THE ENCRUSTING SPONGE HALISARCA-LAXUS - POPULATION-GENETICS AND ASSOCIATION WITH THE ASCIDIAN PYURA-SPINIFERA

The encrusting sponge Halisarca laxus forms a seemingly obligate assoc iation with the stalked solitary ascidian Pyura spinifera. In 1991 we examined spatial variation and short-term temporal variation in this a ssociation at three neighbouring sites in southeastern Australia. This sponge dominated the surface of almost all the 500 individual ascidia ns examined, with mean cover usually exceeding 90%. This pattern was c onsistent among sites and throughout the year of the study. The domina tion of a small isolated patch of habitable substratum by a sponge is most unusual, given that they are regarded as relatively poor recruite rs. To understand how this association might be maintained, we determi ned the underlying genotypic diversity of the sponge population using starch-gel electrophoresis, P, spinifera is a dump-forming ascidian an d usually occurs in clumps of up to 22 individuals, Electrophoretic su rveys, based on six variable allozyme loci, revealed that at a total o f five plots within three neighbouring New South Wales populations, si ngle sponge genotypes may cover entire ascidian clumps; although a clu mp sometimes played host to more than one sponge clone. Allele frequen cies (averaged across four loci that appear to conform to Mendelian in heritance) showed little variation among populations (standardised gen etic variance, F-ST = 0.013). Nevertheless, sponge populations were ge notypically diverse, with samples from 63 of 172 individual clumps dis playing unique ''clonal'' genotypes, Moreover, multi-locus genotypic d iversity within all sites approached the level expected for sexual rep roduction with random mating. Taken together, these data imply that H. laxus produces sexually-derived larvae that are at least moderately w idely dispersed. Given the relatively small size of the patches that t his sponge inhabits, we also conclude that these larvae are good colon ists and good spatial competitors on their ascidian hosts.