burrows in the exposed beach sand
- Size
- Shell: 5–7 cm, Weight: 15–30 g
- Lifespan
- 5–10 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 20 metres depth. Burrows 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. Also found in southern Australia and the Southwest Pacific.
- 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. Sometimes collected by recreational gatherers for eating.
- Population
- Populations are considered stable across most of the species' range. The trough shell is not commercially harvested in New Zealand. It is sometimes collected by recreational gatherers for eating, but its small size makes it less desirable than larger 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 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 small size and thin shell made it less valuable than larger shellfish. Today, it is known to beachcombers, who admire its delicate, translucent shell but rarely give it a second glance.
Waves crash. Named for its trough-like shape, this surf clam lives on exposed beaches, burrowing into the sand where waves break. Its shell is thin and fragile, easily broken by waves or predators. The shell is white or pale grey, sometimes with a purple tinge near the hinge. The surface is covered in fine concentric lines, the growth rings that mark the clam's age.
Several similar species of trough shell live in New Zealand waters, all with similar habits and appearance. Telling them apart requires a careful examination of the hinge teeth and the shape of the shell. To the casual beachcomber, they are all just trough shells, washed up on the high tide line after storms.
They are often found washed up on beaches after storms, where their delicate shells are collected by beachcombers. The shells are thin and translucent, almost like glass. They break easily, so a complete specimen is a lucky find. The shell is light in the hand, a fragile reminder of the clam that lived inside.
Their small size and thin shells make them less popular for eating than larger, thicker-shelled surf clams. The flesh is sweet but tiny, and the shell is so fragile that it often cracks during harvest. It would take dozens of trough shells to make a meal, and the effort of digging them from the surf is not worth the reward.
The trough shell is a common resident of surf beaches, but it is rarely noticed. It lives hidden in the sand, filtering water and feeding on plankton. When the tide goes out, it closes its shell and waits. When the tide returns, it extends its siphon and resumes feeding. It is a simple life, but it works. The trough shell has been living this way for millions of years, and it will probably continue for millions more.