The ecology of hydroids (Hydrozoa: Cnidaria) in Port Phillip Bay, Australia, and their impacts as fouling species in longline mussel culture

© 2011 Dr. Isla FitridgeHydroids (Cnidaria:Hydrozoa) are a frequently abundant component of marine sessile communities associated with natural and artificial habitats. Many hydroids have the potential for economic impacts through extensive fouling of piles, pontoons, vessel hulls and aquaculture facilities. Detrimental effects of hydroid biofouling on mussel aquaculture have been documented overseas, yet little is understood about the relationship between hydroids and juvenile/adult mussels, or of the role of hydroids in biofouling assemblages. In Port Phillip Bay, hydroids displayed seasonal and spatial patterns of distribution, growth, reproduction and larval recruitment. In commercial mussel farms, hydroid morphology was generally found to facilitate recruitment of the blue mussel, Mytilus galloprovincialis, and there was a significant correlation between the presence of the branched hydroid Obelia dichotoma on mussel ropes and mussel recruitment. However, although attractive to mussel larvae, the stolonal hydroid Ectopleura crocea may be negatively affecting mussel production. Significant detrimental effects of fouling on mussel condition were found, particularly on juvenile mussels. This may be a result of filtration interference, as fouling by E. crocea on mussel shells was site specific, mostly occurring on the body, edge and dorsal shell regions. Analysis of the plankton and the trophic ecology of E. crocea determined that competition for food between M. galloprovincialis and E. crocea may also be affecting mussel growth. E. crocea colonies captured an assortment of prey items but the most common items were various crustaceans, diatoms and bivalves. Laboratory feeding studies confirmed that plantigrade larvae (those settling out from the plankton) were consumed more readily than other bivalve life stages. The consistency in prey numbers of many taxa in the diet of E. crocea at each site, regardless of the temporal variation in prey availability, implies some selectivity in hydroid feeding. This thesis used a range of mensurative and experimental approaches to investigate the ecology of hydroids in Port Phillip Bay, Australia, and determine their impacts as fouling species in the commercial longline culture of blue mussels, Mytilus galloprovincialis. To my knowledge, this research represents the first documented assessment of the range of impacts of hydroids such as O. dichotoma and E. crocea in longline mussel culture