armoured crab, lost before it was named

Size
Length: 15–20 cm
Lifespan
5–10 years
Diet
Omnivorous – scavenged mudflats, estuaries, and sandy shallows. A sentinel crab built for the margins – broad-bodied, long-legged, with eyes on stalks that could see in every direction at once. Scavenged the tide line, hunted small invertebrates, and aerated the mud with its constant digging.
Habitat
Mudflats, estuaries, and sandy shallows of the Hauraki Gulf and Manawatū coast. A sentinel crab built for the margins – broad-bodied, long-legged, with eyes on stalks that could see in every direction at once. Scavenged the tide line, hunted small invertebrates, and aerated the mud with its constant digging.
Range
Found on mudflats, estuaries, and sandy shallows of the Hauraki Gulf and Manawatū coast. Described from subfossil remains found in coastal deposits and early naturalist accounts. Last reliably recorded in the 1920s.
Endemism
Endemic
Main Threats
Coastal development was the primary threat. Also threatened by pollution, harbour dredging, and predation by introduced fish and rats. Last reliably recorded in the 1920s. A few preserved specimens remain in museum collections – their carapaces faded, their legs brittle, their estuaries turned to concrete and lawn.
Population
A true giant among sentinel crabs. Estimated carapace width 6–8 centimetres (the largest living New Zealand sentinel crab, Macrophthalmus hirtipes, reaches 3–4 centimetres). Leg span perhaps 15–20 centimetres – a crab the size of a small saucer. Last reliably recorded in the 1920s.
Conservation Status
Extinct
A crab the size of your palm, with eyes on stalks that swivel independently, watching you from two directions at once. A crab that lives in the mud, digging deep burrows, emerging at low tide to scavenge and hunt. That was the giant coastal crab, and it was the phantom of the estuary. Its eyes made it special. Sentinel crabs are named for their long eyestalks – they look like tiny periscopes, scanning the mudflat for predators and prey. The giant coastal crab had eyestalks longer than any other New Zealand crab, giving it a 360-degree view of its world. It could see a bird approaching from above, a fish from the side, a rat from the shore – all at the same time. It was the watchman of the mudflat. It burrowed. Deeply. Sentinel crabs dig spiral burrows that can reach half a metre into the mud. They retreat into these burrows at high tide, plugging the entrance with mud, waiting for the water to recede. When the tide falls, they emerge to feed – picking through the mud for organic matter, scavenging dead fish, hunting small worms and molluscs. Their constant digging aerates the mud, bringing oxygen to the deeper layers and making the estuary healthier for everything else. Crabs are not marathon breeders, but they invest in each clutch of eggs. The female carried her eggs under her abdomen, fanning them with her legs to keep them oxygenated. The larvae drifted in the plankton for weeks before settling back into the mud. That strategy works when the estuary is clean and the predators are balanced. It fails when the mud turns toxic and the fish turn hungry. Coastal development and introduced predators destroyed it. When Europeans arrived, they drained estuaries, filled mudflats, and built ports, seawalls, and suburbs. The giant crab needed extensive, undisturbed intertidal zones with soft mud and clean water. Take away the mudflats, take away the crab. At the same time, introduced fish – especially brown trout and perch – moved into the estuaries. They ate crab larvae by the million. Rats patrolled the shore at low tide, flipping rocks, digging into burrows, eating adult crabs. A crab with long eyestalks can see a rat coming, but it cannot outrun one in a burrow. The smaller sentinel crabs survived. They are faster, more cryptic, better at hiding in shallow burrows. Some still scuttle on our remaining mudflats. But the giant is extinct. A few specimens in a museum jar, a few fragments in coastal shell deposits, and the memory of a crab that used to watch the tide from its periscope eyes. We filled its world. Then we wondered why it left.