Cockles

Cockles are classified as bivalves within the phylum Mollusca. (Almost all shelled marine animals, as well as octopus and squid, are molluscs.)

The New Zealand cockle, also known as tuaki or tuangi, is endemic to New Zealand’s coastal areas. They are not endangered, although there are some concerns about the commercial harvesting of the species.

Habitat

New Zealand cockles live in sheltered coastal areas around the North and South Islands, Stewart Island and the Chatham Islands. Cockles’ preferred habitat is within the soft mud and fine sand in subtidal areas. They favour sediments that have a larger grain size and commonly live in sand flats and beds of eelgrass. In some areas, they have been found at water depths of 20 metres but they generally prefer shallower habitats. Cockles also live in estuaries and can tolerate some changes in salinity.

Cockles are filter feeders and are normally sessile, remaining in the same spot and feeding on microscopic plankton that are floating in the water column. However, both adults and juveniles are able to move to more favourable habitats when required.

Adaptations

The New Zealand cockle is well adapted to its sandy, sheltered habitat. They have a sturdy, heavy shell that provides protection from physical damage, predators and drying out. Cockles normally burrow 2–3 centimetres into the sand. However, they live in a tidal environment, and if waves or currents dislodge them, their shell protects them from damage as they tumble around in the water. Their soft flesh is also protected from many predators by the sturdy shell (although it is not always enough to stop the prying beaks and sharp pincers of birds and crabs that like to eat them!). Drying out is a daily danger for animals that live in tidal environments. A cockle’s shell is able to hold a reservoir of water to sustain it when the tide is out.

Cockles: a popular food source

Cockles are not only a favourite food source for birds and crabs. They have been a popular item for people since early Māori settlement, and cockle shells have been found in middens throughout the country. Recreational harvesting is still widespread, and many people consider cockles a delicacy. Cockles are also commercially harvested in a number of locations including Whangārei Harbour, Golden Bay and Otago Peninsula. Cockles can be harvested all year round, and New Zealand manages the harvest with strict quotas. In 2008/2009, the total allowable commercial catch (TACC) was set at 3,214 metric tonnes.

The role of cockles in the ecosystem

Cockles are filter feeders, and they may accumulate toxins, bacteria and viruses present in the surrounding environment. This has important implications for harvesting, and cockles should only be eaten if they are taken from unpolluted areas. It also means that cockles have the potential to act as environmental indicators and to help gauge the health of an ecosystem.

In their role as filter feeders, cockles also provide an important link in the food web between the primary producers, such as phytoplankton, and smaller carnivores, such as birds, crabs and rock lobsters. Cockles also provide another important food web service – when they are filtering the water, they help prevent blooms of phytoplankton that reduce oxygen availability for fish and many other species.