Mate locating and access behaviour of the parasitic pea crab, Nepinnotheres novaezelandiae, an important parasite of the mussel Perna canaliculus.

Pea crabs are globally ubiquitous symbionts in the marine environment that cause serious economic impact in the aquaculture production of several major bivalve species. However, little is known about their host-parasite interactions, especially the mating behaviour of these parasites that could prove useful for controlling their infestation in aquaculture. In this study, the mate location behaviour of male New Zealand pea crabs, Nepinnotheres novaezelandiae (Filhol, 1885), was observed when dwelling in its preferred host, the commercially important green-lipped mussel, Perna canaliculus. Given the cryptic behaviour of the male crabs, a novel trapping system was developed to determine whether male crabs would exit their mussel hosts in response to an upstream female crab. The presence of receptive female crabs placed upstream successfully attracted 60% of male crabs from their host over 24 h. Observations of the nocturnal mate-finding behaviour of male crabs were made in darkness using infrared video recordings. Males spent on average 49 min on empty hosts and never left a mussel containing a female conspecific once found, spending 200 min on average to gain entry to the mussel. Male crabs were often observed stroking the mantle edge of the mussel whilst attempting to gain entry, successfully increasing mussel valve gape during entry from 3.7 to 5.5 mm. A pheromone-based mate location system is likely used by this crab to greatly reduce the risks associated with the location of females.

Recruitment of the parasitic pea crab Nepinnotheres novaezelandiae into green-lipped mussels Perna canaliculus.

Pea crab species are globally ubiquitous parasites of marine bivalves including several major aquaculture species. However, little is known about the environmental factors that affect their recruitment into aquacultured mussels. The effect of depth and distance from shore on the recruitment of the parasitic pea crab Nepinnotheres novaezelandiae into New Zealand green-lipped mussels Perna canaliculus was examined with a field experiment. The incidence of pea crab infection of mussels over 295 d was nearly double when deployed at 5-10 m depth (1.97%) compared to 20-30 m depth (0.96%), although it was not significantly different due to the overall low period prevalence in the experimental population. The sex ratio of crabs recovered was significantly skewed towards females with a ratio of 1:14 (χ = 11.3, p < 0.001). Infection with pea crabs was found to significantly reduce final mussel shell height on average by 28% (21.0 mm) over 295 d (Mann-Whitney U = 6.0, p < 0.0001). This study confirms that parasitism of green-lipped mussels by pea crabs has a significant impact on the growth of the mussels and suggests that the incidence of pea crabs will be higher in shallower water and when mussels are in closer proximity to the shore. With no control methods available for preventing pea crab infection, these results suggest that moving mussel farms offshore has the potential to reduce the incidence of pea crabs in mussels and warrants larger-scale assessment.

Biological characteristics of parasitic Nepinnotheres novaezelandiae within a Perna canaliculus farm.

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 green-lipped 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 126390 ± 14144 individuals, including 93000 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.