Research Items (15)
Different krill species have a pivotal position in many marine food webs by both preying upon several tro-phic levels and being forage species for consumers. Within these food webs, different krill species coexist, though it remains unclear what mechanisms allow for the coexistence, for instance, of northern krill species in subarctic environments. Here, we hypothesize that the stable coexistence of sympatric krill species is based on trophic niche partitioning related to seasonal trade-offs between their respective food preferences, their energy needs, and prey availability. To test our hypothesis, we analyzed the diets, feeding selectivity, and lipid composition of three coexisting northern krill species (Meganyctiphanes norvegica, Thysanoessa inermis, and Thysanoessa raschii) throughout 1 yr using a multimarker approach. We assessed lipid classes, fatty acids, and stable isotope signatures of krill and potential food sources (27 groups, from phytoplankton to lipid-rich copepods) to elucidate seasonal variation of niche utilization. Results revealed strong trophic niche separation occurring at a very fine trophic scale (species level) throughout the year. The three krill species showed different degrees of food specialization rather than being purely opportunistic as classically proposed. Feeding on copepod prey was important to accumulate energy reserves for overwintering and subsequent rebuilding of energy reserves. Energy reserve utilization might reduce potential competition for the limited available resources, especially under low food conditions.
The Nordic krill Meganyctiphanes norvegica and Arctic krill Thysanoessa raschii both dominate the krill community within the Estuary and Gulf of St. Lawrence system where they are central forage species for its pelagic ecosystem. We developed a species‐specific physiological individual based model that implements the critical physiological processes of growth, molting, and reproduction of female adults as responses to environmental forcing. Key innovations of our approach were the decoupling between the molting schedule and growth, as well as considering two distinct sources of prey (phytoplankton and mesozooplankton). Our simulation results revealed that the details of the feeding process were critical for an accurate representation of the production dynamics of adult individuals from both species. Their specific feeding preferences on phytoplankton and mesozooplankton resulted in distinct species‐specific phenological patterns that reproduced observations. The present study highlights the importance of detailed knowledge of diet and feeding behavior of krill species to improve our understanding of population responses in a rapidly changing environment.
Spatio-temporal variations of the physiological condition and the feeding behaviour of Meganyctiphanes norvegica in the gulf of St. Lawrence
In subarctic ecosystems, the krill communities are composed of many species, ranging from temperate to arctic, with diverse morphology and ecology. These krill species may express differential eco-physiological traits, which in turn may vary with strong seasonal changes in temperature and food supply in these environments. To enhance our capacity to predict energy flow and trophic shifts, understanding of feeding strategies and energy reserve dynamics becomes crucial. Here our goal was to determine how species-specific feeding behaviour could affect performance under strong seasonal environmental changes including winter ice-cover in the subarctic St. Lawrence Gulf, Canada. We monitored throughout one-year the feeding behavior and lipid composition of three coexisting krill species (Meganycthiphanes norvegica, Thysanoessa inermis, and Thysanoessa raschii) using a multi-marker approach, including lipids classes, fatty acids and stable isotopes. Our results confirmed that cold-water adapted species (Thysanoessa spp.) accumulated higher lipid content dominated by wax-esters whereas the temperate species (M. norvegica) stored less energetic lipids mostly composed of triacylglycerols. Thysanoessa spp. started accumulating lipids in spring until their maximum was reached in autumn by feeding mostly on diatoms. In contrast, M. norvegica adopted a different strategy, showing a trophic shift with an increasing proportion of copepods in its diet in winter limiting thus their needs to accumulate energetic lipids. Strikingly, the maximum of lipid content in M. norvegica occurred in winter.
The northern krill community is composed of many species exhibiting diverse morphology and ecology. In the St. Lawrence estuary, three krill species coexist as forage species and play a key role in energy transfer to higher trophic levels. However, the dynamics of their trophic interactions and their feeding behavior are still poorly understood and thus, limit our capability to predict potential shifts in energy flows. Thus, our goal was to determine if diet, feeding behaviour, and trophic position differ among species throughout the seasons using multitrophic markers approach (fatty acid and isotopic markers). Both proxies revealed that diets and feeding behavior were significantly different with a seasonal shift to a higher trophic position during summer. Meganyctiphanes norvegica, which is the biggest species in the SLE, seems to be more of a specialist with a diet mostly based on large copepods whereas Thysanessa spp. fed in higher proportions on phytoplankton and small copepods suggesting a more generalist feeding behavior. Our results will be discussed in regard of how niche partitioning may support stable coexistence of marine invertebrate.
