MEPS 201:211-220 (2000)  -  doi:10.3354/meps201211

ABSTRACT: Dispersal of intertidal mussel propagules was examined by sampling larvae (0.2 to 0.8 mm) of Perna perna in shallow (10 to 20 m) inshore waters on the south coast of South Africa. Mussels were randomly distributed through the water column with no signs of diel vertical migration. Horizontal distribution was sampled on fine scale grids of plankton stations on 5 occasions. Lines of stations ran offshore and were 300 m apart. Within each line, stations were 10 m apart. On each occasion grids were sampled 3 to 4 times in rapid succession so that each station was sampled at intervals of about 1 h. Distribution was patchy and denser clouds of larvae moved in the same direction and at the same speeds as surface currents. Displacement of passive particles moving with wind-driven currents was estimated from wind data. Over periods of 1 mo estimated total displacement was 118 to 220 km depending on month and year. However, because of frequent changes in wind direction, net displacement for any month was 54 to 164 km to the northeast. These estimates were compared with the rate of spread over 4 yr of an invasive intertidal mussel (Mytilus galloprovincialis) dispersing initially from a single-point source. Spread was estimated as the furthest occurrence of M. galloprovincialis from the parent population. After 4 yr, 90% of individuals sampled were <5 km from the original population. Yearly increase in range between 1988 (introduction to the area) and 1992 was strongly directional: 55 to 97 km to the northeast and 12 to 29 km to the southwest. The good match with estimates from wind data implies that mussel larvae in this region are dispersed like passive particles and that dispersal direction and ranges can be predicted from hydrographic data. The results strongly suggest that maximum effective dispersal of mussel larvae in this area is relatively limited (<100 km), with the great majority of successful recruits appearing within <5 km of the parent population.

KEY WORDS: Intertidal · Invasive species · Larval dispersal · Mussels · Mytilus · Perna