, basal CYP2C9-like activity was significantly higher in pollution-adapted fish. These results demonstrate the importance of exposure and evolutionary histories in shaping organisms' responses to pollution and provide significant evidence of sex-specific biotransformation differences in F. grandis populations.

The age factor in optic nerve regeneration: Intrinsic and extrinsic barriers hinder successful recovery in the short-living killifish

As the mammalian central nervous system matures, its regenerative ability decreases, leading to incomplete or non-recovery from the neurodegenerative diseases and central nervous system insults that we are increasingly facing in our aging world population. Current neuroregenerative research is largely directed toward identifying the molecular and cellular players that underlie central nervous system repair, yet it repeatedly ignores the aging context in which many of these diseases appear. Using an optic nerve crush model in a novel biogerontology model, that is, the short-living African turquoise killifish, the impact of aging on injury-induced optic nerve repair was investigated. This work reveals an age-related decline in axonal regeneration in female killifish, with different phases of the repair process being affected depending on the age. Interestingly, as in mammals, both a reduced intrinsic growth potential and a non-supportive cellular environment seem to lie at the basis of this impairment. Overall, we introduce the killifish visual system and its age-dependent regenerative ability as a model to identify new targets for neurorepair in non-regenerating individuals, thereby also considering the effects of aging on neurorepair.

Recognizing the enemy: do predator cues influence hatching in Neotropical annual killifish?

Annual fish species have evolved complex adaptations to survive in temporary wetlands. The main adaptation of these fish is the ability to produce embryos that survive dry periods. Embryo development of this fish can show variation at multiple levels influenced by many environmental factors, such as photoperiod and temperature. Predator cues are another factor that can influence the embryonic stage. One way in which annual fish could adapt to predators is by using risk-spreading strategies (through bet-hedging). Nonetheless, this strategy depends on the coevolutionary history between predators and preys and on the degree of environmental unpredictability, resulting in different responses across different species. This study investigated the influence of predator cues on the embryonic development and hatching of two Austrolebias species that inhabit ponds that present differences in hydroperiod and the risk of predator presence. The results confirmed a differentiated response between the two annual fish species tested, corroborating the modulation of hatching against the risk of predation by native predatory fish. The authors further showed that development times varied between the two annual fish species, regardless of the presence of predators. They highlight that the variation in embryonic development is strongly affected by different levels of hydroperiod unpredictability faced by the two species. To unravel finer-scale local adaptations in the annual fish embryo development, future studies should focus on a region with greater spatial gradient.

Agonistic interactions with asymmetric body size in two adult-age groups of the annual killifish Millerichthys robustus (Miller & Hubbs, 1974)

In this study, the author evaluated two adult age groups of the Mexican rivulus Millerichthys robustus with body size asymmetries to determine the strategies used by an annual killifish during agonistic interactions of different ontogenetic stages. To achieve this goal, the author first characterized the ethogram of agonistic interactions of M. robustus composed of seven behavioural units in males and five behavioural units in females. The author then analysed agonistic interaction strategies used by males and females with body size asymmetries in two groups of different adult ages that represent different ontogenetic stages: (a) just after sexual maturity was reached, at 5 weeks of age, and (b) near natural death, at 24 weeks of age. The agonistic behaviour patterns of M. robustus were compatible with the logic of mutual assessment. Large males had an advantage during their interactions in both age groups, winning all of the encounters. Nonetheless, there was more aggression in 5-week-old fish encounters. In addition, small 24-week-old fish were more aggressive than small 5-week-old fish. These changing strategies may be because of the cost-benefits required during a fight at each ontogenetic stage. In the female encounters, size did not predict winners, as both small and large fish won a similar number of encounters, and some contests remained unresolved regardless of age group. There was a tendency for small females of any age to risk more than males in fights to maintain reproductive fitness.

Exposure to wastewater effluent disrupts hypoxia responses in killifish (Fundulus heteroclitus)

