Complex patterns of habitat use by juvenile spider crabs "Majabrachydactila" revealed by stable isotope analyses

Patterns of habitat use by spider crab juveniles, Maja brachydactila, from two geographic areas in the NW coast of the Iberian Peninsula, were analyzed through the analysis of Carbon and Nitrogen stable isotope relations in two tissues (muscle and hepatopancreas). The main organisms that constitute the diet of spider crab juveniles in rocky and sandy habitats and the different organic matter sources in coastal food webs were analysed too. The isotopic ratios of C and N did not show any difference between habitats. The use of carapace colour and epibiosis as an indicator of habitat use was not supported by our data. These results suggest much more frequent movements between both types of habitats than those suggested by former studies (carried out using behavioural observations and sampling distributions) that hypothesized seasonal and ontogenetic changes. In the coastal food web, two main trophic compartments were identified according to the organic matter source: one based in plankton and seaweeds, that corresponds, mainly, with rocky habitats, and another based in sedimentary particulated organic matter that correspond with sandy bottoms. The relationships observed between the C and N isotopic ratios and the proportions of lipids and proteins in muscle and hepatopancreas seem to indicate a clear relation between the trophic origin of the food consumed by the spider crab juveniles and its energetic condition, and the individuals that spent more time feeding in sandy areas would show a better energetic condition. Juveniles of Maja brachydactila, independently of the habitat of capture, would consume in average approximately two thirds of the preys in rocky habitats and a third in sedimentary habitats. The results obtained indicate that, in exposed environments, large