Larval ecology of an ascidian tropical population in a Mediterranean enclosed ecosystem

Abstract

The larval ecology of a population of the tropical tunicate Ecteinascidia turbinata was studied in a semi-enclosed lagoon on the island of Formentera (Mediterranean Sea). Due to the lagoon's isolation and processes of extinction/regeneration of the population, research was carried out to evaluate some aspects of the species' ecology which permit it to survive in this habitat. Settlement and recruitment were found to be intimately related to the period of sexual reproduction, and determined by the local production of larvae. In this sense, the population's spawning potential (larvae m(-2)) matched a perfect linear relationship (R-2 = 0.9496, p < 0.05) with the settlement and recruitment registered on experimental settlement surfaces. Of the larvae produced by the population 29% settled locally, contributing to the consolidation of populations already established. Only 1.3% of the larval production theoretically calculated developed into colonies at a local level. To determine how Settlement site might contribute to the colonies' survival and thus to population distribution, zooids were transplanted from an appropriate zone to a stressful zone, which showed that there is a risk of inappropriate habitat selection for settlement. The behaviour of the larvae (philopatry) and gregarious settlement in unpredictable environments (spatio-temporally variable) seems advantageous to prevent advection to inappropriate sites. E. turbinata ensures local recruitment and renewal of the established population, but it seems that there is partial dispersal of larvae beyond the parent population. The study shows that there is sufficient recruitment from within the population to maintain it and the timing of larval production and recruitment strongly suggest that recruits are from the population itself.