Association among ascidians: facilitation of recruitment inPyura spinifera

Abstract

Subtidal benthic communities on hard substrata are usually characterised by intense competition for space; neighbouring organisms are generally viewed as a threat to the maintenance of this limited resource. Here, I describe an association between two species of subtidal ascidians where the abundance of one appears to be wholly determined by the abundance of the other.Pyura spinifera is a large subtidal stalked ascidian that is common in south-eastern Australia and usually occurs in clumps. At the base of these clumps the unstalked ascidianCnemidocarpa pedata is often present. Between February and May 1992, I used a removal experiment to test the prediction that the recruitment ofP. spinifera was dependent on the presence ofC. pedata. There were three treatments at two sites: (i)C. pedata were totally removed, (ii)C. pedata densities were reduced by half, and (iii) ascidians were left untouched (control). The removal ofC. pedata significantly reducedP. spinifera recruitment relative to controls; recruitment ofP. spinifera was only observed onC. pedata. Results were consistent between sites. Furthermore, laboratory settlement trials indicated thatP. spinifera settle preferentially in response to cues with which recruits are normally associated, while settlement is poor or does not occur in the absence of such cues. Laboratory trials also indicate that these responses are mediated by water-borne compounds.