Common Spider Crab

Photo: Frank Ashwood · CC BY

Large spider crabs aggregate in The Gully in late summer and autumn when they moult in groups, a behaviour that reduces predation risk by overwhelming potential predators with numbers. Aggregations of dozens can cover the gully floor in August and September, their spiny carapaces camouflaged with attached algae and sponge fragments. Outside aggregation season, individuals forage singly on the gully walls, picking off hydroids and soft invertebrates.