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Shellfish

by  Maggy Wassilieff

New Zealanders often see the shells of mussels, cats’ eyes and limpets on the shoreline, but they might be surprised to know that sea slugs, shipworms and squid are also classified as shellfish. They all belong to the country’s vast mollusc group, which includes the beautiful Arabic volute, and the giant sea snail, whose shell is used by Māori as a trumpet.


The mollusc family

More than just molluscs

In zoological terms, ‘shellfish’ refers to species belonging to the mollusc group. But some people use the term to include edible species that have shells but are not molluscs – such as crayfish, prawns and sea urchins (kina).

Shellfish belong to the large group of marine animals known as molluscs. With more than 80,000 species known worldwide, molluscs are the second largest group in the animal kingdom. They include familiar shellfish that are partially or entirely enclosed within a shell, such as mussels, oysters, snails and limpets. Others possess very small or internal shells –for example, sea hares, shipworms and squid. Sea slugs and octopuses are also molluscs, but lack any type of shell.

Mollusc features

There is no standard shape or body plan for molluscs, although they share some common features:

  • an unsegmented, soft body
  • a mantle (a fold in the body wall, that may produce a shell)
  • a muscular foot or tentacles.

Many molluscs also have:

  • an external shell that completely or partially encloses the animal
  • a radula – a ribbon of teeth for rasping food.

Molluscs in New Zealand

There are over 3,660 species of mollusc living in New Zealand waters. This makes them the country’s largest group of marine animals, far outnumbering the crustaceans (crayfish, crabs, krill) and vertebrates (fish and marine mammals). Scientists working on fossils know of nearly 7,000 other types that once lived around ancient New Zealand.

New Zealand’s named species are found in all seven classes of mollusc:

  • Gastropods – sea snails, sea slugs and limpets (over 3,000 species)
  • Bivalves (436 species)
  • Octopus and squid (129 species)
  • Chitons (56 species)
  • Tusk shells (11 species)
  • Monoplacophorans (2 species)
  • Aplacophorans (2 species).

Distribution

Many of these species are not found anywhere else. Some groups such as ostrich foot shells (Struthiolariidae) have probably been in New Zealand since the break-up of the ancient Gondwana continent 85 million years ago. However, a number of the New Zealand species are widely distributed throughout the South Pacific region, and a smaller group is found in south-east Australia.


Sea snails

Known to scientists as gastropods or univalves, snail-like shellfish are the most prolific of the seven mollusc groups. They are found in all marine habitats, from estuaries and coastal shores to the deepest depths of the ocean floor. The characteristic spiral shell ranges from the flattened ear shape of the native abalone or pāua (Haliotis iris), to the coiled spire of the common turret shell (Maoricolpus roseus). A few New Zealand snails, such as the spiny murex (Poirieria zelandica), have intricately sculptured shells, but there are few of the large, colourful shells found in other countries.

Most sea snails have a well-developed head, with tentacles and eyes. They withdraw their head and foot into the shell when inactive or threatened. For added protection many have a disc (operculum), attached to their foot, that covers the opening when the snail withdraws inside.

Sea snail sizes

At 24 centimetres long, the trumpet shell (Charonia lampas) is New Zealand’s largest sea snail. It is usually seen around rocky coasts in spring when it moves into shallow waters to breed. Take your pick for the smallest sea snails – around 75% of them are smaller than a little fingernail, and hundreds are the size of a pinhead.

Herbivorous snails

Many of the sea snails commonly encountered around the rocky coast eat seaweed. They usually move slowly on their broad, flat foot, scraping away at low growths of seaweed with a rasp-like radula (tooth-lined tongue), or browsing on the fronds of larger seaweeds. Top shells (100 New Zealand species), limpets (50) and turban shells (25) are well represented among the coastal grazers.

Reaching up to 15 centimetres in length, black-footed pāua (Haliotis iris) are New Zealand’s largest herbivorous snails. These limpet-like creatures can attach themselves with such a powerful grip of their muscular foot that divers need a knife to pry them from subtidal rocks. Pāua feed at night on red seaweeds and decomposing pieces of brown seaweed that drift by.

Coastal carnivores

Many of the carnivorous sea snails are instantly recognisable by their shell, which tapers at each end. When searching for food they extend a long tube (siphon), which acts as a sniffer for tasty chemicals in the incoming current of water.

