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Crabs, crayfish and other crustaceans

by  Niel Bruce and Alison MacDiarmid

Crustaceans are diverse – some are less than a millimetre long and others are over half a metre, there are lobsters and those shaped like worms, and some are thin discs and others are totally enclosed in shells. Belonging to the same group as insects, spiders, mites and scorpions, they are the bugs of the sea.


Crustaceans – bugs of the sea

Crabs and crayfish are crustaceans, a subdivision of arthropods – the large group of animals without backbones (invertebrates) that includes insects, spiders, mites, scorpions and springtails. Arthropods have jointed legs and a hard outer shell that acts as a skeleton.

Crustaceans mainly occupy marine habitats, and it is in the world’s oceans that they show their greatest diversity. However, they are well represented on land, notably by woodlice or slaters and some sand hoppers, and in freshwater habitats. Between 50,000 and 67,000 species are known worldwide. However, scientists estimate the total number of crustaceans to be 10–100 times greater than this.

Characteristics

A crustacean has the following features:

  • a segmented body with a hard exterior (known as an exoskeleton)
  • jointed limbs, each often with two branches (termed biramous)
  • two pairs of antennae
  • gills
  • seven or more pairs of appendages for feeding, swimming, walking, respiration and reproduction (clasping, sperm transfer, egg brooding and carrying young).

Moulting

In order to grow, all crustaceans periodically cast off their old exoskeleton, to reveal a new one beneath. This process is called moulting and leaves the crustacean vulnerable to predation and cannibalism. In the common red crayfish (Jasus edwardsii) the first moult occurs soon after hatching, and moulting continues for the rest of its life. The markings on each crayfish are unique and are retained through each moult – the crustacean equivalent of fingerprints.

Diversity

Crustaceans show a greater diversity of body form than any other animal group, and include worm-like slaters (isopods), short-bodied crabs and long-bodied shrimps and prawns. There is a great range in size, from less than a tenth of a millimetre (parasitic species and those that live between sand grains) to nearly half a metre (giant crabs, lobsters and slaters, which can weigh up to 20 kilograms).

Crustacean groups

The most recent classification of crustaceans comprises six classes. One of these (the cave-dwelling Remipedia) is not known in New Zealand, and another (the Cephalocarida) is known only from a single species. The Cephalocarida are believed to be close to the ancestral form from which other crustaceans evolved. The best-known crustaceans are edible species such as crabs, crayfish and shrimps. They belong to the Malacostraca class, along with slaters and sand hoppers.

New Zealand crustaceans

Most of the major crustacean groups are found in New Zealand waters, though many warm-water groups are absent or weakly represented. In 2005 the number of species known in New Zealand was 2,682, though the figure is always increasing. The actual number of species could easily be 10 times that figure because, with the exception of crayfish, crabs and shrimps, many of the groups have not been well studied. A number of species are endemic to New Zealand – unique to the country like kiwi and tuatara.


Crabs

With their stalked eyes, wide bodies, large nippers and sideways scuttling movement, crabs are well represented in New Zealand. There is even a freshwater crab (Halicarcinus lacustris) in the streams and lakes of Auckland and the Waikato.

Characteristics

Crabs have wide, flat bodies with no obvious tail. The head and chest are fused and protected by a shield-like structure called a carapace.

The tail is tucked under its body. Males have a narrow tail; females have a broad, rounded one that supports their eggs.

True crabs have a pair of claws followed by four pairs of walking legs. Various other crustaceans (hermit crabs, porcelain crabs and king crabs) superficially resemble crabs but have only three pairs of walking legs.

Habitat and food

At least 30 species are known from the intertidal realm. Some, such as the purple rock crab or pāpaka nui (Leptograpsus variegatus), spend much of their time out of water, sheltering in rock crevices during the day and venturing onto the shore at night in search of food. The purple rock crab is not a fussy eater and consumes seaweeds as well as live or dead animals, including smaller members of its own species.

Giant spider crab

With a leg span of 80 centimetres, the giant spider crab (Jacquinotia edwardsii) is New Zealand’s largest crab. It is a cold-water species, found around the southern South Island and the subantarctic islands in shallow to relatively deep (600 metres) habitats.

There is a lot of muscle in its body and long legs, but with little demand for crab meat in New Zealand no fishery has developed around this species.

