Abstract
The interstitial meiofauna of sand beaches includes many species that are regarded as cosmopolites, on the basis of records from various sites throughout the world. There is a long‐standing debate about the causes of such a distribution, but for most of these species it is still questionable whether they are in fact true species or ‘merely’ complexes of cryptic species. Here genetic differentiation is examined within and between two species considered typically cosmopolitan, Hesionides arenaria arenaria (Hesionidae) and Stygocapitella subterranea (Parergodrilidae), by RAPD‐PCR analyses of specimens from diverse marine regions on two continents. For S. subterranea at three sites, on the North Sea coast (Sylt) and the US Atlantic (Massachusetts) and Pacific (Washington) coasts, with 14 primers 335 different DNA fragments were found: 20 diagnostic ones for the European animals, 17 for the animals from the American East Coast and 14 for those from the American West Coast. Five cluster procedures were used, all of which significantly distinguished the three populations as separate genetic clades; it is recommended that each be given species status. In contrast, the individual specimens of H. a. arenaria from eight European sites, between Skagerrak and the eastern Mediterranean (including the Canary Islands), and the North American Pacific coast, for which 468 different DNA fragments were amplified with 12 primers, do not form separate genetic clades. For no single population was it possible to demonstrate even one diagnostic character. Therefore the cosmopolitan nature of H. a. arenaria has been confirmed at the DNA level.