rock oyster clinging to warm northern shores

Size
Shell: 5–8 cm, Weight: 20–40 g
Lifespan
10–20 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
Rocky shores and intertidal zones from the mid-tide mark down to the low tide mark. Attaches firmly to rocks using cement-like secretions. Prefers areas with moderate wave action and clear, clean water.
Range
Coastal waters of the North Island and northern South Island from Northland to Marlborough. Most common on rocky shores in northern New Zealand. Also found in Australia and the Pacific Islands.
Endemism
Native
Main Threats
Competition from the introduced Pacific oyster is the primary threat. Habitat loss from coastal development. Climate change affecting intertidal habitats and ocean acidification impacting shell formation.
Population
Populations have declined in many areas due to competition from the introduced Pacific oyster and habitat loss. The rock oyster is no longer common in many formerly productive areas. It is not commercially farmed in New Zealand and is rarely harvested due to its small size.
Conservation Status
Not Threatened
New Zealand's only native oyster is smaller and more delicate than the introduced Pacific oyster, with a rougher, more irregular shell. The shell is often covered with barnacles and other encrusting organisms, a testament to the oyster's longevity. An oyster that has been attached to the same rock for decades becomes a miniature reef, providing habitat for dozens of other species. It attaches firmly to rocks in the intertidal zone, often in dense beds that provide habitat for other small animals. The attachment is permanent. A young oyster settles on a rock, secretes a cement-like substance from its foot, and never moves again. It will spend its entire life on that rock, growing slowly, filtering water, and reproducing. Its numbers have declined since the introduction of the Pacific oyster, which outcompetes it for space and food. The Pacific oyster, introduced in the 1970s for aquaculture, spread rapidly through northern New Zealand. It now dominates the rocky shore, crowding out the native rock oyster. The native oyster cannot compete. It grows more slowly, reproduces less frequently, and is less tolerant of warm water. The Maori names Tio, Tio para and Tio-repe are also used for the Bluff oyster, reflecting the similarity between the two native oyster species. Tio is the general term for oyster. Tio para means mud oyster, referring to the Bluff oyster's habitat on muddy bottoms. Tio-repe may refer to the rough texture of the rock oyster's shell. The rock oyster is still found in some northern estuaries and rocky shores, but it is no longer common. Conservationists worry that it could be pushed to extinction by the introduced Pacific oyster. The two species are now hybridising in some areas, creating a mixed population that may eventually lose the genetic identity of the native species. The rock oyster is fighting for its place on the shore, and it is losing.