red swimming crab hunting in the subtidal shallows

Size
Length: 5–8 cm, Weight: 30–80 g
Lifespan
3–5 years
Diet
Carnivorous. Feeds on small crustaceans, worms, and molluscs. Uses paddle-like back legs to swim. Burrows into sand to ambush prey. Hunts small fish and crustaceans. Feeds most actively during high tide. A fast and agile predator in shallow coastal waters.
Habitat
Sandy and muddy bottoms in shallow coastal waters, harbours, and estuaries from 5 to 50 metres depth. Burrows into sand during day with only eyes showing. Emerges at night to hunt. Prefers sheltered areas with soft substrate.
Range
Found in coastal waters of North and South Islands from Northland to Stewart Island. Most common in sandy bays and harbours. Also found in southern Australia and the Southwest Pacific. Distribution follows suitable sandy habitats.
Endemism
Native
Main Threats
Bycatch in set nets and bottom trawls is primary threat. Recreational collecting in some areas poses risk. Habitat loss from coastal development and dredging affects survival. Climate change impacts near-shore habitats. Often caught by recreational fishers from beaches and wharves.
Population
Populations considered stable across most of species range. Red swimming crab is a common sight on sandy beaches and in harbours. Not targeted commercially in New Zealand but sometimes collected by recreational fishers for bait. No formal stock assessment exists.
Conservation Status
Not Threatened
Paddle-shaped back legs set it apart from other crabs. These flattened appendages are perfect for swimming. They allow the red swimming crab to move through the water column with surprising speed. It is a smaller relative of the common paddle crab. It shares the same ability to swim and burrow. But it lives in slightly deeper water. Depth provides separation. Similarity is superficial. Function is shared. Smooth and shiny, its shell distinguishes it from the rougher, more textured shell of the paddle crab. Two similar species live in New Zealand waters. Nectocarcinus antarcticus and Nectocarcinus bennetti. They are difficult to tell apart without close examination. Both have the same reddish-brown colour. Both have the same paddle-shaped back legs. Both swim, burrow, and hunt in the same way. Distinction requires effort. Observation demands patience. The difference is subtle. The behaviour is identical. Under the sand, it hides during the day. The red swimming crab buries itself in the soft sediment. Only its eyes and antennae remain exposed. It watches the water above. It waits for prey to pass. A small fish, a worm, a crustacean. Anything that moves within reach is fair game. When the prey approaches, the crab explodes from the sand. It grabs the victim with its claws. It drags it down. Ambush is the strategy. Speed is the tool. Surprise is the advantage. High tide brings it out to hunt. The red swimming crab emerges from the sand as the water rises. It swims through the shallows in search of food. It uses its paddle legs to propel itself forward. Movement occurs with a grace that seems impossible for a crab. When it spots prey, it dives toward the bottom. It uses its claws to capture and crush. Efficiency is key. Waste is minimal. The hunt is brief. The meal is secured. Recreational fishers often catch it by accident. The red swimming crab is taken as bycatch in set nets and bottom trawls. It is sometimes collected from beaches and wharves. It is used as bait for snapper and other species. A few people eat it. It is small and yields little meat. The pinch is mild. Not worth worrying about. Value is low. Effort is high. The crab is overlooked. It is not prized. Moulting is regular and risky. The crab sheds its old exoskeleton. It inflates a new, soft one. It hides in the sand until the new shell hardens. The old shell is eaten or breaks down. The crab grows quickly. It reaches adult size within a year. It lives for only three to five years. This fast turnover helps populations recover from localised declines. Time is short. Growth is rapid. Survival is brief. Dredging and coastal development destroy its habitat. The red swimming crab needs sandy and muddy bottoms in sheltered bays and harbours. When these areas are dredged for navigation or filled for development, the crab loses its home. It is adaptable. But it cannot survive where the sand has been replaced by concrete or deep water. For now, it remains common. A small, swimming crab in the shallows. Going about its business in the space between tides. No one told it otherwise.