The New Zealand pea crab (Nepinnotheres novaezelandiae), is a species of small, parasitic crab that lives most commonly inside New Zealand green-lipped mussels. Adult females are about the size and shape of a pea, while adult males are smaller and flatter. Adult New Zealand pea crabs are completely reliant on their host mussel for shelter and food, which it steals from the mussel's gills. These crabs are of concern to green-lipped mussel aquaculture because they reduce the size and growth of mussels, although infected mussels can be harvested and consumed.
Taxonomy
This species was first described and named by Henri Filhol in 1885 as Pinnotheres novae-zelandiae and in 1901 as Pinnotheres schauinslandi (a junior synonym) by H. Lenz. It was later assigned to the genus Nepinnotheres.
Description
thumb|left|Stage I adult male Pinnotheres novaezelandiae
Adult female New Zealand pea crabs have a soft-shelled exoskeleton. Adult male New Zealand pea crabs have a hard, chitinous exoskeleton. Adult males will only leave their host in order to find a mate. The infection rate in wild green-lipped mussel populations can range from 0 to 70%. the mate location behaviour of male New Zealand pea crabs was observed when dwelling in 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. Observations of the nocturnal mate-finding behaviour of male crabs were made in darkness using infrared video recordings. The presence of receptive female crabs placed upstream successfully attracted 60% of male crabs from their host over 24 h. 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. The authors concluded that a pheromone-based mate location system is likely used by this crab to greatly reduce the risks associated with the location of females. These crabs are of concern to green-lipped mussel aquaculture because they reduce the size and growth of mussels by up to 29%.