Cockles of the Inlet
What is the cockle?
Just below the surface of the sediment in the Pāuatahanui Inlet lives the New Zealand cockle. It is the most abundant shore animal in the Inlet.
Known to Māori as tuangi and to science as Austrovenus stutchburyi, the NZ cockle is a shallow-burrowing suspension feeder found in soft mud to fine sand on sheltered beaches around the coasts of New Zealand. It was scientifically described by the English surgeon and naturalist William Wood in 1828. He named it after the geologist Samuel Stutchbury, who collected cockles and other sea shells while he was the naturalist to the Pacific Islands from 1825 to 1827. The cockle is a dominant member of the fauna of estuaries and harbours right around New Zealand. Our last triennial survey, in 2019, estimated the Pāuatahanui Inlet population to be between 350 and 400 million individuals. It is estimated that around 80% of the total intertidal biomass of the Inlet is due to the cockles.
The adult cockle has a rounded heart-shaped bivalved shell (when views side-on) with strong radial ribs and concentric grooves. The cockle’s shape and external structure anchor the animal in the sediment so it can’t be scoured out by waves and currents. The cockle’s large mobile foot allows it to burrow under the surface, although its rounded shape and short siphons mean it burrows only deep enough to cover the shell.
Species: A. stutchburyi
The New Zealand Cockle is technically not a true cockle but instead belongs to a group of bivalve molluscs known as ‘venus clams’.
Veneridae, or venus clams, is a large family of marine, mostly edible, bivalve shellfish with almost 500 living species.
‘The characters used for classifying this group still tend to be superficial, focusing on external features, especially those of the shell. Venerid clams are characterized as bivalves with an external posterior ligament, usually a well demarcated anterior area known as the lunule, and three interlocking structures (called cardinal teeth) in the top of each valve; several of the subfamilies also have anterior lateral teeth, anterior to the cardinal teeth: one in the left valve, and two (sometimes obscure) in the right valve. The inner lower peripheries of the valves can be finely toothed or smooth.‘ (From Wikipedia).
How do cockles feed?
Cockles live throughout the subtidal and intertidal areas of the Inlet but are most abundant between the shallow subtidal (permanently covered by about 1 metre of water) and the middle of the intertidal areas. They are filter feeders, drawing water through their gills to get the food and oxygen they need. Those in the intertidal area must be submerged and, therefore, able to feed for at least three hours per day (1½ hours per tidal cycle).
The filter-feeding habit of cockles makes them essential to the water quality of the Inlet since they remove plankton and minute particles of organic debris and help to keep the water clear. It has been calculated that up to one-third of the volume of each incoming tides passes. population-wide, through their gills.
The cockle is known as a suspension filter feeder. When the tide covers the sand and the cockle is submerged, it opens its shell to expose two siphons, ‘inhalent’ and ‘exhalent’. When cockles are feeding, their siphons can be seen above the surface of the sediment. The foot is used to position the animal at the right depth below the surface of the sand.
Water is drawn water into its outer body cavity through its ‘inhalant’ siphon, straining the water through its gills to remove potential food particles suspended in the water. It then pumps the water out through its ‘exhalant’ siphon. The captured particles are moved down to the base of the gills and on towards the mouth by cilia (fine motile hairs). Large particles are removed by the ‘palps’ and redirected to the exhalant water stream. Small particles are directed into the mouth.
Life Cycle of the Cockle
A few cockles may live for 20 years but the average age at death is 5-8 years for those that survive the high juvenile mortality rate. Age is estimated by counting the annual growth rings in the shell. Age is closely correlated with size only for the first 2-3 years of growth, by which time the cockles have reached a length of 18-20 mm. The largest cockles now seen in the Inlet are 45-50 mm but records from Māori middens indicate that cockles grew much larger in the past.
Cockles spawn from October to December. The sexes are separate and maturity seems to be a function of size rather than age, spawning first at around 18mm. They release eggs and sperm into the water on an outgoing tide. The resulting larvae spend 3-6 weeks in the plankton before settling on the sediment. Since many of the newly spawned larvae are carried out to sea by the ebbing tide it is not known how many of those that settle in the Inlet are derived from its resident population and how many come from neighbouring populations in Cook Strait. Thus the number of juvenile cockles in the Inlet is not necessarily an indication of the reproductive output of the resident population.
After settling, juvenile (up to 10 mm in length and one year old) and pre-adult (10-20 mm) cockles move around the Inlet, either passively through tidal and wave action or actively by dragging themselves over the sediment surface. It is common for settlement to be much higher in some parts of the Inlet than others (the actual areas of high density vary from year to year) but redistribution of the young as they grow results in a more even, (although still patchy) distribution of young adults (18-25 mm) throughout the Inlet.
Juvenile mortality is high. Predation by carnivorous snails, birds and fish may take 70-90% of juveniles in the first three years post-settlement. Once this phase is over the mortality rate from natural causes other than senescence is very low.
Cockle as a source of food
The cockles’ shallow living arrangements make them vulnerable to predators. They are a major food source of fish and shore birds such as the oystercatcher, which feeds primarily on cockles and other shellfish. Small carnivorous snails are important predators of young juvenile cockles.
Cockles have long been a food source for humans as well. Middens around the Inlet show that the cockles were an important source of food for pre-European Māori. An archaeological dig on the south side of the Inlet in 2000 yielded some significant evidence about Māori resource use and the huge size of cockles that grew here in the past.