Hypoxia (low oxygen) often occurs in aquatic ecosystems that receive effluent from municipal wastewater treatment plants (WWTP). The combination of hypoxia and WWTP effluent could impair fish health, because WWTP effluent contains multiple contaminants that could disrupt the physiological pathways fish use to cope with hypoxia, but the interactive effects of these stressors on fish physiology are poorly understood. We have examined this issue by exposing mummichog killifish (Fundulus heteroclitus) to hypoxia (5 and 2 kPa O₂) and/or 100% WWTP effluent for 21 days in a full factorial design. We then measured hypoxia tolerance, whole-animal metabolism, gill morphology, haematology, and tissue metabolites. In clean water, killifish responded to chronic hypoxia with improvements in hypoxia tolerance, as reflected by increases in time to loss of equilibrium at 0.5 kPa (t_LOE). These improvements occurred in association with increases in the exposed surface of gill lamellae that resulted from a regression of interlamellar cell mass (ILCM). Concurrent exposure to wastewater attenuated the increases in t_LOE and gill remodeling in chronic hypoxia, and nearly depleted brain glycogen stores. Therefore, exposure to WWTP effluent can disrupt the physiological mechanisms fish use to cope with chronic hypoxia and impair hypoxia tolerance. Our research suggests that the combination of stressors near WWTPs can have interactive effects on the physiology and health of fish.

Spawning habits and embryonic development of the banded lampeye killifish Aplocheilichthys spilauchen (Duméril 1861) in ex situ fresh and brackish water environments

Aside from ornamental uses, there is growing interest in using killifishes for a multiplicity of purposes including baitfish and mosquito biocontrol. This experiment explored the spawning habits and embryonic development of the banded lampeye, Aplocheilichthys spilauchen in ex situ freshwater (0.04‰) and brackish water (5.01‰) to ascertain the captive breeding prospects for mosquito control in areas where they occur. Significantly higher number of eggs were laid in the brackish water than the freshwater (X²  = 1613.0, P < 0.05), and black mop was the most preferred spawning substrate, followed by green, blue and white mops. Microscopic monitoring of embryos revealed that cleavage occurred within the first 30 min after fertilization, organogenesis commenced on average in the 25th hour and hatching in approximately 230 h. Although freshwater eggs were relatively bigger than brackish water eggs and certain embryonic developmental stages occurred faster in the freshwater than brackish water, these differences were overall not significant and had no effects on the development and hatching. The observed outcome that A. spilauchen can be optimally propagated with black mops in brackish water offers a significant step in its use for the mosquito biocontrol programme, as well as other potential uses not yet explored.

Phylogenetic relationships and ecological niche conservatism in killifish (Profundulidae) in Mesoamerica

The family Profundulidae is a group of small-sized fish species distributed between southern Mexico and Honduras, where they are frequently the only fish representatives at higher elevations in the basins where they occur. We characterized their ecological niche using different methods and metrics drawn from niche modelling and by re-examining phylogenetic relationships of a recently published molecular phylogeny of this family to gain a better understanding of its biogeographic and evolutionary history. We assessed both lines of evidence from the perspective of niche conservatism to set a foundation for discussing hypotheses about the processes underlying the distribution and evolution of the group. In fish clades where the species composition is not clear, we examined whether niche classification could be informative to discriminate groups geographically and ecologically consistent with any of the different hypotheses of valid species. The characterization of the ecological niche was carried out using the Maxent algorithm under different parameterizations and the projection of the presence on the main components of the most relevant environmental coverage, and the niche comparison was calculated with two indices (D and I), both in environmental space and in that projected geographically. With the molecular data, a species tree was generated using the *BEAST method. The comparison of these data was calculated with an age-overlap correlation test. Based on the molecular phylogeny and on niche overlap analyses, we uncovered strong evidence to support the idea that ecologically similar species are not necessarily sister species. The correlation analysis for genetic distance and niche overlap was not significant (P > 0.05). In clades with taxonomic conflicts, we only identified Profundulus oaxacae as a geographically and ecologically distinct group from P. punctatus. All the evidence considered leads us to propose that Profundulidae do not show evidence of niche conservatism and that there are reasons to consider P. oaxacae as a valid species. Our study suggests that niche divergence is a driving evolutionary force that caused the diversification and speciation processes of the Profundulidae, along with the geological and climatic events that promoted the expansion or contraction of suitable environments.

The role of gut microbial community and metabolomic shifts in adaptive resistance of Atlantic killifish (Fundulus heteroclitus) to polycyclic aromatic hydrocarbons

