The Common Cockle

Tuangi

Scientific Name Austrovenus stutchburyi

The common cockle is probably the most common bivalve in New Zealand, living just below the surface of the sand in concentrations of thousands per square metre. They are often abundant tucked into the root-like rhizomes of sea grass (Zostera) beds.

Cockles favour quiet harbour waters, estuary tidal flats and sheltered coastal sand banks. When the tide is in, the cockles extend two short siphons into the water and filter plankton as they circulate the sea water through their gills.

They grow at about 10 mm per year for the first year or two and then slow down, reaching about 30-mm in shell length in 3 to 4 years. Under excellent conditions, cockles may reach 18-mm and become sexually mature in their first year. Their maximum size is about 50-mm in shell length.

There are male and female cockles. The larger females produce the most eggs and so stocks of big adults might be very important to spawning success. Cockles shed their eggs and sperm into the sea water over the summer months. Two peak spawning seasons, in October and March, were found for cockles in Wellington Harbour and the Bay of Islands.

The eggs are fertilised in the sea and may float for a time at the sea surface before developing into a tiny swimming larvae, called veligers. The larvae swim, feeding on plankton, for about three weeks or until they reach a suitable habitat for settlement. The larvae are attracted to chemical signals put out by adults. Once the larvae settle on the substrate they can swim off again if conditions are not suitable. If the signals are good, they transform into a tiny cockles and dig in to the sand.

Once they undergo metamorphosis, they must survive in their chosen beach. During the first two years, however, they can move up and down the beach - often migrating together in large numbers.

Most of the young cockles don't survive long. Only a few of the millions spawned survive to become adults.

Predators

People are one of the main predators on cockles. The legal limit is 150 per day per person but many people take far more than their share.

The variable Oystercatcher (Haematopus unicolor) can gobble up 200 cockles a day.

Some gastropods, such as the Speckled Whelk (Cominella adspersa) and the Mud Whelk (Cominella glandiformis) eat cockles.

They are also killed by starfish, flounder and eagle rays and may be buried by shifting sediment or smothered by very fine silt or killed by pollution.

Other Cockles

There are several other cockles in New Zealand. The Purple Cockle (Venericardia purpurata) lives on open beaches and sometimes in the same locations as the common cockle. It is about 30-mm in shell length and has a purple or pink colour in its shell. The Northern Cockle, (Venericardia reinga) is much less common and is restricted to the far north of New Zealand. It has a flat edge on the upper end of the shell, while the common cockle's shell is evenly rounded.


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