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.
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.
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.
