BIOCHEMICAL BIOGEOGRAPHY AND EVOLUTIONARY RELATIONSHIPS AMONG THE SIX KNOWN POPULATIONS OF STENASELLUS RACOVITZAI (CRUSTACEA, ISOPODA) FROM TUSCANY, CORSICA AND SARDINIA.

Allozymic variation at 15 enzymatic loci was studied in six populations of the stygobiont isopod Stenasellus racovitzai. This species belongs to the exclusively stygobiont family Stenasellidae, characterised by morphological, ecological and biogeographical features indicating an extremely ancient separation from a presumed surface ancestor. Here we present allozymic data for six populations of S. racovitzai: two from Tuscany, two from Corsica and two from Sardinia. Levels of genetic divergence indicate differentiation at the species level for four distinct groups of S. racovitzai (one from Tuscany, one from Corsica and two from Sardinia) despite their basic morphological uniformity. In addition, genetic data for the two Corsican populations support the hypothesis of a differential spatial distribution of young offspring and adults. Phylogenetic relationships indicate a more ancient separation of the two Sardinian species, while the Corsican and Tuscan species seem to be more recent and related to each other. Relationships among the populations agree quite well with the palaeogeographical events that affected the sampling area and strongly support the hypothesis that the present distribution resulted from vicariance rather than dispersal.