Variablility of stable isotope signatures (delta C-13 and delta N-15) in two spider crab populations (Maja brachydactyla) in Western Europe

The ontogenic variations of nitrogen and carbon stable isotopic signatures (delta N-15 and delta C-13) were investigated in two spider crab (Maja brachydactyla) populations inhabiting in different biotopes of Western Europe. The Iroise Sea population is localized in Western Brittany and characterized by a seasonal migration occurring on a large bathymetric and habitat gradient while the Seine Bay population, in the Eastern English Channel, remains in a more homogeneous environment during its migration. In the Iroise Sea population, delta C-13 Values increased significantly both with body size and age, revealing a shift towards "benthic-component" prey with spider crab growth. On the contrary, neither body size nor ageing gave rise to a significant trophic level change (derived from the delta N-15). In this M. brachydactyla population, the seasonal migrations from coastal waters in summer to offshore habitats in winter involved significant but slight differences in both delta C-13 and delta N-15. In the Seine Bay population, low variations for both carbon and nitrogen were recorded related to either sex or size or seasonal migration. Thus, the delta C-13 and delta N-15 variability in the spider crab depends on the availability and diversity of prey in its different living habitats, as well as on the morphological aptitudes of individuals to feed on prey (individual's size)