LONG-DISTANCE DISPERSAL OF A SUB-ANTARCTIC BROODING BIVALVE (GAIMARDIA-TRAPESINA) BY KELP-RAFTING

The probability of successful dispersal by sessile benthic invertebrat es is thought to strongly influence their geographic distribution and population genetics. Generally, species with long-lived planktonic lar vae are expected to exhibit wider distribution patterns than those spe cies which brood their young, due to their presumably greater potentia l for dispersal. In some cases, however, brooding species exhibit broa d distributions and show evidence of genetic exchange with geographica lly distant populations. One potential factor that has been invoked as an explanation is dispersal by floating and rafting of adults and egg masses. Several studies have shown that it is possible for sessile ad ults to disperse on the order of several to many thousand kilometers b y rafting on debris in ocean currents. With very few exceptions, howev er, direct evidence of rafting in the open ocean has been lacking. We present evidence of long-distance (1300 to 2000 km) dispersal of a bro oding pelecypod, Gaimardia trapesina (Lamarck, 1819), in the Southern Ocean in the vicinity of Cape Horn, the Falkland Islands, and the anta rctic island South Georgia (54 degrees S; 37 degrees W). Data on survi val and fecundity rates of G. trapesina and the prevalence of kelp raf ts collected during the austral winter of 1993 indicate that dispersal by rafting can occur over ecologically relevant time scales and could potentially serve as a significant means of genetic exchange between populations.