Pea crab
Encyclopedia
The New Zealand pea crab, Pinnotheres novaezelandiae, is a small, parasitic crab
Crab
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...

 that lives most commonly inside New Zealand green-lipped mussel
New Zealand green-lipped mussel
The New Zealand green-lipped mussel, is a bivalve mollusc in the family Mytilidae. P. canaliculus has great importance as a cultivated species for New Zealand.-Distribution:Perna canaliculus has a wide distribution which covers all of New Zealand's mainland. P...

s. 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. The New Zealand pea crab is found throughout New Zealand and can infect up to 70% of natural populations. These crabs are of concern to green-lipped mussel aquaculture
Aquaculture
Aquaculture, also known as aquafarming, is the farming of aquatic organisms such as fish, crustaceans, molluscs and aquatic plants. Aquaculture involves cultivating freshwater and saltwater populations under controlled conditions, and can be contrasted with commercial fishing, which is the...

 because they reduce the size and growth of mussels, although infected mussels can be harvested and consumed.

Description

Adult female New Zealand pea crabs have a soft-shelled exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

.Their carapace
Carapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...

 is oval in shape, ranging in size from 9.3 to 20.2 mm (0.366141732283465 to 0.795275590551181 ) wide. Sexually mature adult females almost always have eggs that are tucked under their abdomens, giving them a more spherical appearance. Adult females are opaque white in colour. Developing eggs change colour from red to orange to yellow before they hatch, giving the brooding mother a different tint at each stage. Adult male New Zealand pea crabs have a hard, chitinous exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

. Their carapace
Carapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...

 is smaller and more dorso-ventrally flattened than that of the female, ranging in size from 3.2 millimetre wide. Adult males are a creamy white colour with distinctive orange markings.

Ecology

Female New Zealand pea crabs spend their entire adult lives within a single host. Adult males will only leave their host in order to find a mate. The hard exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

 and flattened body shape of the male New Zealand pea crab helps with this endeavour. New Zealand pea crabs are completely reliant on their host for food, shelter and a place to mate. The New Zealand pea crab collects food by sitting on the gills of the green-lipped mussel and stealing food strands from the mussel. The relationship between the New Zealand pea crab and the green-lipped mussel is one of parasitism
Parasitism
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...

 because the crab damages the mussel's gills when taking food. Infected mussels are also smaller and slower growing than uninfected mussels.

The New Zealand pea crab is endemic to New Zealand and is common throughout the country, inhabiting the North Island, South Island, Stewart Island and the Chatham Islands
Chatham Islands
The Chatham Islands are an archipelago and New Zealand territory in the Pacific Ocean consisting of about ten islands within a radius, the largest of which are Chatham Island and Pitt Island. Their name in the indigenous language, Moriori, means Misty Sun...

. The New Zealand pea crab lives most commonly in green-lipped mussels (Perna canaliculus), but can also be found in many other bivalve molluscs including the blue mussel
Blue mussel
The blue mussel, Mytilus edulis, is a medium-sized edible marine bivalve mollusc in the family Mytilidae. In spite of its specific name edulis, it is not the sole edible Mytilus species.-Distribution:...

 (Mytilus edulis aoteanus), the Pacific oyster
Pacific oyster
The Pacific oyster, Japanese oyster or Miyagi oyster , is an oyster native to the Pacific coast of Asia. It has become an introduced species in North America, Australia, Europe, and New Zealand.- Etymology :...

 (Crassostrea gigas) and a species of clam (Chione stutchburyi). The infection rate in wild green-lipped mussel populations can range from 0 to 70%.

Edibility

Pea crabs are edible and were once a delicacy in the United States. George Washington used to love having pea crabs floating in his oyster soup. Mussels infected by pea crabs are also edible.
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