Whelks, rock shells and oyster borers are common carnivores around New Zealand’s rocky coast, preying on barnacles and other molluscs. Whelks are one of the largest groupings of sea snails, with over 75 species recorded from New Zealand waters.

Small and deadly, oyster borers (Haustrum scobina) force open the protective plates of barnacles with their muscular foot, and insert their proboscis into the flesh to consume it. When attacking shellfish such as oysters, they drill the shell with their radula, taking anywhere between 45 minutes to two days to reach the succulent meat inside.

The beautifully marked Arabic volute (Alcithoe arabica) dwells under sandy sediments during the day, emerging at night to seek shellfish such as cockles, pipi, and Dosinia species.

Floating snails

Violet snails (Janthina species) float upside down in the open sea. They secrete a raft of bubbles that keeps them afloat as they drift with the currents, and feed on jellyfish and Portuguese man-of–war colonies. After strong storms their shells wash ashore, especially on Northland coasts.


Sea slugs

Some of New Zealand’s most beautiful animals are the sea slugs. They have no shell, but scientists group them with the gastropod group of molluscs, which are snail-like shellfish. Two main types of sea slug frequent coastal waters:

  • nudibranchs: colourful, patterned slugs
  • sea hares: brownish slugs, resembling seaweed.

Nudibranchs

Nudibranch means ‘naked gill’ and refers to the feathery growths near the rear end of the animal, which function as breathing apparatus. Over 80 species are known from New Zealand. The warm waters around the Poor Knights Islands marine reserve, in the north, are one of the best places to see the spectacular blue and brown gem nudibranch (Dendrodoris denisoni), and Verco’s nudibranch (Tambja verconis), resplendent in gold and blue.

In contrast, New Zealand’s largest species, the Wellington nudibranch (Archidoris wellingtonensis), is an ugly creature. Capable of reaching 20 centimetres in length, its orange-brown body is covered in large wart-like growths.

Defensive strategies of nudibranchs

Nudibranchs are carnivores and feed specifically on sponges, sea firs (hydroids) and sea squirts. Because they lack the protection of a hard shell, it might be thought that their soft bodies would be vulnerable to hungry predators such as fish and starfish. But most predators avoid them, probably because they contain unpleasant chemicals in their bodies. However, the Roboastra luteolineata preys on fellow nudibranchs.

A remarkable defensive system is used by the pink and white Jason’s nudibranch (Jason mirabilis). They feed on hydroid polyps, and by some unknown means incorporate the hydroid’s stinging cells into growths (cerata) on their backs. Any predator biting into these growths will trigger the stinging cells and receive a dose of poison.

Test for toxins

Scientists think the bright colours of nudibranchs are an adaptive strategy that warn predators of poisons or nasty-tasting chemicals. But how can you test for them? Marine scientist John Morton suggested that you cautiously touch the animal with your tongue. This may not be a good idea, as in 2009 dogs died after eating the rather drab-coloured nudibranch species Pleurobranchaea maculata, washed up on Auckland beaches. Testing found that the nudibranchs contained the toxin tetrodotoxin, which if ingested would also be fatal in humans. Signs were put up warning people not to touch the nudibranchs and to watch their dogs and children.

New Zealand has eight species of sea hare. This group of fleshy slugs has prominent tentacles that could, with some imagination, be thought to resemble the ears of a hare. Some species have a small internal shell protecting the gills. All are herbivores, feeding on succulent seaweeds, which afford good camouflage. They shoot out a poisonous purple dye if a predator gets close.

Sea hares

Sea hares are hermaphrodites (each animal has both male and female sex organs). Mating often occurs in groups, with chains of animals lining up to fertilise the released eggs of the animal in front. Sea hares release their eggs in a colourful tangle resembling spaghetti or knitting wool.


Bivalve molluscs

Most of the familiar edible shellfish such as mussels, cockles, oysters, pipi and scallops belong to a group of molluscs known as bivalves. The term bivalve refers to their two hinged shells (technically valves). Both shells are usually of the same shape, as in mussels, but in oysters and scallops one is flat and one curved.

Bivalves are adapted to living in a confined space and feeding by straining plankton from the water. Even though they have no head or radula (rasping tongue), bivalves are a successful group, with 436 species known from around New Zealand. They are also prolific. Some species can reach phenomenal densities: over 20,000 per square metre.