Altered gut microbiomes may play a role in rapid evolution to anthropogenic change but remain poorly understood. Atlantic killifish (Fundulus heteroclitus) in the Elizabeth River, VA have evolved resistance to polycyclic aromatic hydrocarbons (PAHs) and provide a unique opportunity to examine the links between shifts in the commensal microbiome and organismal physiology associated with evolved resistance. Here, 16S rRNA sequence libraries derived from fish guts and sediments sampled from a highly PAH contaminated site revealed significant differences collected at similar samples from an uncontaminated site. Phylogenetic groups enriched in the libraries derived from PAH-resistant fish were dissimilar to their associated sediment libraries, suggesting the specific environment within the PAH-resistant fish intestine influence the gut microbiome composition. Gut metabolite analysis revealed shifts between PAH-resistant and non-resistant subpopulations. Notably, PAH-resistant fish exhibited reduced levels of tryptophan and increased levels of sphingolipids. Exposure to PAHs appears to impact several bacterial in the gut microbiome, particularly sphingolipid containing bacteria. Bacterial phylotypes known to include species containing sphingolipids were generally lower in the intestines of fish subpopulations exposed to high concentrations of PAHs, inferring a complex host-microbiome relationship. Overall, killifish microbial community shifts appear to be related to a suppression of overall metabolite level, indicating a potential role of the gut in organismal response to anthropogenic environmental change. These results on microbial and metabolomics shifts are potentially linked to altered bioenergetic phenotype observed in the same PAH-resistant killifish populations in other studies.

Towards improved fish tests in ecotoxicology - Efficient chronic and multi-generational testing with the killifish Nothobranchius furzeri

As many freshwaters are chemically polluted, one of the challenges for policy makers is to determine the potential impact of these pollutants on ecosystems and to define safe concentrations. Common practice is the use of ecotoxicological assays to assess the response of model organisms from different trophic levels such as algae, invertebrates and fish during exposure to dilutions of a specific compound. Ideally, ecotoxicological assessments of (pseudo-)persistent chemicals should be performed across the life-cycle or even multiple generations for an accurate risk assessment. Multigenerational tests with fish are, however, impractical and costly given the long lifespan and generation time of classic model species. Here, we suggest a framework for more relevant, time- and cost-efficient fish-based testing in ecotoxicology and align it with accredited test guidelines. Next, we introduce an upcoming fish model, the turquoise killifish Nothobranchius furzeri, and show how it facilitates such research agendas due to a short lifespan and generation time. Through a review of fish-based exposure studies with a set of reference toxicants, we position N. furzeri as a sensitive species, suitable for screening effects of different pollutant types. Ultimately, we perform a cost-benefit analysis and propose a plan of action for the introduction of N. furzeri into accredited test guidelines.

PLHC-1 topminnow liver cells: An alternative model to investigate the toxicity of plastic additives in the aquatic environment

Plasticizers are widespread environmental contaminants that have been described as obesogens in terrestrial vertebrates. However, its effects on fish lipids homeostasis are almost unknown. This work explores the use of PLHC-1 cells as an alternative model to assess the disruption of hepatic lipids by plastic additives and to gather information on the mode of action of these chemicals in fish. PLHC-1 lipid extracts were analyzed by flow injection coupled to high resolution mass spectrometry (FIA-ESI(+/-)-Orbitrap-Exactive) after 24 h exposure of the cells to the selected plasticizers: dibutyl phthalate (DBP), di-(2-ethylhexyl) phthalate (DEHP), bisphenol A (BPA), bisphenol F (BPF), and chlorinated bisphenol A diglycidyl ether (BADGE·2HCl). The analysis of the culture medium and the intracellular concentration of the chemicals revealed the highest bioconcentration of BADGE·2HCl, DBP and DEHP, which was in agreement with the strongest alteration of the cells lipidome. BADGE·2HCl induced a significant depletion of triacylglycerides (TGs), while DEHP and DBP stimulated the accumulation of TGs. Exposure to BPF induced the generation of reactive oxygen species in PLHC-1 cells and a significant depletion of phosphatidylcholine (PC)- and phosphatidylethanolamine (PE)-plasmalogens, and TGs (cell depots of polyunsaturated fatty acids). Overall, this study evidences different modes of action of plastic additives in topminnow liver cells, describes differential lipidomic signatures, and highlights the higher lipotoxicity of BADGE·2HCl and BPF compared to BPA.

Effects of crude oil vapors on the cardiovascular flow of embryonic Gulf killifish

Direct contact with toxicants in crude oil during embryogenesis causes cardiovascular defects, but the effects of exposure to airborne volatile organic compounds released from spilled oil are not well understood. The effects of crude oil-derived airborne toxicants on peripheral blood flow were examined in Gulf killifish (Fundulus grandis) since this model completes embryogenesis in the air. Particle image velocimetry was used to measure in vivo blood flow in intersegmental arteries of control and oil-exposed embryos. Significant effects in oil-exposed embryos included increased pulse rate, reduced mean blood flow speed and volumetric flow rate, and decreased pulsatility, demonstrating that normal-appearing oil-exposed embryos retain underlying cardiovascular defects. Further, hematocrit moderately increased in oil-exposed embryos. This study highlights the potential for fine-scale physiological measurement techniques to better understand the sub-lethal effects of oil exposure and demonstrates the efficacy of Gulf killifish as a unique teleost model for aerial toxicant exposure studies.

