Exploratory analysis of talitrid population genetics as an indicator of the quality of sandy beaches

Allozymic variation was studied at 23 loci in 10 populations of Talitrus saltator, two populations of Talorchestia deshayesii and two populations of Talorchestia brito from the Mediterranean and Baltic Seas. Levels of genetic differentiation among the three species showed a close relationship between T. saltator and T. deshayesii, whereas T. brito was more genetically differentiated. Within T. saltator, it was possible to recognise two main clusters, the first grouping consisting of Baltic, Tyrrhenian and North African populations, the second comprising Adriatic and East Mediterranean populations. Talitrus saltator populations were generally characterised by low levels of gene flow (Nm < 1). The relationship between Nm and geographic distances was analysed to test for an isolation by distance pattern in the spatial genetic variation. Isolation by distance was detected at the scale of the whole Mediterranean, with geographic distance explaining about 50% of the variance in gene flow. The North African populations showed an island model of genetic structuring; this result is mainly due to the anomalous genetic pattern of a population from a Tunisian beach (Tabarka) highly exploited for tourism. Human activities, which may influence T. saltator population dynamics, are believed to be responsible for the observed pattern of genetic variation in North Africa.