Roles of larval behaviour and microhabitat traits in determining spatial aggregations in the ascidian Pyura chilensis

摘要

ABSTRACT: The large edible ascidian Pyura chilensis Molina, 1782 is a sessile suspension feeder that occurs in intertidal and subtidal habitats along the Chilean and Peruvian coasts. In these habitats the species, although solitary, is commonly found in highly aggregated assemblages and is seldom isolated. The results of a study to determine whether the observed field aggregations could be explained by larval behaviour, microhabitat with quiescent hydrodynamics, or both, are reported. Our laboratory experiments showed that P. chilensis larvae have a short pre-settlement period when maintained in the presence of artificial substratum conditioned with adult conspecific extract. Moreover, P. chilensis larvae tended to settle in shaded areas. In the control treatment without conspecifics, or in illuminated areas, low numbers of P. chilensis settlers were found. In a 5-armed radial maze, under still water and low seawater flow regimens, larvae of P. chilensis tended to swim toward and settle in chambers that contained water in which conspecific adults had been placed. In the field, we tested the hypothesis that larval settlement in P. chilensis differs depending on microhabitat traits such as water flow, mechanical disturbance and light intensity. We conducted quadrat sampling on low rocky platforms in contrasting microhabitats, such as around holdfasts of the kelp Lessonia nigrescens, kelp whiplash zones and between holdfasts. The majority of P. chilensis settlers were found on top of conspecific adults inhabiting shaded microhabitats and around kelp holdfasts. These results suggest that protected habitats and chemical cue-mediated larval response to waterborne conspecifics during the settlement period contribute to the formation of intertidal and shallow subtidal aggregations of P. chilensis. Given that the edible ascidian P. chilensis supports an important small-scale fishery and hosts an important associated community of macro- and micro-invertebrates, we suggest that our results should be considered when determining fisheries regulatory and management practices for this species.