Genetic divergence and origin of Mediterranean populations of the River Blenny Salaria fluviatilis (Teleostei : Blenniidae)

The current distribution of the River Blenny Salaria (= Blennius) fluviatil is, one of the two freshwater representatives of a large, cosmopolitan mari ne fish family, poses an interesting biogeographical problem because this s pecies inhabits widely separate circum-Mediterranean watersheds. Potential scenarios of its dispersal were examined using allozyme analysis of several populations from the Iberian and Greek peninsulas. Based on Nei genetic di stances, the most divergent populations were the populations inhabiting lak es, Lake Trichonis in Greece, and Ruidera Lakes in Spain. Their high diverg ence suggests their early isolation from the main ingroup populations. In c ontrast, low genetic distances were found among river populations regardles s of geographic location. There was a correlation between genetic distance and geographic distance among Iberian river populations, suggesting that di spersal following the colonization of fresh water occurred via the sea to n earby, unconnected river basins. The ancestor of S. fluviatilis may have be en a euryhaline species, allowing incursions into fresh water and subsequen t dispersal via the sea. This dispersal scenario could theoretically be com bined with multiple colonization episodes. The two old lake populations sha red a unique allele at the Pgdh-A locus in high frequency with its closest relative S. pave, which was absent from other populations. This may indicat e two initial incursions into fresh water by a wide-ranging marine ancestor that possessed this allele. Differential selection on this allele in lake habitats or convergence are less likely possibilities. Thus, the present di stribution of S. fluviatilis appears to stem from a combination of "raceme" origins (i.e., more than one colonization episode) and subsequent dispersa l and divergence in new watersheds.