Reprint of: High prey-predator size ratios and unselective feeding in copepods: A seasonal comparison of five species with contrasting feeding modes

There has been an upsurge of interest in trait-based approaches to zooplankton, modelling the seasonal changes in the feeding modes of zooplankton in relation to phytoplankton traits such as size or motility. We examined this link at two English Channel plankton monitoring sites south of Plymouth (L4 and E1). At L4 there was a general transition from diatoms in spring to motile microplankton in summer and autumn, but this was not mirrored in the succession of copepod feeding traits; for example the ambushing Oithona similis dominated during the spring diatom bloom. At nearby E1 we measured seasonality of food and grazers, finding strong variation between 2014 and 2015 but overall low mesozooplankton biomass (median 4.5 mg C m−3). We also made a seasonal grazing study of five copepods with contrasting feeding modes (Calanus helgolandicus, Centropages typicus, Acartia clausi, Pseudocalanus elongatus and Oithona similis), counting the larger prey items from the natural seston. All species of copepod fed on all food types and differences between their diets were only subtle; the overriding driver of diet was the composition of the prey field. Even the smaller copepods fed on copepod nauplii at significant rates, supporting previous suggestions of the importance of intra-guild predation. All copepods, including O. similis, were capable of tackling extremely long (>500 µm) diatom chains at clearance rates comparable to those on ciliates. Maximum observed prey:predator length ratios ranged from 0.12 (C. helgolandicus) up to 0.52 (O. similis). Unselective feeding behaviour and the ability to remove highly elongated cells have implications for how copepod feeding is represented in ecological and biogeochemical models

Variabilité et plasticité de la nutrition des méduses à zooxanthelles : apports expérimentaux et de terrain

Whereas most jellyfishes are strictly heterotrophic organisms, some of them undergo a photosymbiosis with autotrophic Dinophyceae (“zooxanthellae”). These zooxanthellate jellyfishes, as holobionts, are mixotrophic deriving nutrition from both predation and photosynthesis. However, the relative importance of autotrophic and heterotrophic nutrition can vary as a function of ontogeny, phylogeny and ecology. Such variations of nutrition have important consequences for the population dynamics of these organisms. It is therefore central to characterize the variability and the plasticity of the nutrition of zooxanthellate jellyfishes to understand their ecology. In this thesis, the nutrition of zooxanthellate jellyfishes was investigated using laboratory experimental systems and field studies. A first experiment allowed to confirm previous findings that autotrophic nutrition is of small importance for the polyp of zooxanthellate jellyfishes. A second experiment assessed how elemental and isotopic compositions of zooxanthellate jellyfishes could be used to study their nutrition. The findings of this experiment are then confronted with results from the field: The nutrition of zooxanthellate Mastigias papua medusae was studied in their natural environments (Palau) through the use of isotopic, elemental but also fatty acids compositions. These field results demonstrate the wide plasticity of the nutrition of Mastigias papua ranging from pure heterotrophy to dominant autotrophy.The existence of such a wide plasticity in the nutrition of zooxanthellate jellyfishes helps to understand some crucial aspect of their ecology such as their generally low ability to bloom relative to non-zooxanthellate jellyfishes, or their reactions to temperature-induced bleaching.Alors que la majorité des méduses sont des hétérotrophes strictes, certaines vivent en photosymbiose avec des Dinophyceae autotrophes (« zooxanthelles »). Ces méduses à zooxanthelles, en tant qu’holobiontes, sont mixotrophes, dérivant leur nutrition à la fois de la prédation et de la photosynthèse. Toutefois, l’importance relative de l’autotrophie et de l’hétérotrophie dans la nutrition peuvent varier en fonction de l’ontogénie, de la phylogénie, ou de l’écologie. De telles variations ont d’importantes conséquences pour la dynamique des populations de ces organismes. Il est donc important de pouvoir caractériser la variabilité et la plasticité de la nutrition des méduses à zooxanthelles pour comprendre leur écologie. Au cours de cette thèse, la nutrition des méduses à zooxanthelles a été étudiée par le biais d’expériences de laboratoire et d’observations de terrain. Une première expérience a permis de confirmer des résultats précédents suggérant que la nutrition autotrophe est de faible importance pour les polypes des méduses à zooxanthelles. Une seconde expérience a mis en évidence comment les compositions isotopiques et élémentaires des méduses à zooxanthelles peuvent être utilisées pour étudier leur nutrition. Ces résultats sont ensuite confrontés aux observations de terrain : la nutrition de la méduse à zooxanthelles Mastigias papua a été étudiée dans son environnement naturel (Palaos) via l’étude de leur compositions isotopiques, élémentaires, mais aussi en acides gras. Ces résultats de terrain démontrent l’importante plasticité de la nutrition de Mastigias papua, pouvant aller de la pure hétérotrophie, une autotrophie dominante. L’existence d’une telle plasticité dans la nutrition des méduses à zooxanthelles aide à comprendre certains aspects centraux de leur écologie, tels que leur tendance à former moins de blooms que les méduses sans zooxanthelles, ou leurs réactions aux évènements de blanchissement induit par la température

