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.