Pea crabs are commercially significant parasites in the aquaculture production of bivalves in many parts of the world. However, there is scant information available on the biology of these important parasites in aquaculture. The population structure, sex ratio, and breeding status were determined for the pea crab Nepinnotheres novaezelandiae residing in a typical greenlipped mussel Perna canaliculus farm in New Zealand. Of the 324 crabs randomly sampled, there were significantly more female (82.4%) than male (17.6%) crabs found. The vast majority of crabs (87.0%) were sexually mature and of these, females comprised 86.4% and males 13.6%. However, the sex ratio of immature crabs was relatively even, suggesting that male crabs may have higher mortality while searching for mates. Crab size was highly variable, indicating that recruitment to mussels in the farm was continuous. Carapace width ranged from 4.00 to 11.5 mm, with males tending to be smaller with a mean (± SE) carapace width of 6.31 ± 0.16 mm versus females with a carapace width of 8.03 ± 0.06 mm. The crabs did not show any preference for parasitizing mussels of different sexes. Despite the fact that the mussels were only 10 mo old, most female crabs were sexually mature (Stage V) and 89.3% were gravid. The mean (± SE) clutch size was 2592 ± 579 and clutch size was directly associated with female carapace width. The mussel farm pea crab population was estimated at 126 390 ± 14 144 individuals, including 93 000 gravid females carrying a total of over 241 million eggs. Overall, the results show that pea crabs rapidly colonize farmed mussels and mature quickly to establish a significant breeding population within the mussel farm, with larval output capable of infecting nearby mussel farms as well as wild populations of bivalves.KEY WORDS: Pea crab · Green-lipped mussel · Greenshell™ · Parasite · Mussel farm · Brachyura
Resale or republication not permitted without written consent of the publisherDis Aquat Org 101: 61-68, 2012 62 that pea crab infection was responsible for a loss in production across the nation's mussel aquaculture industry of US $2.16 million annually (Trottier et al. 2012).Infection levels in wild mussel populations by Nepinnotheres novaezelandiae vary widely and have been reported to range from 4 to 70% (Jones 1977a, Hickman 1978, Baxter 1981, Bierbaum & Ferson 1986). Within green-lipped mussel aquaculture an infection level of 5.3% has been reported, although higher levels are suggested to occur (Trottier et al. 2012).The life history of Nepinnotheres novaezelandiae has been partially described, with most larval stages inferred from related species (Bennett 1964, Jones 1977b, Wear & Fielder 1985. After hatching from eggs, the larval development begins with an unknown number of zoeal stages which are presumably followed by a megalopal stage that has never been observed (Wear & Fielder 1985). The first crab instar is thought to be the invasive stage and is followed by a series of soft carapace stages for both sexes (Jones 197...