flat disc hiding under every stone

Size
Length: 1–2 cm
Lifespan
2–3 years
Diet
Plankton.
Habitat
Under rocks in intertidal and shallow subtidal zones.
Range
Throughout New Zealand on rocky shores.
Endemism
Native
Main Threats
Habitat disturbance and pollution.
Population
Common in rocky areas throughout New Zealand.
Conservation Status
Not Threatened
Human Risk
harmless
Handling Note
fragile limbs detach easily when handled, causing stress to animal
Conservation Note
Native porcelain crab common on rocky shores throughout New Zealand, not subject to formal NZTCS threat classification.
Te Ao Māori
Half crabs have no specific Māori name. They are often grouped with other small crustaceans. Their flat shape is distinctive. Like a coin. Like a button. Hidden under the stone. A secret of the shore. Waiting to be found. Or left alone.
Its body is flat. Disc-like. It looks like half a crab. Hence the name. The Half Crab is actually a porcelain crab. Petrolisthes elongatus. It hides under rocks. It filters water. It is fragile. Its legs break easily. This is a defence. Autotomy. Lose a leg. Save a life. Regrow later. It is a calculated loss. A strategic sacrifice. It works. Range covers both main islands. It is found in rocky intertidal zones. Sheltered bays. Harbours. It is common. But hidden. You must lift the rock. Look under. See the white disc. See the waving arms. It is feeding. Filtering. Living. Threats include habitat disturbance. Humans turn over rocks. Expose the crab. Birds see it. Eat it. Pollution harms water quality. It needs clean water. Plankton. Currents. Without these, it starves. It suffocates. It dies. Habitat is the underside of stones. Dark. Damp. Cool. It clings tightly. It does not move far. It is sedentary. It waits. It filters. It survives. It is a master of stillness. Of patience. Of hiding. Diet is plankton. Microscopic food. It uses feathery legs to catch it. Sweep. Retract. Eat. Repeat. It is a gentle feeder. Non-aggressive. Passive. It takes what the tide gives. It asks for nothing more. Life span is two to three years. Growth is slow. Moulting is dangerous. Soft shell means risk. It hides until hard. Then it returns to the filter. To the rock. To the dark.