Austrovenus stutchburyi, commonly known as the New Zealand cockle or New Zealand little neck clam, is a marine bivalve in the family Veneridae that occurs only in New Zealand. Its Māori name is in the North Island or in the South Island. The shell is between in length, but may reach up to . The outside is dirty white; the inside is porcelain white and often has purple stains. A. stutchburyi is widespread throughout the coasts of New Zealand, including the Chatham Islands and subantarctic Auckland Islands. It is found in the intertidal-to-low-tide zone in muddy and fine sands. A. stutchburyi feeds via suspension feeding, where food particles are filtered out of the water. The species is often harvested for food, active commercial harvesting occurring in the Dunedin and Tasman regions. It is a traditional food source for Māori people, who have an archaeological history of harvesting A. stutchburyi and have customary fishing rights to harvest it in areas of cultural significance.
Spawning occurs in spring and summer; the larvae (juvenile stage) attach to a surface within twenty days. The rate at which A. stutchburyi grows depends heavily on environmental conditions such as salinity, food availability and temperature. A. stutchburyi may support primary production (the conversion of inorganic carbon to organic compounds) by releasing ammonium ions within ecosystems. Its predators include crabs, wading birds, large whelks (carnivorous shellfish) and stingrays. The clam is known to be infected by the larvae of the trematodes Curtuteria australis and Acanthoparyphium, which damage the ability of A. stutchburyi to burrow into sediment. A. stutchburyi can cause neurotoxic shellfish poisoning after exposure to red tides, which release toxins into the water that are subsequently absorbed by the clams.
Many populations of A. stutchburyi have been depleted by overharvesting or human-mediated changes to their habitat. Excessive amounts of sediment can also threaten the clam by smothering it. As such, there have been efforts to restore depleted populations by transplanting individuals from healthy ecosystems. The customary Māori principles of , reserves and , a series of resource-management practices incorporated into New Zealand law can be used to regulate the species' harvesting. Since A. stutchburyi accumulates heavy metals and other contaminants, it has been considered a potential bioindicator.
Taxonomy Austrovenus stutchburyi is commonly referred to as New Zealand cockle or New Zealand little neck clam. In the indigenous Māori language, it can be referred to as in the North Island or in the South Island, among other names. It was first described using the modern system of nomenclature as Venus stutchburii by English zoologist William Wood. The specific name refers to Samuel Stutchbury, who collected the species from the Bay of Islands in 1826. Since that 1828 description, it has been mistakenly recognised as a new species several times. It was transferred to the genus Chione in 1906, then briefly to the genus Austrovenus in 1926 before being transferred back to Chione in 1927. It was finally placed in Austrovenus again in 1977. The lectotype (a single specimen designated as the type specimen when none was specified) is stored in the Natural History Museum of London.
It is the only living species in Austrovenus; the two other species (Austrovenus northlandica and Austrovenus tamakiensis) are only known from fossils. In the New Zealand fossil record, A. stutchburyi is common in Cenozoic Era deposits. The earliest fossil record of the species is from the Opoitian stage 5.33 ma (million years ago) in the early Pliocene. Most fossil records fall between the Mangapanian stage, which began roughly 3 ma, and early Haweran Stage (beginning 0.34 ma).
Description 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 long, but can occasionally exceed . 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.
Distribution and habitat
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 east of the mainland and the subantarctic Auckland Islands 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 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 COI gene is used in population genetics studies because it has a predictable and informative rate of mutations that often provides clear indications of when populations have diverged. The study found that the genetic population is more subdivided than open-ocean fauna in the country and that there are six distinct subpopulations. Genetic composition changed significantly near Cook Strait, which separates the North and South Islands. The study suggested that genetic subdivision in A. stutchburyi may reflect limited dispersal in its estuarine habitat relative to open-ocean species. It also noted that A. stutchburyi could disperse long distances to some extent. Another possible explanation for the subdivision is due to historical dispersal barriers that no longer exist, such as glaciation or sea level changes. A third possibility is that some haplotypes are better suited to environmental factors that vary slightly over the country, such as temperature.
Ecology and behaviour
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. stutchburyi is a suspension feeder, meaning it filters organic particles from the water to feed on; inorganic particles (which are inedible) are expelled. In one laboratory study of A. stutchburyi, individuals were subjected to different concentrations of suspended sediment (which the species filters for food). After two days, individuals exposed to intermediate concentrations of suspended sediments had the highest water filtration rates. After fourteen days of exposure, the filtration rates were identical regardless of what concentration of suspended sediment the individuals were exposed to, indicating that the initial differences observed on day two were no longer present. This suggests that the feeding response of A. stutchburyi to increased suspended sediment concentrations (such as those caused by sediment runoff) varies over time.
Life history A. stutchburyi is gonochoristic, meaning it has separate sexes. It is inferred to undergo spawning during at least spring and summer. In a laboratory study, A. stutchburyi larvae developed into the veliger stage and settled by attaching to a solid substrate around 20 days after fertilisation. Based on this developmental period, the authors inferred that settlement (also referred to as spatfall) in the ocean most likely occurs between February and May. The juvenile can disperse from unsuitable habitat by crawling with its foot or simply allowing the current to roll it along the surface. The round, robust shape of the shell may assist it in passively rolling with water currents. It can control dispersal by either staying buried or emerging to the surface. It has been inferred that A. stutchburyi can live up to 20 years, but more research is needed to confirm this.
Physiology Shell growth rates vary with environmental conditions, leading to considerable differences in shell length at a given age. In one study, A. stutchburyi at in length could be less than two years old at one site, but over eight years old at another site. Some environmental conditions that influence growth rate include salinity (amount of salt dissolved in the water), food availability, temperature and shore level. As A. stutchburyi grows, growth bands can appear in the shell, forming a series of macro and micro growth bands, the macro rings seemingly forming once each year during winter months. Analysis of the shell growth patterns found that A. stutchburyi grows more during summer than winter, and that summer growth was greatest during spring tides and lowest during neap tides. Another study found that A. stutchburyi grew at a rate of per month; factors like salinity, concentration of chlorophyll-a (a proxy for photosynthetic microorganisms) and temperature during low tide were the main influences.
A. stutchburyi is able to tolerate a wide range of salinities. In one study, A. stutchburyi struggled to survive in low salinity (1.4%) water with low phytoplankton concentrations, but if phytoplankton concentrations were high, it was able to survive in salinity as low as 0.7%. In contrast, the habitat they were collected from typically had salinity ranging from 2.8% to 3.4%.