Colour variation and crypsis in relation to habitat selection in the males of the crab spider Xysticus sabulosus (Hahn, 1832) (Araneae : Thomisidae)

The crab spider Xysticus sabulosus (HAHN, 1832) is a sit-and-wait predator, typical for sandy habitats in Europe and Flanders. In the Flemish coastal dunes, the species is very abundant in grey and blond dunes. Its abdominal and cephalothorical colour varies from almost completely white to dark brown. During autumn 2001, we investigated crypsis as a function of the occupied micro- and macrohabitat. Our results indicate that (1) Colouration differs between the populations. The species is darker in a grey dune completely covered with dried (brown) mosses and in a humid dune slack covered with algae, than in populations from a sea inlet and from a blond dune. The colouration of a population from a grey dune with mosses, lichens and bare sand is intermediate. (2) Individual cephalothoracal colouration is, in contrast to abdominal colouration, related to microhabitat selection : individuals with darker cephalothoraxes occupy hunting sites with a higher coverage of mosses, while those with a pale one are found in microhabitats with a high amount of nude sand. The observed spider colour-environment covariation between populations is probably the result of natural selection or colour alteration during the juvenile development. Further research on these possible underlying mechanisms remains, however, necessary.