Estimation of the uptake and gut retention of microplastics in juvenile marine fish: Mummichogs (Fundulus heteroclitus) and red seabreams (Pagrus major)

We investigated the impact of microplastics (MPs) on marine fish by estimating the uptake and retention by mummichogs (Fundulus heteroclitus) and red seabreams (Pagrus major) of MPs similar in size (≥0.25 mm) and composition (polyethylene) to MPs detected in fish intestines. Results revealed a correlation between MP concentrations in aquarium water and the content of MPs in the gastrointestinal tracts of exposed fish. More than 95% were excreted from both species within 25 h; this retention time is similar to the processing time of food items. The rate of excretion showed little dependence on MP size, but there was some dependence on fish species and MP shape. These results suggest that MPs similar to those we studied have little direct adverse impact on these two marine fish species.

GABA metabolism is crucial for long-term survival of anoxia in annual killifish embryos

In most vertebrates, a lack of oxygen quickly leads to irreparable damages to vital organs, such as the brain and heart. However, there are some vertebrates that have evolved mechanisms to survive periods of no oxygen (anoxia). The annual killifish (Austrofundulus limnaeus) survives in ephemeral ponds in the coastal deserts of Venezuela and their embryos have the remarkable ability to tolerate anoxia for months. When exposed to anoxia, embryos of A. limnaeus respond by producing significant amounts of γ-aminobutyric acid (GABA). This study aims to understand the role of GABA in supporting the metabolic response to anoxia. To explore this, we investigated four developmentally distinct stages of A. limnaeus embryos that vary in their anoxia tolerance. We measured GABA and lactate concentrations across development in response to anoxia and aerobic recovery. We then inhibited enzymes responsible for the production and degradation of GABA and observed GABA and lactate concentrations, as well as embryo mortality. Here, we show for the first time that GABA metabolism affects anoxia tolerance in A. limnaeus embryos. Inhibition of enzymes responsible for GABA production (glutamate decarboxylase) and degradation (GABA-transaminase and succinic acid semialdehyde dehydrogenase) led to increased mortality, supporting a role for GABA as an intermediate product and not a metabolic end-product. We propose multiple roles for GABA during anoxia and aerobic recovery in A. limnaeus embryos, serving as a neurotransmitter, an energy source, and an anti-oxidant.

Antidepressant exposure reduces body size, increases fecundity and alters social behavior in the short-lived killifish Nothobranchius furzeri

Social and mating behavior are fundamental fitness determinants in fish. Although fish are increasingly exposed to pharmaceutical compounds that may alter expression of such behavior, potential effects are understudied. Here, we examine the impact of lifelong exposure to two concentrations (0.7 and 5.3 μg/L) of the antidepressant fluoxetine on fecundity and social behavior (i.e. sociability and male-male aggression) in the turquoise killifish, Nothobranchius furzeri. When exposed to the highest concentration of fluoxetine (5.3 μg/L), fish were smaller at maturation but they more frequently engaged in mating. In addition, in both fluoxetine treatments females roughly doubled their overall fecundity while egg fertilization rates were the same for exposed and unexposed fish. Although aggression of male fish was not impacted by fluoxetine exposure, exposed male fish (5.3 μg/L) spent more time in the proximity of a group of conspecifics, which implies an increased sociability in these individuals. Overall, the results of this study indicate that exposure to fluoxetine may result in disrupted male sociability, increased mating frequency and an increased reproductive output in fish populations.

No water, no problem: stage-specific metabolic responses to dehydration stress in annual killifish embryos

Annual killifish survive in temporary ponds by producing drought-tolerant embryos that can enter metabolic dormancy (diapause). Survival of dehydration stress is achieved through severe reduction of evaporative water loss. We assessed dehydration stress tolerance in diapausing and developing Austrofundulus limnaeus embryos. We measured oxygen consumption rates under aquatic and aerial conditions to test the hypothesis that there is a trade-off between water retention and oxygen permeability. Diapausing embryos survive dehydrating conditions for over 1.5 years, and post-diapause stages can survive for over 100 days. Diapausing embryos respond to dehydration stress by increasing oxygen consumption rates while post-diapause embryos exhibit the same or reduced rates compared with aquatic embryos. Thus, water retention does not always limit oxygen diffusion. Aerial incubation coupled with hypoxia causes some embryos to arrest development. The observed stage-specific responses are consistent with an intrinsic bet-hedging strategy in embryos that would increase developmental variation in a potentially adaptive manner.