δ13C, δ15N, and C:N ratios as nutrition indicators of zooxanthellate jellyfishes: insights from an experimental approach

Some jellyfish host zooxanthellae in their tissues (mostly from the family Symbiodiniaceae; Dinophyceae) and supplement their heterotrophic nutrition with their symbiont's photosynthates. The mixotrophy of zooxanthellate jellyfishes (as holobionts) renders the study of their nutrition, growth, and population dynamics complicated. Here, we used an experimental approach to assess how carbon and nitrogen stable isotopes (δ13C and δ15N) as well as the elemental composition (C:N ratios) of zooxanthellate jellyfishes are affected by variations in nutrition sources: i. e. predation (heterotrophic) versus photosynthesis (autotrophic). Our laboratory experiment, conducted on the zooxanthellate jellyfish Cassiopea sp. medusae (including symbionts) in the presence or absence of light and prey during 24 days, showed conclusive results. Presence of light decreased δ15N, increased δ13C and C:N ratios, whereas presence of prey increased δ15N, and decreased δ13C and C:N ratios. The medusae incubated with both light and prey had intermediate δ15N, δ13C and C:N ratios. Variations in zooxanthellate jellyfishes' nutrition sources (autotrophy vs. heterotrophy) are thus reflected by their isotopic and elemental composition. By disentangling the effects of autotrophy and of heterotrophy on zooxanthellate jellyfish isotopic and elemental compositions, these results would help to interpret the values of δ13C, δ15N and C:N ratios that can be observed on these organisms in fieldwork studies

A (very) First Glance

International audienc

Review of the diversity, traits, and ecology of zooxanthellate jellyfishes

WOS:000495633200009Many marine organisms form photosymbioses with zooxanthellae, but some, such as the medusozoans, are less well known. Here, we summarize the current knowledge on the diversity of zooxanthellate jellyfishes, to identify key traits of the holobionts, and to examine the impact of these traits on their ecology. Photosymbiosis with zooxanthellae originated at least seven times independently in Medusozoa; of these, five involve taxa with medusae. While most zooxanthellate jellyfishes are found in clades containing mainly non-zooxanthellate members, the sub-order Kolpophorae (Scyphozoa: Rhizostomeae) is comprised-bar a few intriguing exceptions-of only zooxanthellate jellyfishes. We estimate that 20-25% of Scyphozoa species are zooxanthellate (facultative symbiotic species included). Zooxanthellae play a key role in scyphozoan life-cycle and nutrition although substantial variation is observed during ontogeny, or at the intra- and inter-specific levels. Nonetheless, three key traits of zooxanthellate jellyfishes can be identified: (1) zooxanthellate medusae, as holobionts, are generally mixotrophic, deriving their nutrition both from predation and photosynthesis; (2) zooxanthellate polyps, although capable of hosting zooxanthellae rarely depend on them; and (3) zooxanthellae play a key role in the life-cycle of the jellyfish by allowing or facilitating strobilation. We discuss how these traits might help to explain some aspects of the ecology of zooxanthellate jellyfishes-notably their generally low ability to outbreak, and their reaction to temperature stress or to eutrophication-and how they could in turn impact marine ecosystem functioning

