Nemertean, copepod, and amphipod symbionts of the dimorphic ascidian Pyura stolonifera near Melbourne, Australia: specificities to host morphs, and factors affecting prevalences

Abstract

On the central coast of Victoria. Australia, the dimorphic ascidian Pyura stolonifera (Heller, 1878) harbors three endosymbionts: the nemertean Gononemertes australiensis Gibson, 1974, the copepod Doropygus pulex (Thorell, 1859), and the amphipod Paraleucothoe novaehollandiae (Haswell, 1880). The specificities of these symbionts to two host colour morphs were studied during 1989 to 1991 as part of a multidisciplinary investigation aimed at determining whether the two morphs are genetically distinct. Distributional surveys revealed that nemerteans and copepods occur only in yellow and brown ascidians, respectively, and that amphipods live in both forms. These specificities held true not only when the two morphs were in allopatry, but also in sympatry. These observations, especially the sympatric data, suggest that the two host morphs might be genetically distinct. For example, the two morphs might have different genetically encoded internal milieus that favour the survival of nemerteans in yellow ascidians, and copepods in brown hosts. In transplant experiments, which involved moving ascidian morphs within and between habitats, the “wrong” symbionts never colonised the “wrong hosts”. These results, although consistent with the hypothesis of genetic maintenance of specificity, were deemed inconclusive because of the difficulty of establishing reliable controls (i.e. vacant hosts). The relationships between symbiont prevalences and several factors (season, year, site within host, host individual, host habitat, host size/age, host breeding condition, and co-occurrence of other symbiont species) were also analysed. Both simple (e.g. greater prevalences for large hosts) and complex (e.g. prevalence x season x gonad state of host) interactions were detected for all three symbiont species. These are among the very few quantitative analyses of factors affecting prevalences of ascidicolous nemerteans and amphipods. The present report identifies one of very few definite nemertean-ascidian symbioses. Since no differences in gross condition were ever noticed between occupied and vacant hosts, it is suggested that all three symbionts are commensals rather than parasites or mutuals.