Potential biological control agents for the european green crab, Carcinus maenas in Australian waters

Marine species are invading new ports and waterways threatening biological diversity and contributing to environmental changes which are difficult to reverse or remediate. The European green crab, Carcinus maenas (Linnaeus, 1758) is a successful invasive marine species which has spread from Europe to adversely impact the marine ecosystems of South Africa, and the continents of North and South America and Australia. Methods for controlling this crab are sought and biological control is one possible method. This thesis examines the parasite fauna of native and introduced shore and near-shore crabs from the temperate coastlines of Victoria, Flinders Island and Tasmania, Australia, to search for potential biological control agents for the introduced pestiferous European green crab, C. maenas. Collections were made from the intertidal zone by hand and trap and from shallow sub-tidal (< 5m depth) zones using traps set from boats. The study area surveyed both established populations (first recorded > 100 years ago) and recently arrived populations (first recorded 12 years ago). This survey revealed a number of potential biological control agents against C. maenas, including two species of trypanorhynch tapeworm, Dollfusiella martini (Beveridge, 1990) and Trimacracanthus aetobatidis (Robinson, 1959), and a new species of rhizocephalan. Field observations of high larval trypanorhynch loads in individual C. maenas showed evidence of gross pathology which was histologically studied and described. Physiological impairment of C. maenas was indirectly examined through digestive enzyme analysis of the parasitised digestive gland. Histology and digestive enzyme analysis revealed that C maenas with high intensity trypanorhynch infections suffered digestive gland damage and possibly impaired digestive enzyme function. Taxonomic relatedness of native hosts with C. maenas was shown to be more important than ecological overlap for parasites to transfer to C. maenas. Consequently, attempts were made to cross-infect green crabs with a rhizocephalan found on the confamilial native crab, Nectocarcinus integrifrons (Latreille, 1825), in the laboratory. These attempts failed, however, the unnamed rhizocephalan was described and named as Sacculina nectocarcini, a congeneric to Sacculina carcini (Thompson, 1836) ‚ÄövÑvÆ a well documented parasitic castrator of C. maenas in its native European range. An analysis of mitochondrial cytochrome oxidase I (COI) DNA of nominal S. carcini, parasitising three species of portunid crab, revealed that S. carcini is capable of parasitising at least two other species of portunid crab in addition to C. maeanas. The use of S. carcini as a biological control agent must be treated with caution. COI analysis proved to be a useful tool for resolving spatial heterogeneity of S. carcini. In conclusion, larval trypanorhynch tapeworms offer some potential for control against C. maeans. However, the many unknown trophic links required to complete the lifecycles of these parasites will make field application unpredictably difficult. S. nectocarcini directly attacks its host, making field management as a biological control agent simpler and more effective. The host specificity of this parasite needs to be resolved to determine whether it will switch to C. maenas. The host specificity of S. carcini, a rhizocephalan parasitising C. maenas in its native range, is too lax for it to be released safely in Australia.