Habitat discontinuity and genetic structure in populations of the estuarine species Hediste diversicolor (Polychaeta: Nereididae)

Relationships between patterns of allozyme polymorphisms and habitat discontinuity were investigated in the estuarine species Hediste diversicolor (Polychaeta: Nereididae). Populations (50!n!99) were collected from six estuaries along the Adriatic coast of Italy, 9–175 km apart. Large scale patterns of differentiation (across 2500 km) were tested by comparing populations from the Adriatic coast with populations (nZ35) from two Tyrrhenian estuaries. Sixteen enzymatic systems corresponding to 21 allozymic loci were analysed by means of cellulose acetate electrophoresis. As in previous genetic studies, low levels of allozymic variation were observed within populations of H. diversicolor (mean Hobs range: 0.025–0.064). F-statistics revealed the occurrence of high levels of genetic structuring, consistent with the limited dispersal capabilities of the species and its discontinuous distribution along the coast. The multidimensional scaling plot based on Cavalli-Sforza and Edward’s genetic distances showed the occurrence of two major clusters reflecting geographical distances between populations from the Adriatic and Tyrrhenian coasts. A positive correlation between pairwise coancestry coefficients (q) and shoreline distances (km) was observed when Adriatic and Tyrrhenian populations were analysed together ( gZ3.934, p!0.01). Conversely, among estuarine habitats of the Adriatic coast, patterns of genetic structuring of H. diversicolor were not related to shoreline distances. Results reveal the lack of isolation by distance among estuarine environments on the Adriatic coast. The combined effects of adaptation to local environmental conditions and isolation appear to be among the major factors influencing the genetic structuring of H. diversicolor on a local scale. Differentiation over a larger spatial scale may be related to different evolutionary histories of the Adriatic and Tyrrhenian populations.