Isotopic evidence for the dietary difference between Rhizostomeae Nemopilema nomurai and Semaeostomeae Cyanea nozakii.

Blooms of the Rhizostomeae Nemopilema nomurai and the Semaeostomeae Cyanea nozakii have become more prominent in the coastal waters of China since the end of the 20th century. However, the trophic ecology of these jellyfish species remain incompletely understood. In this study, the trophic characterizations of N. nomurai and C. nozakii populations were assessed using stable isotope analysis (SIA), with a focus on the important bloom area offshore of the Yangtze Estuary. Our results indicated obvious trophic differences between two scyphomedusae. The higher trophic position of the C. nozakii population in the coastal planktonic food web was reflected by its relatively large δ15N value compared to that of N. nomurai. The MixSIAR model indicated that small copepods (<1000 µm) and seston were important food sources for N. nomurai, and showed a stable dietary, irrespective of N. nomurai size. Conversely, C. nozakii exhibited a more diverse diet composition, and gelatinous organisms also were an important part of the diet of C. nozakii. Moreover, a pronounced ontogenetic shift in the diet of C. nozakii was observed, consisting of an increase in the proportion of zooplanktonic prey (excluding seston) in the C. nozakii diet with diameter. This study provides isotopic evidence of the substantial difference in trophic ecology between N. nomurai and C. nozakii, which resulted from the variations in SI values and diet compositions. Inconsistent size-based variation patterns were observed in trophic ontogenetic shifts within the N. nomurai and C. nozakii groups, highlighting a need for further investigation. These results will give insights into the characteristics of trophic ecology and functional roles of Rhizostomeae and Semaeostomeae, and indicate the need for a more careful consideration of the representations of Rhizostomeae and Semaeostomeae in coastal ecosystems, so as not to underestimate the knowledge of taxon-specific ecological effects on energy flow.

Trophic effects of jellyfish blooms on fish populations in ecosystems of the coastal waters of China.

Jellyfish play an important role in the material cycling and energy flow of food webs, and massive aggregations may have deleterious consequences for local fisheries; yet a theoretical framework of the trophic effects of jellyfish blooms on coastal fisheries is unclear. To address this knowledge gap, we assessed the trophic interactions between cooccurring bloom jellyfish and dominant fish groups (omnivorous fish and piscivorous fish) in the coastal waters of China (CWC) via stable isotope analysis; we subsequently discussed how jellyfish blooms may affect energy flow through coastal ecosystems. Our results indicate a considerable degree of trophic overlap (mean ratio > 65 %) between jellyfish and small omnivorous fish (< 10 cm), highlighting a similarity in feeding habits, while the overlap ratio decreased to <55 % of the large omnivorous fish group (> 10 cm). Relatively higher trophic levels and smaller overlaps of large omnivorous fish were found in the ecosystem with high jellyfish biomass, which suggested that they may reinforce the ontogenetic trophic shift pattern to alleviate the potential for resource competition with jellyfish under conditions of jellyfish explosion. The smallest trophic overlap (< 20 %) highlighted the strong trophic differentiation between jellyfish and piscivorous fish. Additionally, our study suggested that a massive aggregation of jellyfish can negatively influence zooplankton but may not transfer energy further up efficiently, implying a weak trophic coupling between jellyfish and upper-trophic levels in CWC ecosystems. Thus, we speculate that jellyfish play an important role in shaping pathways involved in the energy transfer of food webs and that large blooms may negatively affect fisheries through bottom-up control affecting prey availability. In general, these results hold strong potential to further improve the understanding of the trophic interactions between jellyfish and fish populations. Furthermore, this study provides valuable data for predicting the consequences of jellyfish blooms on ecosystems, and is crucial for ecosystem-based management of coastal fisheries.

Toxicology assessment of deep-sea mining impacts on Gigantidas platifrons: A comparative in situ and laboratory metal exposure study.

Deep-sea toxicology is essential for deep-sea environmental impact assessment. Yet most toxicology experiments are conducted solely in laboratory settings, overlooking the complexities of the deep-sea environment. Here we carried out metal exposure experiments in both the laboratory and in situ, to compare and evaluate the response patterns of Gigantidas platifrons to metal exposure (copper [Cu] or cadmium [Cd] at 100 µg/L for 48 h). Metal concentrations, traditional biochemical parameters, and fatty acid composition were assessed in deep-sea mussel gills. The results revealed significant metal accumulation in deep-sea mussel gills in both laboratory and in situ experiments. Metal exposure could induce oxidative stress, neurotoxicity, an immune response, altered energy metabolism, and changes to fatty acid composition in mussel gills. Interestingly, the metal accumulating capability, biochemical response patterns, and fatty acid composition each varied under differing experimental systems. In the laboratory setting, Cd-exposed mussels exhibited a higher value for integrated biomarker response (IBR) while in situ the Cu-exposed mussels instead displayed a higher IBR value. This study emphasizes the importance of performing deep-sea toxicology experiments in situ and contributes valuable data to a standardized workflow for deep-sea toxicology assessment.

Distribution and biomass of gelatinous zooplankton in relation to an oxygen minimum zone and a shallow seamount in the Eastern Tropical North Atlantic Ocean.

