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.
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 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.
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.
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.
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.