Most bivalves are sedentary or slow-moving animals. Some, such as pipi and cockles, spend their life buried in seafloor sediment, while others like the oyster and mussel remain anchored to one spot.

Sediment burrowers

Bivalve shellfish are adapted for burrowing into ocean sediments, pulling themselves down with their foot. Different species live at different depths beneath the sea floor. Burrowing bivalves extend a pair of siphons which act as their feeding, breathing, and waste-removal tubes. Water is drawn in through one siphon and passes over the shellfish’s gills, where food particles are strained off and oxygen is absorbed. The water is then expelled through the other siphon.

Bridge over troubled waters

Shipworms, the scourge of wooden sailing ships for centuries, still wreak havoc today. They caused the collapse of a rail bridge spanning the Nūhaka River, on the East Coast, in 2005. Supporting timbers below the high-water level were riddled with the burrowing shellfish.

Rock and wood borers

A few species – angel wing (Barnea similis), date mussel (Zelithophaga truncata) and piddocks (Pholadidea suteri and P. tridens) – bore into soft coastal rocks to form their burrows. Like the sediment burrowers, they filter feed by means of long siphons.

The peculiar shipworm (Bankia species) tunnels into submerged wood. With a greatly reduced shell at the front of their worm-like body, shipworms are barely recognisable as bivalve shellfish.

Mussels

Mussels attach themselves to rocks and solid structures with strong, elastic threads secreted from a gland in their foot. When young, mussels can separate from their threads and move about a little. New Zealand has 22 species of true mussel, of which the commercially farmed green shell mussel (Perna canaliculus) is the best known. Along with the blue mussel (Mytilus galloprovincialis), it is common around rocky coasts from low tide to subtidal depths.

Big mussels

Horse mussels (Atrina zelandica) are the largest living shellfish found in New Zealand. They live for about six years and usually grow 30–35 centimetres long. One enormous specimen at the Auckland Museum measures 40.5 centimetres. But it is no match for the 1.5-metre Magadiceramus rangatira giant mussel that lived in New Zealand waters 100 million years ago.

Oysters

Rock oysters (Saccostrea glomerata) and Pacific oysters (Crassostrea gigas) attach one shell to a solid surface and remain fixed in place. Pacific oysters are commercially farmed in northern North Island estuaries.

Scallops

Unlike other bivalves, scallops can swim. Most of the time they lie on the sandy or muddy sea floor with their shells slightly open, filter-feeding. But if danger beckons in the form of an octopus or starfish, they contract a powerful muscle that pulls their shells together, shoots out water and moves the scallop forward. Using this form of jet propulsion, scallops can manoeuvre quickly through the water.

Two types of scallop are commercially harvested in New Zealand: the large Pecten novaezelandiae from coastal waters in Golden Bay at the top of the South Island, and the vividly coloured queen scallop Zygochlamys delicatula, from deep water off the Otago coast.


Dredge oysters

Variously known in New Zealand as Bluff oyster, dredge oyster and flat oyster, or by its Māori name tio, this much-loved delicacy is really the Chilean oyster – Ostrea chilensis.

Found all around the New Zealand coast in small patches to depths of 35 metres or more, dredge oysters grow in extensive beds in the South Island’s Foveaux Strait, Golden Bay and Tasman Bay. The pear-shaped adult is a heavy-shelled bivalve, some 6–10 centimetres long, with a cupped lower shell and a flat upper shell.

Life cycle

A dredge oyster spends its first 18–30 days as a larva within its parent’s body. It is then released into the water and settles a few centimetres away. It immediately cements the lower shell to some surface, often the shell of another oyster.

After about two years the cement bonding breaks down and the oyster lies on the muddy sea floor. It reaches sexual maturity around this time. A dredge oyster may be a hermaphrodite, producing eggs and sperm at the same time, or it may be in a definite male or female phase. It can alternate between each sex.

Dredge oysters can live for eight years or more and reach harvest width of 58 millimetres in four to six years.

Foveaux Strait oyster fishery

Foveaux Strait oyster beds have been dredged since 1863. The first beds were small, and were soon depleted. A large bed discovered in eastern Foveaux Strait was dredged from 1888 until the 1950s. When this proved uneconomic, the focus moved to the centre of the strait. In the early 1930s, the oysters were struck by disease and many died. Other disease outbreaks followed in the early 1960s and between 1986 and 1992. The culprit was Bonamia exitiosus, a parasite that lives inside the oyster’s blood cells.

