deepwater clam dredged from offshore sandy plains
- Size
- Shell: 10–15 cm, Weight: 500–1000 g
- Lifespan
- 100–150 years
- Diet
- Phytoplankton and organic particles. Filters food from the water using its gills. Draws water in through its long siphon and extracts microscopic algae. Feeds continuously when submerged.
- Habitat
- Sandy and muddy bottoms in coastal waters from 10 to 100 metres depth. Burrows deep into sediment with only its long siphon extending above the surface. Prefers areas with clean, stable sand.
- Range
- Deep waters around the North and South Islands from Northland to Otago. Most common on sandy bottoms of the continental shelf. Endemic to New Zealand. Not found in Australia or elsewhere.
- Endemism
- Endemic
- Main Threats
- Bycatch in bottom trawl fisheries is the primary threat. Habitat damage from dredging and bottom trawling. Extremely slow growth and late maturity make populations highly vulnerable to disturbance.
- Population
- Population trends are poorly understood due to the species' deep-water habitat and extremely long lifespan. Caught as bycatch in trawl fisheries for flatfish. Its extreme longevity means any population decline would take many decades to reverse.
- Conservation Status
- Not Threatened
New Zealand's equivalent of the famous geoduck of the Pacific Northwest has a long, fleshy siphon that can extend up to half a metre from its buried shell. This remarkable organ draws in water, filtering out microscopic algae for food. The siphon is so long because the clam lives deep in the sand, sometimes half a metre below the surface. It must reach up to the water above to feed. The siphon is also the clam's only vulnerability. It is soft and edible, and predators like starfish and crabs will attack it if they can reach it.
This species is one of the longest-living animals in New Zealand waters, capable of surviving for more than a century. Scientists can determine its age by counting the growth rings on its shell, much like counting tree rings. A deepwater clam that is 150 years old was born when New Zealand was still a British colony. It has lived through two world wars, the invention of the aeroplane, and the digital revolution, all while sitting motionless on the seafloor, filtering water and growing slowly.
The deepwater clam lives buried deep in the sand of the continental shelf, rarely seen by humans. It is not targeted by any commercial fishery, but it is caught accidentally as bycatch in bottom trawl fisheries for flatfish and other bottom-dwelling species. A trawl net dragged across the seafloor will rip the clams from their burrows, crushing their fragile shells. Because they grow so slowly and reproduce so late in life, a single trawl can wipe out clams that have been growing for a century.
Its extreme longevity makes it highly vulnerable to disturbance. A population that takes a hundred years to reach maturity cannot recover quickly from fishing pressure. Scientists know very little about this species. Its deep-water habitat makes it difficult to study. No formal stock assessment exists. We do not know how many deepwater clams live on the continental shelf, or how fast they are declining. What we do know is that they are long-lived, slow-growing, and vulnerable. That is usually a recipe for conservation concern.