Krill as forage species play a key role in energy transfer to higher trophic levels in marine ecosystems. In the St. Lawrence estuary, three krill species (Meganyctiphanes norvegica, Thysanoessa raschii, Thysanoessa inermis) coexist and dominate the macrozooplankton biomass. Although, these species are ecologically important, the dynamics of their trophic interactions with the lower trophic food web are still poorly understood. Our goal was to evaluate if diets, feeding behavior and trophic position differ among the 3 krill species throughout the seasons. We monitored variability of these parameters throughout one-year, 2014 to 2015 (including winter icecover), using stable isotopes and fatty acids composition in the lipids neutral fraction. Both proxies revealed that diets and feeding behavior of these species were significantly different. M. norvegica always showed a diet mostly based on copepods while Thysanessa spp. fed a higher proportion of phytoplankton. Moreover, lipid content at least of M. norvegica seemed closely related to copepod availabilities suggesting that this species is more “carnivorous” whereas Thysanoessa spp. seemed to be omnivorous with a preference for phytoplankton. Therefore, M. norvegica showed a relatively stable and higher trophic position than Thysaonessa spp. throughout the year. However, the trophic position of T. inermis was surprisingly high in the St. Lawrence estuary compared compared to Arctic regions. All species showed a seasonal shift to a higher trophic position during summer. The resulting trophic niche partitioning may support stable coexistence of these krill species in the St. Lawrence estuary.
The St. Lawrence estuarine transition zone includes the interface between the freshwater and the brackish. The euryhaline and dominant copepod Eurytemora affinis occurs throughout the entire transition zone, but E. affinis is a sibling species complex with two morphologically similar but genetically distinct clades. The E. caroleeae and the North-Atlantic clades show differences in their distribution pattern across the fresh and the brackish water interface. We hypothesized that niche separation occurs between the two clades. To test this hypothesis we 1) characterised food sources and physiological condition of the copepods in each habitat, and 2) performed “in situ” reciprocal transplant experiments in the freshwater and the brackish water habitats. To quantify the ecophysiological condition of both clades, we determined their fatty acids composition and concentration, their survival rate and reproduction efficiency. Results showed similar food composition between habitat, but suggest different feeding behaviour for each clade. E. carolleeae showed similar results in the transplant experiment between both habitats, whereas the North-Atlantic clade showed high mortality in the Atlantic clade habitat (freshwater). Despite the fact that total fatty acid content did not change significantly during the transplantation, the North-Atlantic clade showed a strong decrease of their reproduction rate in the freshwater habitat, suggesting a trade-off between survival and reproduction in order to maximize available energy for survival. Thus the Atlantic clade had a larger performance range across the interface compared to the North-Atlantic clade which was much more limited to its brackish water habitat. The Atlantic clade can be considered as a generalist whereas the North-Atlantic clade acts as specialist and thus, limiting intrusion of the latter in freshwater habitat.
The lower St. Lawrence estuary (LSLE) is influenced by river discharge and saltwater inflow. Together with arctic water inflow and ice cover in winter, a strong stratification occurs resulting in a cold intermediate layer (CIL) from spring to autumn. This stratification provides thermal habitats. Here, we focus on two krill species Thysanoessa raschii and Meganyctiphanes norvegica that aggregate in the CIL and the warmer deep water layer, respectively. Both species are known to migrate into the surface layer to feed and thus transferring energy through the food web by linking lower with higher trophic levels. However, their specific feeding biology and trophic interactions are poorly understood. We tested the following hypotheses: (1) the diets vary throughout the season depending on food availability, (2) similar to thermal habitat separation, trophic niche separation between T. raschii and M. norvegica occurs, characterized by herbivory in T. raschii and carnivory in M. norvegica. Trophic position and feeding behavior of these krill populations were monitored throughout the year 2014-2015 using a stable isotope approach. The two species showed a seasonal shift in their trophic position being at a higher trophic level in summer than in spring and autumn, which did not correspond to phytoplankton or zooplankton availability. Within the trophic space of carbon and nitrogen stable isotopes M. norvegica showed a relatively stable position, whereas T. raschii covered a much larger space throughout the year. M. norvegica always showed a higher trophic position than T. raschii. Results of a stable isotope mixing model (SIAR) revealed 50% phytoplankton and 50% copepods in the diet of M. norvegica, while T. raschii showed a higher proportion of ca. 70% phytoplankton in its diet. Both species exploited several trophic levels, but in different proportions, thereby minimizing diet overlap, suggesting a further mechanism to enhance stable co-existence of these krill species in the LSLE.