Nutrient limitation during diatom growth Influence the copepod grazing and the carbon export

International audienceWithin the marine realm, diatoms play a major role in trophic food web and carbon export. On the other hand, Zooplankton grazing pressure on diatoms may contribute to carbon export through faecal pellets emission in the surface mixed layer or after vertical migration in the mesopelagic layers. Change in the water column stratification induced by global warming should increase the nutrients limitation in the surface ocean. Nitrogen and Silicon limitation during two diatoms growth monospecific culture (Chaetoceros neogracile and Skeletonema marinoi) conducted to a quick modulation of the diatom Si/C ratio and cell chain length of S. marinoi. To assess the influence of zooplankton, grazing incubation with both diatoms and copepods has been performed. Change in Si/C ratio and cell chain length influenced the ingestion rate of three copepods species (Acartia clausi, Calanus helgolandicus and Euterpina acutifrons) and faecal pellets emission and composition. The influence nutrient limitations during diatoms growth on copepods grazing could modulate biological pump of carbon through an increase of aggregates formation within the surface water and export more particles

Isotopic and elemental compositions reveal density‐dependent nutrition pathways in a population of mixotrophic jellyfish

Mixotrophic organisms are increasingly recognized as important components of ecosystems, but the factors controlling their nutrition pathways (in particular their autotrophy–heterotrophy balance) are little known. Both autotrophy and heterotrophy are expected to respond to density‐dependent mechanisms but not necessarily in the same direction and/or strength. We hypothesize that the autotrophy–heterotrophy balance of mixotrophic organisms might therefore be a function of population densities. To investigate this relationship, we sampled mixotrophic jellyfish holobionts (host, Mastigias papua etpisoni; symbiont, Cladocopium sp.) in a marine lake (Palau, Micronesia) on six occasions (from 2010 to 2018). Over this period, population densities varied ~100 fold. We characterized the nutrition of the holobionts using the δ13C and δ15N signatures as well as C:N ratios. δ13C values increased and δ15N values decreased with increasing population densities (respectively, R2 = 0.86 and 0.70, P  P > 0.05). This indicates that the autotrophy–heterotrophy balance tends toward autotrophy when population densities increase. We propose that the availability of zooplanktonic prey is the main driver of this pattern. These results demonstrate that the autotrophy–heterotrophy balance of mixotrophic jellyfishes can be tightly regulated by density‐dependent mechanisms

Comment. What drives plankton seasonality in a stratifying shelf sea? Some competing and complementary theories

The Plymouth L4 time plankton series in the Western English Channel is a textbook example of a shallow, stratifying shelf sea system. Over its 30 yr of weekly sampling, this site has provided a diverse and contrasting suite of numerical and conceptual models of plankton bloom formation, phenology, and seasonal succession. The most recent of these papers, Kenitz et al. (2017) has initiated this comment, partly because we feel that it has presented a slightly misleading picture of the plankton composition at this site, and of a robust, recurring seasonal succession. We address this by illustrating the extent of inter-annual variability in phenology that occurs at the site, and which needs to be captured better within models. However our main aim is to foster a much better integration of the variety of top-down and bottom-up processes that have all been suggested to be key in driving seasonal succession. Some of these, particularly the multiple grazing and growth controls contributing to the so-called "loophole hypothesis" may be complementary, but others, such as the role of copepod feeding traits in driving species succession (Kenitz et al. 2017) offer testable competing hypotheses. The basic assumptions and outputs of all these models need to be validated more critically, both against time series data and process studies that include the finding of unselective feeding. We suggest that the variability in plankton phenology (and not just mean timing and amplitude) could be used to diagnose the performance of alternative models of plankton succession