Mullock reefs

Fishermen knew that the densest beds of oyster were associated with reefs of material known as mullock. Resembling coral, mullock reefs are built up from the skeletons of assorted bryozoans, shellfish, polychaete worms and sponges. They are common around New Zealand where strong currents flow through shallow channels, and they may stretch for several kilometres.

After 140 years of dredging, most mullock reefs in Foveaux Strait have been destroyed by heavy equipment, and few sites are available for young oysters to settle. The oyster population in the eastern Foveaux Strait has never recovered since the 1950s, and neither have the mullock reefs. Some environmentalists believe the fishery will collapse if destruction of the reefs continues.


Chitons, tusk shells and rare classes

Chitons

Looking rather like a miniature armadillo encircled by a snake, chitons can usually be found on coastal rocks along with limpets, where they graze on rock-hugging growths of seaweed.

A chiton is easily recognised by the eight overlapping shell plates on its back, which have a leathery rim. They cling to rocks with a muscular grip and move slowly over surfaces at night when the tide is in. They have relatively flexible bodies and can squeeze into narrow crevices – unlike limpets, whose rigid shell prevents entry into such spaces. If detached from rocks, they roll into a ball just as a slater (woodlouse) does, their soft body protected by the shelly plates.

Despite minimal development of a head, and no sensory tentacles, some chitons know exactly where they are on the shore, and return to a home site after feeding. They are sensitive to pressure and detect light with numerous shell eyes. These are tiny holes in their shell plates that connect to structures containing a lens, a retina and pigment cells.

New Zealand chitons

Most of New Zealand’s 56 species of chiton are coastal animals, but a few are known in deep water. New Zealand’s most common species, the snakeskin chiton (Sypharochiton pelliserpentis), grows to 4 centimetres and is found on all rocky shores. Less commonly seen because it favours subtidal, shaded locations is the noble chiton (Eudoxochiton nobilis). At 10 centimetres, it is New Zealand’s largest species.

Tusk shells

Tusk shells, named for their resemblance to elephant’s tusks, are carnivorous snails with a tubular, tapering shell. They are burrowing animals, living partially buried in the sea floor. Both ends of their shells are open: the broad end is buried and the narrow end sticks out into the water.

Tusk shell snails are simple creatures: they lack eyes, gills and a heart. They have a tiny head that consists of a proboscis with a mouth and threadlike tentacles that sweep the sediments and trap micro-organisms for food. The best-known tusk shell in New Zealand, Anatalis ana, inhabits coastal muds as well as deeper sediments. In pre-European times Māori collected tusk shells that washed ashore, and threaded them together for necklaces and anklets.

Rare classes

Two rare classes of shellfish, the Monoplacophorans and Aplacophorans, are uncommon in New Zealand waters. Monoplacophorans are deep-water shellfish that superficially resemble limpets. An eagle-eyed scientist must have been at work the day the first New Zealand monoplacophoran was discovered in sediments trawled up from a depth of 1 kilometre. Micropilina tangaroa is just 1.5 millimetres long, and only one specimen was found. Since then a second related monoplacophoran has been discovered off the Southland coast.

Aplacophorans are small worm-like molluscs that live and feed on deep-water corals and hydroids. They have no shell, eyes or tentacles. Two species are known from New Zealand waters. One (Dorymenia quincarinata) has been found at shallow depths in Lyall Bay, Wellington, and at depths of 240 metres near the Chatham Islands. The other species (Neomenia naevata) was collected from the Taiaroa Canyon, off the Otago coast, at a depth of 380 metres.


Food and other uses

Seafood

Shellfish were, and continue to be, an important part of the diet of Māori living on the coast. Traditionally, shellfish-gathering was the work of women and girls, who harvested a variety of species such as toheroa, tuatua and pipi (all Paphies species), pāua (Haliotis species), tuangi (Austrovenus stutchburyi), rock oyster (Saccostrea cucullata), dredge oyster (Ostrea chilensis), mussels (Perna canaliculus, Mytilus edulis) and scallops (Pecten novaezelandiae). These are still commonly eaten today. Many other coastal shellfish that were once eaten, such as Cook’s turban (ngāruru, Cookia sulcata) and ringed dosinia (harihari, Dosinia anus), are seldom gathered by Māori or Europeans today.