Intra-species differences in population size shape life history and genome evolution

The evolutionary forces shaping life history divergence within species are largely unknown. Turquoise killifish display differences in lifespan among wild populations, representing an ideal natural experiment in evolution and diversification of life history. By combining genome sequencing and population genetics, we investigate the evolutionary forces shaping lifespan among wild turquoise killifish populations. We generate an improved reference genome assembly and identify genes under positive and purifying selection, as well as those evolving neutrally. Short-lived populations from the outer margin of the species range have small population size and accumulate deleterious mutations in genes significantly enriched in the WNT signaling pathway, neurodegeneration, cancer and the mTOR pathway. We propose that limited population size due to habitat fragmentation and repeated population bottlenecks, by increasing the genome-wide mutation load, exacerbates the effects of mutation accumulation and cumulatively contribute to the short adult lifespan.

Fluctuating environments during early development can limit adult phenotypic flexibility: insights from an amphibious fish

The interaction between developmental plasticity and the capacity for reversible acclimation (phenotypic flexibility) is poorly understood, particularly in organisms exposed to fluctuating environments. We used an amphibious killifish (Kryptolebias marmoratus) to test the hypotheses that organisms reared in fluctuating environments (i) will make no developmental changes to suit any one environment because fixing traits to suit one environment could be maladaptive for another, and (ii) will be highly phenotypically flexible as adults because their early life experiences predict high environmental variability in the future. We reared fish under constant (water) or fluctuating (water-air) environments until adulthood and assessed a suite of traits along the oxygen cascade (e.g. neuroepithelial cell density and size, cutaneous capillarity, gill morphology, ventricle size, red muscle morphometrics, terrestrial locomotor performance). To evaluate the capacity for phenotypic flexibility, a subset of adult fish from each rearing condition was then air-exposed for 14 days before the same traits were measured. In support of the developmental plasticity hypothesis, traits involved with O2 sensing and uptake were largely unaffected by water-air fluctuations during early life, but we found marked developmental changes in traits related to O2 transport, utilization and locomotor performance. In contrast, we found no evidence supporting the phenotypic flexibility hypothesis. Adult fish from both rearing conditions exhibited the same degree of phenotypic flexibility in various O2 sensing- and uptake-related traits. In other cases, water-air fluctuations attenuated adult phenotypic flexibility despite the fact that phenotypic flexibility is hypothesized to be favoured when environments fluctuate. Overall, we conclude that exposure to environmental fluctuations during development in K. marmoratus can dramatically alter the constitutive adult phenotype, as well as diminish the scope for phenotypic flexibility in later life.

Plasticity, repeatability and phenotypic correlations of aerobic metabolic traits in a small estuarine fish

Standard metabolic rate (SMR), maximum metabolic rate (MMR), absolute aerobic scope (AAS) and critical oxygen tension (P_crit) were determined for the Gulf killifish, Fundulus grandis, an ecologically dominant estuarine fish, acclimated to lowered salinity, elevated temperature and lowered oxygen concentration. Acclimation to low salinity resulted in a small, but significant, elevation of P_crit (suggesting lower tolerance of hypoxia); acclimation to elevated temperature increased SMR, MMR, AAS and P_crit; acclimation to low oxygen led to a small increase in SMR, but substantial decreases in MMR, AAS and P_crit Variation in these metabolic traits among individuals was consistent and repeatable when measured during multiple control exposures over 7 months. Trait repeatability was unaffected by acclimation condition, suggesting that repeatability of these traits is not context dependent. There were significant phenotypic correlations between specific metabolic traits: SMR was positively correlated with MMR and P_crit; MMR was positively correlated with AAS; and AAS was negatively correlated with P_crit In general, within-individual variation contributed more than among-individual variation to these phenotypic correlations. The effects of acclimation on these traits demonstrate that aerobic metabolism is plastic and influenced by the conditions experienced by these fish in the dynamic habitats in which they occur; however, the repeatability of these traits and the correlations among them suggest that these traits change in ways that maintain the rank order of performance among individuals across a range of environmental variation.

