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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.
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