Fast grower

The only commercially harvested crab is the paddle crab (Ovalipes catharus), which is a coastal species that supports a small and relatively low-value fishery. Its body grows very quickly – from 3 centimetres to 10 centimetres in a year, and when fully grown it can be up to 15 centimetres wide.

Hermit crabs

Hermit crabs inhabit empty shells of sea snails, which they cart about with them as they scuttle over the sea floor. They do this to protect the soft, vulnerable abdomen, which is offset to one side of the body. As the hermit crab grows, it needs to find larger shells for protection. Sixty species are known in New Zealand.

King crabs

King crabs are the giants of New Zealand’s crustaceans, and they live in deep water. Scientists group them with hermit crabs rather than true crabs, for like hermit crabs they only have three pairs of walking legs and an offset abdomen. Our largest king crab (Lithodes murrayi) has a body width of 20 centimetres and leg span of up to 1 metre.


Crayfish

Crayfish or rock lobster?

Popularly known as crays, crayfish resemble lobsters but lack the lobster’s large crushing pincers on their first pair of walking legs. They inhabit rocky reefs at depths of 5 to 275 metres.

Overseas, New Zealand crayfish have been marketed as rock lobster, and this name now has official status. To add to the confusion, two species occur around the coast. Red crayfish (Jasus edwardsii) are more common, although the larger green packhorse crayfish (Sagmariasus verreauxi) are widespread. Red crayfish are also known as spiny rock lobsters because of the spiny growths on the sides of their tail. In contrast, packhorse crayfish are sometimes called smooth-tailed rock lobsters.

Two other marine crayfish occur in New Zealand waters: the deepwater species Projasus parkeri and the tropical group Panuliris, confined to the reefs around the Kermadec Islands.

Body and senses

Red crayfish grow to about 45–50 centimetres long and typically weigh around 2–3 kilograms, although 8-kilogram individuals have been caught. Packhorse crayfish grow up to 60 centimetres long and reach weights of 15 kilograms.

Crayfish are well-endowed with features that help them explore and respond to their surroundings. A pair of long antennae project from their heads and can be rotated in all directions to touch and explore their rocky habitat. These also serve as defensive lances, as they have serrated edges and can be used to poke or frighten off predators.

Between the antennae is a pair of short, jointed feelers that function as sniffers, detecting chemicals in the water. Crayfish can detect the slightest of movements when under water, but bright sunlight may damage their eyes when they are pulled from the water.

Adult life

For much of their life red crayfish are social animals with quite complex behaviour. During the day they hide in caves and crevices, and at night they venture out in search of food. Sea stars, kina (sea urchins), crabs and shellfish make up the bulk of their diet.

Crayfish reach maturity around 7–11 years of age. Mating occurs in late summer and autumn. They signal their readiness to mate by releasing urine. A male deposits sperm onto the female’s abdomen as she releases her eggs. She gathers the fertilised eggs and attaches them to long hairs under her tail, where they remain for three to five months before hatching.

Larvae

A newly hatched crayfish larva makes a one- to two-year journey out into the South Pacific Ocean, where it floats about feeding and undergoing numerous moults before swimming back to the coast. It has a few changes of identity along the way.

It begins life as a small spidery creature called a naupliosoma larva. After hatching, the naupliosoma swims up toward the surface and undergoes another moult into a leaf-like larva, known as a phyllosoma. In this form, the crayfish larva spends an extended period floating in ocean currents that carry it far from shore.

The final phyllosoma moult transforms the larva into a miniature (2.5 centimetres) transparent version of the adult, known as a puerulus or postlarva. It is at this stage that the crayfish swims back to the coast. It is a mystery how the postlarvae know their way back.

Trans-Tasman crossing

Red crayfish are also found around southern Australia. Scientists think that many crayfish phyllosoma larvae drifting in the Tasman Sea might have hatched in Australia. If they survive the crossing, these little Aussie reds develop into kiwi crays.

Migratory juveniles

Sometimes juvenile crayfish embark on extraordinary migrations along the east coasts of the North and South islands. Crayfish originally tagged in Otago have been caught in Fiordland – after a journey of 850 kilometres. They walk along the sea floor against the current, down the Southland coast, around the southern tip of Stewart Island then up the Fiordland coast towards South Westland, where their journey ends.

Mass migrations are exceptional. One occurred in 1970 and the next in 1993, but each year a few crayfish migrate over large distances. The reason they undertake these trips is unknown, although it has the effect of spreading the population around the coast.


