burrows deep in exposed beach sand
- Size
- Shell: 7–10 cm, Weight: 30–60 g
- Lifespan
- 10–15 years
- Diet
- Phytoplankton and organic particles. Filters food from the water using its gills. Draws water in through its siphon and extracts microscopic algae. Feeds continuously when submerged.
- Habitat
- Sandy bottoms on exposed beaches from the low tide mark down to 30 metres depth. Burrows deep into sand with only its siphon showing. Prefers areas with clean, stable sand and strong wave action.
- Range
- Coastal waters of the North and South Islands from Northland to Otago. Most common on exposed sandy beaches. Endemic to New Zealand. Not found in Australia or elsewhere.
- Endemism
- Endemic
- Main Threats
- Bycatch in trawl fisheries. Habitat loss from coastal development. Climate change affecting near-shore habitats. No targeted commercial fishery for this species in New Zealand. Sometimes collected by recreational gatherers for eating.
- Population
- Populations are considered stable across most of the species' range. The large trough shell is not commercially harvested in New Zealand. It is sometimes collected by recreational gatherers for eating, but its tough flesh is less desirable than that of other surf clams.
- Conservation Status
- Not Threatened
- Human Risk
- harmless
- Handling Note
- large surf clam, heavy shell requires care when handling
- Conservation Note
- Endemic mollusc; not assessed by NZTCS as marine invertebrates are outside the scope of current threat classifications.
- Te Ao Māori
- The large trough shell has no traditional Māori name, as it was likely not distinguished from other surf clams by early Māori. It would have been gathered occasionally for food, but its tough flesh and deep burrowing habit made it less desirable than other shellfish. Today, it is known to beachcombers and recreational gatherers, who value its large size and its heavy, solid shell.
It is not small. Named for its trough-like shape and its large size, this is the largest of New Zealand's trough shells, reaching up to 12 centimetres in length. The shell is thick and heavy, built to withstand the pounding of the surf. It lives on exposed surf beaches, burrowing deep into the sand where waves break. A large trough shell can be buried 30 centimetres down, hidden from view.
The shell is white or pale grey, sometimes tinged with brown. The surface is covered in fine concentric ridges, the growth rings that mark the clam's age. A 15-year-old trough shell has 15 rings, each one representing a year of survival in the harsh surf zone. The rings are closer together in winter, when food is scarce, and further apart in summer, when the clam grows faster.
Its strong, heavy shell is well adapted to survive the pounding of the surf. The weight of the shell helps anchor the clam in the sand, preventing the waves from washing it away. The shape is elongated and slightly curved, allowing the clam to burrow efficiently. The front end is rounded, the back end is pointed, and the hinge is strong.
It is often found washed up on beaches after storms. The waves tear it from its burrow, tumble it in the surf, and deposit it on the high tide line. There it lies, bleached by the sun, until a beachcomber picks it up. The shell is heavy in the hand, a solid reminder of the power of the sea.
Its tough, chewy flesh is less popular for eating than that of other surf clams, but it is still collected by some beachgoers. The meat must be tenderised before cooking, pounded with a mallet or soaked in a marinade. Even then, it is an acquired taste. The large trough shell is a clam for the patient, for those willing to work for their meal.