Plasticity, repeatability and phenotypic correlations of aerobic metabolic traits in a small estuarine fish

Standard metabolic rate (SMR), maximum metabolic rate (MMR), absolute aerobic scope (AAS) and critical oxygen tension (Pcrit) were determined for the Gulf killifish, Fundulus grandis, an ecologically dominant estuarine fish, acclimated to lowered salinity, elevated temperature and lowered oxygen concentration. Acclimation to low salinity resulted in a small, but significant, elevation of Pcrit (suggesting lower tolerance of hypoxia); acclimation to elevated temperature increased SMR, MMR, AAS and Pcrit; acclimation to low oxygen led to a small increase in SMR, but substantial decreases in MMR, AAS and Pcrit Variation in these metabolic traits among individuals was consistent and repeatable when measured during multiple control exposures over 7 months. Trait repeatability was unaffected by acclimation condition, suggesting that repeatability of these traits is not context dependent. There were significant phenotypic correlations between specific metabolic traits: SMR was positively correlated with MMR and Pcrit; MMR was positively correlated with AAS; and AAS was negatively correlated with Pcrit In general, within-individual variation contributed more than among-individual variation to these phenotypic correlations. The effects of acclimation on these traits demonstrate that aerobic metabolism is plastic and influenced by the conditions experienced by these fish in the dynamic habitats in which they occur; however, the repeatability of these traits and the correlations among them suggest that these traits change in ways that maintain the rank order of performance among individuals across a range of environmental variation.

Hypoxia acclimation alters reactive oxygen species homeostasis and oxidative status in estuarine killifish (Fundulus heteroclitus)

Hypoxia is common in aquatic environments, and exposure to hypoxia followed by re-oxygenation is often believed to induce oxidative stress. However, there have been relatively few studies of reactive oxygen species (ROS) homeostasis and oxidative status in fish that experience natural hypoxia-re-oxygenation cycles. We examined how exposure to acute hypoxia (2 kPa O2) and subsequent re-oxygenation (to 20 kPa O2) affects redox status, oxidative damage and anti-oxidant defenses in estuarine killifish (Fundulus heteroclitus), and whether these effects were ameliorated or potentiated by prolonged (28 days) acclimation to either constant hypoxia or intermittent cycles of nocturnal hypoxia (12 h:12 h normoxia:hypoxia). Acute hypoxia and re-oxygenation led to some modest and transient changes in redox status, increases in oxidized glutathione, depletion of scavenging capacity and oxidative damage to lipids in skeletal muscle. The liver had greater scavenging capacity, total glutathione concentrations and activities of anti-oxidant enzymes (catalase, glutathione peroxidase) than muscle, and generally experienced less variation in glutathiones and lipid peroxidation. Unexpectedly, acclimation to constant hypoxia or intermittent hypoxia led to a more oxidizing redox status (muscle and liver) and it increased oxidized glutathione (muscle). However, hypoxia-acclimated fish exhibited little to no oxidative damage (as reflected by lipid peroxidation and aconitase activity), in association with improvements in scavenging capacity and catalase activity in muscle. We conclude that hypoxia acclimation leads to adjustments in ROS homeostasis and oxidative status that do not reflect oxidative stress, but may instead be part of the suite of responses that killifish use to cope with chronic hypoxia.

The influence of environmental instability on the reproductive strategy of the critically endangered Acıgöl killifish (Aphanius transgrediens)

To investigate the variability in the reproductive tactics of Acıgöl killifish (Aphanius transgrediens), one of the most seriously threatened freshwater teleost species, this study compared its reproductive ecology in two contrasting habitats that differ substantially in terms of stability of environmental parameters, particularly salinity regime (stable vs. unstable). Fish were sampled monthly from October 2013 to September 2014 with the aim of testing whether the reproductive life-history response of fish to stable and unstable conditions differed. The reproductive effort (gonad weight) of both sexes did not differ significantly between the two habitats, but females in the unstable habitat had significantly lower fecundity and larger eggs. The relationship between fecundity and fish size was stronger in the stable habitat, whereas the relationship was quite variable and uncertain in the unstable habitat. Fish born in the unstable habitat reached their first maturity at a smaller size than those in the stable habitat. The gonado-somatic index and the duration of hydrated eggs showed that reproduction continued from February to May in both habitats; nonetheless, a second spawning event occurred during July and August in the unstable habitat, which included the reproductive contributions of YOY individuals and older generations. This study's results suggest that A. transgrediens employs varying reproductive strategies against environmental instability in its restricted unique range. This may have further implications for the ways in which habitat-specific conservation methods are used.