Crayfishing

Early years

Crayfish, or kōura, have always been an important food for Māori. Traditionally they were caught by hand or taken in baited pots that were lowered around coastal reefs. Round or beehive-shaped pots, known as pouraka, were constructed from the stems of a native vine, pirita or supplejack, which were lashed together with harakeke (flax) fibre. Pots were set during the day and left overnight.

Commercial crayfishing was slow to develop. A small canning industry operated from 1906 until the 1930s, but the demand for crayfish was not high until 1948. In that year some crayfish tails were sent to the United States and an export market quickly developed.

Boom and bust

Large volumes of crayfish were caught from the Fiordland coast and around the Chatham Islands in the 1950s and 1960s. With high prices being paid overseas for crayfish tails, many fishermen decided to go crayfishing. They used steel-frame and wire-mesh cages in preference to the traditional supplejack pots.

By the late 1960s, more than 230 crayfishing boats were operating around the Chatham Islands. There was enormous waste of crayfish meat at this time as only the tails were frozen and exported; the bodies were usually dumped at sea or buried on land.

The boom years ended in the 1970s as crayfish landings in the Chatham Islands dropped from a peak of 5,945 tonnes in 1968, to around 510 tonnes in 1974. With a long period until maturity, the crayfish population was unable to recover quickly from such an intensive harvest.

Managing the fishery

In 1980 and 1981 the government attempted to control the number of boats harvesting crayfish by declaring certain areas to be controlled fisheries. In 1990 the Quota Management System (New Zealand’s system for monitoring commercial fish stocks) was applied to the crayfish industry.

In 2004, crayfish were New Zealand’s third-largest seafood export earner. Since 1991 there has been a steady export trade of live crayfish to Taiwan, Hong Kong, Japan and the United States, worth about NZ$110 million each year.

Recreational crayfishing

Around 400–500 tonnes of crayfish are legally caught by recreational and customary fishermen each year. Regulations govern the size of animals that may be taken, and there is a limit of six crayfish per person per day. Females carrying eggs have to be returned to the sea.

Keystone species

Crayfish play an important role in determining the type of habitat that prevails in an area. Animals and plants that play this pivotal role are known as keystone species. In areas where crayfish are fished to low levels, grazing kina (sea urchins) numbers often increase dramatically. With few predators controlling their numbers, the kina eat their way through kelp ‘forests’ so effectively that they destroy the kelp beds. When marine reserves are created, the density of crayfish and other predators increases, kina numbers fall and kelp forests are re-established.


Lobsters, prawns and krill

Scampi

Scampi (Metanephrops challengeri) are deep-water burrowing lobsters. They have been commercially harvested since the 1980s from the Bay of Plenty, off the Chatham Rise, and around the subantarctic Auckland Islands. They are slender pink animals, up to 30 centimetres long and weighing 100 grams. Scampi are cooked and eaten in the same manner as prawns.

Scampi scandal

This little crustacean was the subject of a 2003 parliamentary select committee inquiry. There had been allegations of corruption against senior officers of the Ministry of Fisheries regarding scampi quotas. Although the inquiry found there was no foundation to the allegations, it did find that the ministry had given preferential treatment to the fishing company holding most of the scampi quota.

Squat lobsters

Squat lobsters are eaten in Europe but not in New Zealand, even though they occur in large numbers in New Zealand waters. Munida gregaria (often incorrectly called whale krill) is the most common New Zealand squat lobster. During its bright red phase in summer it can be found swarming close inshore in enormous numbers, from Cook Strait southwards.

Prawns and shrimps

Prawns and shrimps, while superficially similar in body form, are classified quite separately. Like crabs, crayfish and lobsters, they are decapods – 10-legged crustaceans. Shrimps resemble miniature lobsters or scampi, with distinct claws or pincers, and range in size from 2 to 25 centimetres. Prawns differ from shrimps in their breeding habits and in their leg and gill structure – they do not retain eggs, but shed them directly into the water; prawns lack the conspicuous 'claws' that are present in most shrimps.

One of the most familiar shrimps is the semi-transparent Palaemon affinis, a common inhabitant of rock pools.

Shrimp engineers

Some crustaceans are like ecosystem engineers. Each year the burrowing ghost shrimp (Callianassa filholi) churns over 96 kilograms of sediment per square metre of coastal soil. This lets oxygen and organic material get deep into the sediment, which provides the right conditions for very small sediment-dwelling organisms. However, it is detrimental to some shellfish and amphipod species.