High trophic plasticity in the mixotrophic Mastigias papua-Symbiodiniaceae holobiont: implications for the ecology of zooxanthellate jellyfishes

International audienceThe trophic ecology of mixotrophic, zooxanthellate jellyfishes potentially spans a wide spectrum between autotrophy and heterotrophy. However, their degree of trophic plasticity along this spectrum is not well known. To better characterize their trophic ecology, we sampled the zooxanthellate medusa Mastigias papua in contrasting environments and sizes in Palau (Micronesia). We characterized their trophic ecology using isotopic (bulk δ 13 C and δ 15 N), elemental (C:N ratios), and fatty acid compositions. The different trophic indicators were correlated or anti-correlated as expected (Pearson’s correlation coefficient, r P > 0.5 or < -0.5 in 91.1% of cases, p < 0.05), indicating good agreement. The sampled M. papua were ordered in a trophic spectrum between autotrophy and heterotrophy (supported by decreasing δ 13 C, C:N, proportion of neutral lipid fatty acids (NLFA:TLFA), n-3:n-6 and increasing δ 15 N, eicosapentaenoic acid to docosahexaenoic acid ratio (EPA:DHA)). This trophic spectrum was mostly driven by sampling location with little influence of medusa size. Moreover, previous observations have shown that in a given location, the trophic ecology of M. papua can change over time. Thus, the positions on the trophic spectrum of the populations sampled here are not fixed, suggesting high trophic plasticity in M. papua . The heterotrophic end of the trophic spectrum was occupied by non-symbiotic M. papua , whereas the literature indicates that the autotrophic end of the spectrum corresponds to dominant autotrophy, where more than 100% of the carbon requirement is obtained by photosynthesis. Such high trophic plasticity has critical implications for the trophic ecology and blooming ability of zooxanthellate jellyfishes

High trophic plasticity in the mixotrophic Mastigias papua-Symbiodiniaceae holobiont: implications for the ecology of zooxanthellate jellyfishes

The trophic ecology of mixotrophic, zooxanthellate jellyfishes potentially spans a wide spectrum between autotrophy and heterotrophy. However, their degree of trophic plasticity along this spectrum is not well known. To better characterize their trophic ecology, we sampled the zooxanthellate medusa Mastigias papua in contrasting environments and sizes in Palau (Micronesia). We characterized their trophic ecology using isotopic (bulk δ13C and δ15N), elemental (C:N ratios), and fatty acid compositions. The different trophic indicators were correlated or anti-correlated as expected (Pearson’s correlation coefficient, rP > 0.5 or < -0.5 in 91.1% of cases, p < 0.05), indicating good agreement. The sampled M. papua were ordered in a trophic spectrum between autotrophy and heterotrophy (supported by decreasing δ13C, C:N, proportion of neutral lipid fatty acids (NLFA:TLFA), n-3:n-6 and increasing δ15N, eicosapentaenoic acid to docosahexaenoic acid ratio (EPA:DHA)). This trophic spectrum was mostly driven by sampling location with little influence of medusa size. Moreover, previous observations have shown that in a given location, the trophic ecology of M. papua can change over time. Thus, the positions on the trophic spectrum of the populations sampled here are not fixed, suggesting high trophic plasticity in M. papua. The heterotrophic end of the trophic spectrum was occupied by non-symbiotic M. papua, whereas the literature indicates that the autotrophic end of the spectrum corresponds to dominant autotrophy, where more than 100% of the carbon requirement is obtained by photosynthesis. Such high trophic plasticity has critical implications for the trophic ecology and blooming ability of zooxanthellate jellyfishe