Size-dependent zoogeographical distribution of gelatinous thaliaceans associated with current velocity and temperature

Thaliaceans are globally distributed and play an important role in the world's biological carbon pump and marine ecosystems by forming dense swarms with high feeding rates and producing large amounts of fecal pellets and carcasses. The contribution of thaliacean swarms to the downward transport of carbon depends not only on their abundance but also on their body size. However, the key factors influencing the distribution of different-sized thaliaceans remain unstudied. To discriminate thaliacean assemblages and examine the key factors determining the zoogeographical distribution and abundance of different-sized thaliaceans during different monsoon periods, we conducted three cruises in the South China Sea from before the southwest monsoon to the peak of the northeast monsoon. Our results revealed that high thaliacean abundance corresponded to high chlorophyll a concentration, which were associated with hydrodynamic processes, such as upwelling and eddies. Hierarchical partitioning and niche difference analyses demonstrated that current velocity and temperature are key factors that shaped the zoogeographical distribution of different-sized thaliaceans. The global dataset indicated that small-sized thaliacean species tend to occur in coastal areas where the current velocity is generally high, while large-sized species tend to occur in open ocean areas where the current velocity is generally low. The results revealed that global warming-induced changes in surface current velocity and temperature may alter the zoogeographical distribution and abundance of thaliaceans with different sizes, thereby affecting the biological carbon pump and surrounding marine ecosystem. Overall, this study sheds light on the potential responses of pelagic tunicates to global climate change through changes in their hydrodynamic conditions.

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.

Stack of cellular lamellae forms a silvered cortex to conceal the opaque organ in a transparent gastropod in epipelagic habitat

Background

Gelatinous zooplankton in epipelagic environments often have highly transparent bodies to avoid detection by their visual predators and prey; however, the digestive systems are often exceptionally opaque even in these organisms. In a holoplanktonic gastropod, Pterotrachea coronata, the visceral nucleus is an opaque organ located at the posterior end of its alimentary system, but this organ has a mirrored surface to conceal its internal opaque tissue.

Results

Our ultrastructural observation proved that the cortex of the visceral nucleus comprised a stack of thin cellular lamellae forming a Bragg reflector, and the thickness of lamellae (0.16 µm in average) and the spaces between the lamellae (0.1 µm in average) tended to become thinner toward inner lamellae. Based on the measured values, we built virtual models of the multilamellar layer comprising 50 lamellae and spaces, and the light reflection on the models was calculated using rigorous coupled wave analysis to evaluate their properties as reflectors. Our simulation supported the idea that the layer is a reflective tissue, and the thickness of the lamella/space must be chirped to reflect sunlight as white/silver light, mostly independent of the angle of incidence.

Conclusions

In P. coronata, the cortex of the visceral nucleus comprised multicellular lamellae that form a chirped Bragg reflector. It is distinct in structure from the intracellular Bragg structures of common iridophores. This novel Bragg reflector demonstrates the diversity and convergent evolution of reflective tissue using reflectin-like proteins in Mollusca.

Comparison of the polarization contrast of gelatinous zooplankton and a transparent single-use plastic bag-Implications for marine animals

Plastic pollution in the ocean is an increasingly detrimental issue for marine organisms. As a form of polarized light pollution, transparent plastic debris may be more visible and pose additional threats to organisms that can detect and interpret polarized light. Plastic can mimic the visual features of common marine prey items, such as transparent gelatinous zooplankton, which may lead to more significant plastic ingestion. We measured, in situ, the polarization and radiance contrast between a transparent plastic bag and gelatinous zooplankton with an underwater video polarimeter. The plastic bag had significantly higher polarization contrast than the gelatinous zooplankton, yet both shared similar radiance contrasts. This higher polarization contrast may contribute to the observed high ingestion rates of transparent plastic by marine organisms. Further study into the connection between polarization-sensitive organisms and plastic ingestion is recommended.

Microplastics ingestion in the ephyra stage of Aurelia sp. triggers acute and behavioral responses

For the first time, we report a correspondence between microplastics (MP) ingestion and ecotoxicological effects in gelatinous zooplankton (Cnidarian jellyfish). The ephyra stage of the jellyfish Aurelia sp. was exposed to both environmental and high concentrations of fluorescent 1-4 μm polyethylene MP (0.01-10 mg/L). After 24 and 48 h, MP accumulation, acute (Immobility) and behavioral (Frequency pulsation) endpoints were investigated. MP were detected by confocal and tomographic investigations on gelatinous body and mouth, either attached on the surface or ingested. This interaction was responsible for impairing ephyrae survival and behavior at all tested concentrations after 24 h. Acute and behavioral effects were also related to mechanical disturbance, caused by MP, triggering a loss of radial symmetry. Contaminated ephyrae exposed to clean seawater showed full recovery after 72 h highlighting the organisms without the microspheres, attached on body jellyfish surface around the mouth and lappets. In conclusion, short-term exposure to MP affects ephyrae jellyfish health, impairing both their survival and behavior. Polyethylene MP temporarily affect both Immobility and Frequency of pulsation of Aurelia sp. jellyfish. This study provides a first step towards understanding and clarifying the potential impacts of MP contamination in gelatinous zooplankton.

