Ascidian reproductive patterns related to long-term population dynamics

Abstract

The reproductive patterns and population dynamics of four coexisting ascidian species, Ciona intestinalis, Ascidia mentula, Pyura tessellata, and Boltenia echinata, are compared. The population variables were obtained by stereophotographical monitoring of vertical subtidal rock walls at 20 m depth along the Swedish west coast through a 12-year period. Although all four species are hermaphroditic, solitary and coexisting, exploiting the same food resource, and possessing a similar lecithotropic larva, four different reproductive types with characteristic population dynamics were distinguished. Adult longevity stabilized population density, and was correlated with increased investment in a protective tunic and a decrease in effective fecundity (recruitment). Longevity varied with environmental factors. The complexity of reproductive patterns within ecologically proximate species indicates that ‘strategy’ comparisons based exclusively on the classical division of larval development types among marine invertebrates (planktotrophy, lecithotrophy, and direct development) seem too simplified.