Biology and impact of the New Zealand pea crab (Nepinnotheres novaezelandiae) in aquacultured greenlipped mussels (Perna canaliculus)

Abstract

Pea crabs are globally ubiquitous parasites of marine bivalves that cause serious economic impact among several major aquaculture species. However, little is known about the biology of pea crabs in aquaculture as well as their impact on the hosts and aquaculture production. There are many information gaps including host-parasite interactions and pea crab mating behaviour that could potentially prove useful for controlling their infection in aquaculture. Research presented in this thesis investigated the biology, abundance, distribution, impact, and mate location behaviour of Nepinnotheres novaezelandiae within aquaculture of its preferred host the New Zealand green-lipped mussel (Perna canaliculus). Systematic sampling of a typical green-lipped mussel farm revealed an overall farm-wide infection rate of 5.3 % of mussels. Crab abundance increased gradually with decreasing water depth beneath the farm and greater distance from the shoreline. Infection with a pea crab caused a 30 % reduction in total wet weight of the host mussel with shell dimensions also significantly reduced compared to mussels from the same farm without a pea crab. Extrapolating size losses to current total mussel aquaculture of green-lipped mussels in New Zealand represents an estimated loss of US$2.16 million annually. Sampling revealed a strong sex bias toward females (82.4 %) in the population of pea crabs surveyed on the mussel farm. Most crabs (87 %) recovered were sexually mature and of these 86.4 % were mature females. The sex ratio of immature crabs was relatively even indicating high post maturation mortality among male crabs. The pea crab population with the mussel farm was estimated at 126,390 with 93,000 gravid females carrying over 241 million eggs. These results indicate the rapid colonization and maturation of pea crabs in farmed mussels that are able to establish a significant breeding population with the potential for infecting nearby mussel farms as well as wild populations of bivalves. Behaviour experiments revealed that male crabs always entered a host through the widest gap in between the valves at the inhalant siphon of the host mussel. Male crabs were often observed stroking the mantle edge in this region of the mussel whilst attempting to gain entry and significantly increasing valve gape as a result. A natural diurnal rhythm was observed in P. canaliculus with decreased sensitivity to touch at night when crabs are almost exclusively active. This research indicates the likely use of a pheromone-based mate location system to greatly reduce the risks for males associated with locating mates. Overall, these results provide us with a better understanding of the biology and impact pea crab parasitism has on aquaculture. They also indicate potential targets that may be able to be used in biocontrol and management practices to help lower infection levels of this economically important parasite in green-lipped mussel aquaculture.