Since the 1990s Asian immigrants have begun harvesting less well-known coastal shellfish, such as limpets and cat’s eyes, for food.

Traditional uses

Shells were used by Māori in a variety of ways:

  • Fish hooks. Cook’s turban shells were used to fashion the tips of hooks, and the iridescent inner shell of pāua was used for fishing lures.
  • Scrapers and cutters. Mussel shells were used for cutting hair and for scraping flax leaves in order to expose the fibres below the outer green layer. Tuatua shells were used for scaling fish.
  • Trumpets. Trumpets known as pūtara were made from New Zealand’s largest sea snail, Charonia lampas, by fitting a wooden mouthpiece to the top of an empty shell. A loud note is produced because the sound is amplified within the shell. The large, rare triton shell (Charonia tritonis) was also used. Live tritons are not found in New Zealand, although sometimes their dead shells wash up on northern shores.
  • Adornment. Tusk shells were used as anklets and necklaces, and pieces of pāua shell were sometimes hung on skirts.
  • Inlay in carvings. Pieces of pāua shell were used as inlay in wooden and bone carvings, often representing eyes.
  • Bowls and containers. Dosinia species and scallop shells were used to hold pigments for tattooing. Pāua ashtrays, used by Māori and Pakeha alike, came into their own after the 1950s.

Commercial farming

In New Zealand, green-lipped mussels and Pacific oysters are commercially farmed with aquaculture methods. Pāua, scallops, queen scallops and dredge oysters are commercially harvested from the wild.

Pāua jewellery

For nearly 20 years after the end of the Second World War, pāua earrings, necklaces, rings and souvenirs were manufactured solely by disabled servicemen. Since this monopoly on the shell has been lifted, there has been a huge increase in the range of pāua jewellery, often set in silver, from craftspeople using both contemporary and traditional designs.

Pāua pearls have been produced since the mid-1990s. They are grown as half-round pearls inside the living pāua. The two halves are glued together to produce the finished product.

Crushed shell

Crushed shells are used as poultry food additives, as loose paving in gardens, and for decoration. In the past, they were taken from beach deposits around the country, such as at Quail Island in Canterbury and the extensive shell ridges in the Firth of Thames. Another source is the by-product of oyster, mussel and scallop harvests.


Shell collecting

Early collectors

For a short period in the late 1700s, following Captain Cook’s voyages to the South Seas, there was a demand for distinctive New Zealand shells from British and European shell collectors. Some of the desired specimens included pāua (Haliotis iris), top shells (Calliostoma punctulatum, C. selectum and Cantharidus purpureus or C. opalus), Cook’s turban (Cookia sulcata) and the most prized – the circular saw shell (Astraea heliotropium). None of these is particularly rare, and they are no longer highly sought after.

Information and clubs

A wealth of information is available for people interested in learning about New Zealand shells. The first popular book on shell collecting, Beautiful shells of New Zealand, appeared in 1908, and many others have followed. The definitive reference work is A. W. B. Powell’s New Zealand Mollusca (1979).

Important reference collections of fossil and present-day shells are held by Auckland Museum, Wellington’s Museum of New Zealand Te Papa Tongarewa, and the Institute of Geological and Nuclear Sciences.

There are three shell clubs in New Zealand – at Whāngārei, Auckland and Wellington – where enthusiasts identify shells and organise field trips. In the 21st century there is an emphasis on photographing and studying live shellfish in their natural habitat, rather than taking live specimens for personal collections.

Shell kitsch

The largest display of pāua shells is at Pāua House, once a well-known tourist attraction in Bluff, Southland, and now on display in the Canterbury Museum. Fred and Myrtle Flutey had transformed their bungalow by covering it with polished pāua shells.

In the 1980s the owners of ‘shell wonderland’, Nola and Berry Edwards, ran a tearoom and museum that displayed numerous objects decorated with shells, including the family car.


Hononga, rauemi nō waho

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How to cite this page: Maggy Wassilieff, 'Shellfish', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/mi/shellfish/print (accessed 15 November 2019)

He kōrero nā Maggy Wassilieff, i tāngia i te 12 Jun 2006