Physical and topographic characteristics can structure pelagic habitats and affect the plankton community composition. For example, oxygen minimum zones (OMZs) are expected to lead to a habitat compression for species with a high oxygen demand, while upwelling of nutrient-rich deep water at seamounts can locally increase productivity, especially in oligotrophic oceanic waters. Here we investigate the response of the gelatinous zooplankton (GZ) assemblage and biomass to differing oxygen conditions and to a seamount in the Eastern Tropical North Atlantic (ETNA) around the Cape Verde archipelago. A total of 16 GZ taxa (>1100 specimens) were found in the upper 1000 m with distinct species-specific differences, such as the absence of deep-living species Atolla wyvillei and Periphylla periphylla above the shallow seamount summit. Statistical analyses considering the most prominent groups, present at all stations, namely Beroe spp., hydromedusae (including Zygocanna vagans, Halicreas minimum, Colobonema sericeum, Solmissus spp.) and total GZ, showed a strong positive correlation of abundance with temperature for all groups, whereas oxygen had a weak negative correlation only with abundances of Beroe spp. and hydromedusae. To account for size differences between species, we established length-weight regressions and investigated total GZ biomass changes in relation to physical (OMZ) and topographic characteristics. The highest GZ biomass was observed at depths of lowest oxygen concentrations and deepest depth strata at the southeastern flank of the seamount and at two stations south of the Cape Verde archipelago. Our data suggest that, irrespective of their patchy distribution, GZ organisms are ubiquitous food web members of the ETNA, and their habitat includes waters of low oxygen content.

Seasonal variability of the fatty acid composition in Aurelia aurita (Cnidaria: Scyphozoa): implications for gelativore food web studies.

Jellyfish population play an important role in aquatic food chains, and many animals predate on this 'mostly water containing' organisms. However, what gelativores predators could gain from their prey is still poorly understood. This study provides insight into the nutritional value of the moon jelly (Aurelia aurita) by means of its fatty acid (FA) composition, while investigating seasonal variability and differences between its free-swimming life stages. A biweekly sampling was carried out in a temperate coastal ecosystem, the Kiel Fjord, Germany and during two consecutive years. FA profile of A. aurita showed significant seasonal variability, while mature medusae (due to reproductive tissues) possessed highest FA content. In addition, moon jelly contained several essential FAs (i.e. arachidonic acid, 20:4ω6; eicosapentaenoic acid, 20:5ω3; docosahexaenoic acid, 22:6ω3), which likely support predator's vital physiological functions. Even though total FA contents proved to be low (7 × 10-3-34 × 10-3% per g dry weight), evidence supporting A. aurita's capability to meet the dietary requirements of predators such as fish and crustaceans is provided. Finally, implications for gelativore and future food web configurations are discussed, while proposing that jellyfish are likely to be, and become, more than an opportunistic prey to many organisms.

Food quality matters: Interplay among food quality, food quantity and temperature affecting life history traits of Aurelia aurita (Cnidaria: Scyphozoa) polyps.

Understanding the interaction between organisms' life history traits and environmental factors is an essential task in ecology. In spite of the increasing appreciation of jellyfish as an important component in marine ecosystem, there are still considerable gaps in understanding how the phase transition from the benthic polyp to the pelagic medusa stage is influenced by multiple environmental factors, including nutrition. To investigate survival, growth, and phase transition of Aurelia aurita polyps, we designed a factorial experiment manipulating food quantity (20µg C, 5µg C and 1.5µg C polyp-1 every other day), food quality (Artemia salina and two dietary manipulated Acartia tonsa), and temperature (13°C, 20°C, and 27°C). Temperature was the key factor determining phase transition of polyps and negatively affecting their survival rate and growth at 27°C, which reflected a summer heatwave scenario. Furthermore, at polyps' optimum tolerance temperature (20°C) in our study, budding reproduction benefits from high food concentrations. Interestingly, polyps fed with food containing high level highly unsaturated fatty acid (HUFA) were able to compensate for physiological stress caused by the extreme temperature, and could enhance budding reproduction at optimum temperature. Moreover, benthic-pelagic coupling (strobilation) was determined by temperature but affected significantly by food conditions. Mild temperature together with optimum food conditions contributes to inducing more polyps, which may potentially bring about great ephyrae recruitments during overwintering. In contrast, heatwave events can potentially regulate plankton community structure accompanied by changes of nutritional conditions of primary and secondary producers and thus, negatively affect the population dynamics of polyps. We suggest a novel polyp tolerance curve, which can help to understand jellyfish population dynamics in different seasons and ecosystems. This sets up a baseline for understanding how anticipated global warming and food conditions may affect the population size of benthic polyps and consequently pelagic medusae.

Food availability drives plastic self-repair response in a basal metazoan- case study on the ctenophore Mnemiopsis leidyi A. Agassiz 1865.

Many marine invertebrates including ctenophores are capable of extensive body regeneration when injured. However, as for the invasive ctenophore Mnemiopsis leidyi, there is a constant subportion of individuals not undergoing whole body regeneration but forming functionally stable half-animals instead. Yet, the driving factors of this phenomenon have not been addressed so far. This study sheds new light on how differences in food availability affect self-repair choice and regeneration success in cydippid larvae of M. leidyi. As expected, high food availability favored whole-body regeneration. However, under low food conditions half-animals became the preferential self-repair mode. Remarkably, both regenerating and half-animals showed very similar survival chances under respective food quantities. As a consequence of impaired food uptake after injury, degeneration of the digestive system would often occur indicating limited energy storage capacities. Taken together, this indicates that half-animals may represent an alternative energy-saving trajectory which implies self-repair plasticity as an adaptive trade-off between high regeneration costs and low energy storage capacities. We conclude that self-repair plasticity could lead to higher population fitness of ctenophores under adverse conditions such as in ships' ballast water tanks which is postulated to be the major vector source for the species' spreading around the globe.