New Zealand Cockle

The A. stutchburyi shell has two valves, essentially two halves of a single shell, joined by an elastic material (the ligament), as is typical for bivalves. The hinge line, where the two valves meet, is rather wide. Each valve is 15 to 65 mm (0.59 to 2.56 in) long, but can occasionally exceed 90 mm (3.5 in). The shell's overall size can vary slightly throughout New Zealand, being slightly larger in the southern parts of the country. The shell's outer surface has a dirty white colour, at times with rust-coloured stains. The shell's interior is porcelain white and frequently stained purple towards one end of the shell. Occasionally, most of the interior is purple. The shell's interior presents circular scars impressed into the surface where the adductor muscles (which close the shell) attach to it. The outer surface has distinctive ribs running down and across the shell; those running down the shell become less prevalent towards the margin. After veliger larvae settle on substrate, they are at least 180 μm in length. The hinge joining the two larval shells has a projection on the right shell that opposes two projections on the left. There are also ridges on the margins of the left shell that slot into the right shell. Overall, the veliger larvae have an appearance typical of Veneridae larvae.

A. stutchburyi is endemic to (only found in) New Zealand and occurs throughout the two mainland islands, as well as Stewart Island (just south of the South Island), the Chatham Islands 800 km (500 mi) east of the mainland and the subantarctic Auckland Islands 465 km (289 mi) south of the mainland. It lives along the shoreline in sheltered areas with muddy sediment and fine sands, such as in estuaries and harbours. It typically occurs from upper mid-intertidal zones to just below the lowest tide level reached during spring tides. In these habitats, A. stutchburyi can be extremely abundant with population density varying between sites. At Akaroa Harbour in Canterbury, the species was estimated to have a population density of 1,200 per m2 at one study site and 500 per m2 at another nearby study site. A. stutchburyi are usually buried just below the sediment surface or protrude slightly. They rarely burrow more than 50–100 mm (2.0–3.9 in) below the surface. In palaeontological assessments, fossils of A. stutchburyi can be used to infer the historical presence of a brackish sheltered environment, although water currents can move shells away from these environments.

Population genetics. In one study of A. stutchburyi population genetics, the haplotypes (groups of genetic variants) of the cytochrome oxidase I (COI) gene from 29 estuaries were sequenced. …

The excretions of A. stutchburyi are a source of ammonium ions, which support primary production (the conversion of inorganic carbon to organic compounds) in the water by photosynthetic microbes when production is limited by nitrogen abundance. It has also been proposed that these photosynthetic microbes promote oxygen uptake by the sediment. There is a correlation between density of A. stutchburyi and seagrass (Zostera muelleri) density. It has been proposed that the two species benefit from shared space, the seagrass providing shelter to A. stutchburyi, thus reducing mortality, while A. stutchburyi releases ammonium ions, which supports growth of the seagrass. However, abundant populations of each can also occur without the other. Mats of seaweed sometimes overlap in habitat with A. stutchburyi. In laboratory conditions, mats of the red algae Gracilaria chilensis and green algae Ulva lowered the concentration of dissolved oxygen in the sediment by preventing microbes from photosynthesising. As a result, A. stutchburyi buried itself at a shallower depth. In experiments where the dissolved oxygen in the sediment was artificially lowered to mimic these conditions, A. stutchburyi tended to move to the surface, and died after about eleven days. This suggests that seaweed mats have a negative effect on populations of A. stutchburyi.

Diet. A. …

A. stutchburyi is a traditional food source for Māori people. Middens (old dump sites for domestic waste) dating back to pre-European contact often contained shells of A. stutchburyi. The shells in middens can be used to make inferences about historical Māori activity and changes in the species' populations. In the Bay of Islands, middens are rich in remains of A. stutchburyi, suggesting that in was a dietary staple in the area. The shells are also notably larger than in current day, indicating that the modern populations are being hindered by increased sedimentation and reduced resistance to parasites. In another study, the growth rates of A. stutchburyi from six midden sites around New Zealand were all found to have decreased from AD 1300 to present day. Like in the Bay of Islands, it was suggested that this was due to increased sedimentation. Since 1998, Māori often have customary fishing rights to harvest A. stutchburyi in some areas of cultural significance.