- Jan 2016
Le copepode calanoide Eurytemora affinis est un complexe d’especes cles des reseaux trophiques estuariens. En dominant les communautes planctoniques dans la plupart des estuaires de l’hemisphere nord, E. affinis permet un transfert d’energies vers les niveaux trophiques superieurs. Il influence directement le recrutement des especes de poissons et constitue une espece cle des zones d’alevinage. E. affinis est en realite un complexe d’especes cryptiques compose de 6 clades morphologiquement similaires, mais possedant des histoires evolutives distinctes. Dans la zone de transition estuarienne du Saint-Laurent, 2 clades (Atlantique et Nord-Atlantique) sympatriques, spatialement segregues, dominent la communaute zooplanctonique. Chacun des 2 clades semble exploiter differents types d’habitats en fonction de sa tolerance physiologique a la salinite et a la disponibilite de nourriture. Nous discuterons des etudes recentes en mettant l’accent sur la repartition, la differenciation genetique, mais aussi l’ecologique de ces especes du complexe d’E. affinis afin de mieux comprendre leurs influences respectives sur le fonctionnement et la productivite de l’ecosysteme unique que represente la zone d’alevinage de l’estuaire du Saint-Laurent.
- Jan 2015
The cryptic species complex, Eurytemora affinis is an important component of the mesozooplankton of estuarine transition zones (ETZs). In the St Lawrence ETZ, two clades, the Atlantic and the North-Atlantic clades co-occur. However, their distribution along environmental gradients is spatially segregated, potentially because of differential habitat exploitation. In this study, we tested two hypotheses on the geographic disparity of two E. affinis populations, each belonging to one of the clades. First, habitats differ in food composition and availability, and second, each of the two clade populations shows a distinct feeding behaviour, both, influencing their physiological condition. To achieve this goal, we used a fatty acid (FA) approach to determine FA composition of food sources and E. affinis. Its physiological condition was estimated by the concentration of accumulated neutral FAs. Our results revealed that food composition and availability were similar between clade habitats, suggesting that resource conditions did not contribute to the spatial segregation of the two clade populations. The FA composition of both clade populations was characterized by high levels of docosahexaenoic acid and eicosapentaenoic acid, suggesting that they predominantly ingested phytoplankton. However, significantly different FA profiles of the two populations suggested differential feeding behaviour, which may optimize accumulation of energy reserves in relation to their physiological needs. Both clade populations of E. affinis seem to show good physiological conditions, being well adapted to the dynamic environment of the ETZ.
The family Oscarellidae is one of the two families in the class Homoscleromorpha (phylum Porifera) and is characterized by the absence of a skeleton and the presence of a specific mitochondrial gene, tatC. This family currently encompasses sponges in two genera: Oscarella with 17 described species and Pseudocorticium with one described species. Although sponges in this group are relatively well-studied, phylogenetic relationships among members of Oscarellidae and the validity of genus Pseudocorticium remain open questions. Here we present a phylogenetic analysis of Oscarellidae using four markers (18S rDNA, 28S rDNA, atp6, tatC), and argue that it should become a mono-generic family, with Pseudocorticium being synonymized with Oscarella, and with the transfer of Pseudocorticium jarrei to Oscarella jarrei. We show that the genus Oscarella can be subdivided into four clades, each of which is supported by either a small number of morphological characters or by molecular synapomorphies. In addition, we describe two new species of Oscarella from Norwegian fjords: O. bergenensis sp. nov. and O. nicolae sp. nov., and we compare their morphology, anatomy, and cytology with other species in this genus. Internal anatomical characters are similar in both species, but details of external morphology and particularly of cytological characters provide diagnostic features. Our study also confirms that O. lobularis and O. tuberculata are two distinct polychromic sibling species. This study highlights the difficulties of species identification in skeleton-less sponges and, more generally, in groups where morphological characters are scarce. Adopting a multi-marker approach is thus highly suitable for these groups.
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