Distribution and Dynamics of an Intertidal Ascidian Pseudopopulation

Molgula occidentalis, an abundant ascidian in the shallow subtidal zone of the northern Gulf of Mexico, occurs seasonally on intertidal sandbars. One such population was studied over a 5-year period on Bay Mouth Bar, Franklin County, Florida. Most years, ascidians recruited during the spring and summer months, then died during early morning low tides in January and February. Predation by Fasciolaria hunteria (Gastropoda) accounted for a negligible portion of the mortality. Most mortality occurred when strong north winds caused freezing temperatures and prolonged exposure in the intertidal. Molgula occidentalis aggregate at several spatial scales. They occur most abundantly in areas where seagrasses (predominantly Ruppia maritima) stabilize the sediment. Sand movements and excessive siltation have adverse effects on adults and juveniles. Although larvae are capable of attaching to unconsolidated sand, they prefer to settle on the consolidated sands from the adult habitat. Small-scale distribution is also related to elevation. Few ascidians occur in the bottoms of deep pools or on high sand ridges. Highest densities are attained near the edges of pools, no more than a few cm above or below the seawater table at low tide. Because adults can survive in the bottoms of pools, it is suspected that this distribution pattern is established by larval choice andjuvenile mortality. Intertidal Molgula occidentalis are not true populations because they are not self-sustaining. Recruitment depends entirely on larvae emigrating from subtidal populations.