Mercury and selenium concentrations in Scyphozoan jellyfishes and pyrosomes from Monterey Bay National Marine Sanctuary

Gelatinous zooplankton including jellyfishes, pyrosomes, and salps serve as important prey items for a number of marine species; however, relatively few studies have examined contaminant concentrations in these animals. Scyphozoans (Aurelia sp., Chrysaora colorata, C. fuscescens, and Phacellophora camtschatica) and Thaliaceans (Pyrosoma sp.) were collected from 2009 to 2011 from Monterey Bay National Marine Sanctuary and analyzed for total mercury and selenium concentrations. In general, mercury (0.0001-0.0016 μg/g wet weight) and selenium (0.009-0.304 μg/g wet weight) concentrations of the sampled organisms were low; however, the two Pyrosoma sp. had total mercury and selenium concentrations that were one order of magnitude higher than the Scyphozoans. There was a significant positive relationship between mercury and selenium concentrations in jellyfishes and pyrosomes, suggesting a potential detoxification mechanism in these lower trophic level organisms. This study provides evidence that trophic transfer of mercury and selenium likely occurs through ingestion of gelatinous prey.

Floating nurseries? Scyphozoan jellyfish, their food and their rich symbiotic fauna in a tropical estuary

Background

The anthropogenic modification of trophic pathways is seemingly prompting the increase of jellyfish populations at the expense of planktivorous fishes. However, gross generalizations are often made because the most basic aspects of trophic ecology and the diverse interactions of jellyfish with fishes remain poorly described. Here we inquire on the dynamics of food consumption of the medusoid stage of the scyphozoan jellyfish Stomolophus meleagris and characterize the traits and diversity of its symbiotic community.

Methods

S. meleagris and their associated fauna were sampled in surface waters between November 2015 and April 2017 in Málaga Bay, an estuarine system at the Colombian Pacific. Stomach contents of medusae were examined and changes in prey composition and abundance over time analysed using a multivariate approach. The associated fauna was identified and the relationship between the size of medusae and the size those organisms tested using least-square fitting procedures.

Results

The presence of S. meleagris medusa in surface waters was seasonal. The gut contents analysis revealed that algae, copepods and fish early life stages were the more abundant items, and PERMANOVA analysis showed that the diet differed within the seasons (P_(perm) = 0.001) but not between seasons (P_(perm) = 0.134). The majority of the collected medusae (50.4%) were associated with individuals of 11 symbiotic species, 95.3% of them fishes, 3.1% crustaceans and 1.6% molluscs. Therefore, this study reports 10 previously unknown associations. The bell diameter of S. meleagris was positively related to the body sizes of their symbionts. However, a stronger fit was observed when the size relationship between S. meleagris and the fish Hemicaranx zelotes was modelled.

Discussion

The occurrence of S. meleagris was highly seasonal, and the observed patterns of mean body size through the seasons suggested the arrival of adult medusae to the estuary from adjacent waters. The diet of S. meleagris in the study area showed differences with previous reports, chiefly because of the abundance of algae that are seemingly ingested but not digested. The low number of zooplanktonic items in gut contents suggest the contribution of alternative food sources not easily identifiable. The observed changes in the composition of food in the guts probably reflect seasonal changes in the availability of prey items. The regular pattern in the distribution of symbionts among medusae (a single symbiont per host) and the positive host-symbiont size relationship reflects antagonistic intraspecific and interspecific behaviour of the symbiont. This strongly suggest that medusa represent an “economically defendable resource” that potentially increases the survival and recruitment of the symbionts to the adult population. We argue that, if this outcome of the symbiotic association can be proven, scyphozoan jellyfish can be regarded as floating nurseries.

Decomposition of jellyfish carrion in situ: Short-term impacts on infauna, benthic nutrient fluxes and sediment redox conditions

Jellyfish often form blooms that persist for weeks to months before they collapse en masse, resulting in the sudden release of large amounts of organic matter to the environment. This study investigated the biogeochemical and ecological effects of the decomposition of jellyfish in a shallow coastal lagoon in New South Wales, Australia. Catostylus mosaicus carrion was added to the surface of shallow sub-tidal sediments and biogeochemical parameters and macrofaunal abundance immediately below the jellyfish carrion were measured over three days. Sediment plots without jellyfish served as controls. Sediment oxygen demand and carbon and nitrogen efflux increased by up to 60-fold in the jellyfish plots, compared to control plots, and dissolved organic nutrient fluxes were more sustained than in previous studies due to the use of fresh rather than frozen biomass. The decomposing jellyfish progressively altered sediment redox conditions, indicated by an increase in porewater iron (II) and sulfide concentrations measured by high-resolution in situ diffusive samplers. Abundance of some macrofaunal taxa in the jellyfish plots decreased relative to controls, however, the abundance of a carnivorous gastropod, which was presumably feeding on the carrion, increased in the jellyfish plots. While jellyfish carrion may be a food source for some macrofauna, low oxygen conditions coupled with the accumulation of toxic dissolved sulfides in the near-surface sediments may explain the overall change in the macroinfaunal community.