Krill

A number of krill species are known from New Zealand waters. Typically smaller than shrimps, they reach prodigious numbers in the seas around and south of New Zealand in summer months when their plankton food is plentiful. In turn, they are an important food for squid, fish, seabirds and some whale species.


Barnacles and copepods

The Maxillopoda is a class of crustaceans that includes barnacles, which are immobile, and copepods, which can move about. These unlikely classmates are placed together by scientists because of their similar body segmentation.

Filter-feeding barnacles

Among the best known and widely encountered members of this group are the filter-feeding barnacles. The barnacle zone is a familiar site on many rocky shores – a white zone of little shelly pyramids occupying the highest reaches of the tide. Goose barnacles are stalked and can attach to driftwood and other floating objects in the sea. Rocky-shore barnacles are encased in a set of calcium carbonate plates and, externally, lack the appearance of typical crustaceans. However, their jointed limbs and shrimp-like bodies reveal their crustacean nature.

Two pounds of flesh

Few, if any, New Zealanders eat barnacles nowadays, but they once featured in the diet of some Māori. The species Epopella plicata was found in coastal middens around the Aotea Harbour in south Auckland, along with pipi, cockles and tuatua shells. It takes a lot of barnacles to get a good meal: 10,000 Epopella yield about 1 kilogram (two pounds) of meat.

Symbiotic barnacles

Many barnacles are symbionts: animals that live with, on, or in other animals. Some barnacle parasites of crabs cause feminisation of their host. These parasites can be identified as barnacles only when in the larval stage. Others live on the heads of whales. These form large aggregations but do not apparently harm their host. Some are host specific: the barnacle Coronula diadema is a typical associate of humpback whales.

Copepods

The copepods are a highly diverse group, and they occur in nearly all aquatic habitats, including water trapped in the bracts of rain forest plants and the leaf litter of forest floors. There is no widely used common name for this group of generally small (less than 5 millimetres) crustaceans, although the pelagic (ocean swimmers) are occasionally referred to as oar-footed bugs. These animals can occur in vast numbers and are important in the oceans’ food chains, being a major source of food for commercial fish species such as sardines and herrings.

Bottom-dwelling copepods

Another large group lives on the sea floor. These benthic or bottom-dwelling copepods are often flat and roughly circular. Along with benthic isopods, they are highly sensitive to habitat disturbance and adversely affected by pollution. They are probably important food for the larval and juvenile stages of fish. When these tiny crustaceans disappear, so too do the adult fish whose larval stages feed on them: small does not necessarily mean insignificant.


Lesser known groups

Slaters

Slaters and sand hoppers constitute about 20% of all crustacean species. Slaters (isopods) exhibit a greater range of body form than all the other crustaceans combined. Marine slaters range in appearance from wafer-like discs, to worm-like creatures and pill bugs that can roll into a ball. Most are small (3–10 millimetres), although the giant scavenging Bathynomus, a deep-water slater found off the Gulf of Mexico and Australia’s coast, reaches nearly half a metre in length.

Many marine slaters are scavengers – a scourge for commercial fishermen, as they attack dying fish in nets. Others are parasites and live permanently on or in the fish on which they feed. They show most of the typical traits of parasites such as loss of pigment, reduced eyes, strong attaching limbs and high reproductive capacity.

Sand hoppers

With a body flattened from side to side, sand hoppers (amphipods) not only look like fleas, but jump like them too. They are not restricted to beaches but may be found in many marine habitats as well as some inland sites.

Tongue worms

Tongue worms (pentastomids) are scarcely recognisable as crustaceans, except when larvae. The adult form is like a worm or leech – a result of their highly evolved parasitic life in the respiratory tracts of reptiles and birds (and some fish).

Seed shrimps

Seed shrimps (ostracods) bear little resemblance to shrimps. They are tiny crustaceans with a distinct bivalved outer shell (usually referred to as the carapace), and they resemble miniscule cockles or clams. Because of their microscopic size, the group is not often featured in popular accounts and field guides. This is a predominantly marine group occurring in most benthic habitats in all the world’s oceans. In New Zealand, approximately 300 species are known.


Hononga, rauemi nō waho

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How to cite this page: Niel Bruce and Alison MacDiarmid, 'Crabs, crayfish and other crustaceans', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/mi/crabs-crayfish-and-other-crustaceans/print (accessed 6 December 2019)

He kōrero nā Niel Bruce and Alison MacDiarmid, i tāngia i te 12 Jun 2006