Planktonic drifting dispersal of early juvenile Trochita calyptraeaformis Born 1778 [Gastropoda: Calyptraeidae]

Abstract A study was made on the dispersal of postlarval Trochita calyptraeaformis Born 1778 juveniles after termination of their direct development within capsules brooded by sedentary adult females. Field and laboratory evidence suggested that the postlarval juveniles crawled away from incubational substrates and assumed a drifting existence in the water column, as demonstrated by their recruitmernt on experimentally placed off-bottom fouling plates between May and December 1989 at Herradura Bay, Coquimbo, northern Chile. Further observations were made on recruitment of this species into mesh-bag scallop spat collectors 1.5 km away from the inshore brooding areas. Planktonic dispersal was observed for individuals of this calyptraeid primarily in the 1–3 mm size range. Most specimens were caught on collectors suspended 2 m above the bottom, and the recruitment was continuous throughout the study period, with a significant peak between September and October. Long distance dispersal of the juvenile calyptraeids was inferred from their appearance in the deep, off-shore scallop spat collectors, where 2200 individuals were counted, with greater abundance in shallow than in near-bottom collectors. The mechanisms promoting dispersal of this species included seasonal water movement which swept newly hatched juveniles into the water column, morphological attributes of the snails which enhanced their capacity for drifting and, particularly, the presence of a gas-bubble retained between the juvenile mantle cavity and the anterior end of the foot which produced buoyancy. The drifting habit of this species is a remarkable feature when compared with life cycles of other gastropod species having direct development, since postlarval juveniles typically recruit to the benthos near their sites of incubation. These findings were interesting from the standpoint of fisheries management, as T. calyptraeaformis is a commercially exploited species undergoing significant harvesting pressure.