DIVELOG JUNE 406

SPIDER CRAB ANNUAL MIGRATION TO RYE PIER T he annual migration of spider crabs to Rye Pier in Australia is a mesmerising natural spectacle that draws in both researchers and tourists alike. :: images and text by DAVE BAXTER

This phenomenon, characterised by thousands of crabs moulting their exoskeletons, offers a unique glimpse into the life cycle and behaviour of these fascinating creatures. Understanding the significance of this event requires a deeper dive into the biology and ecology of spider crabs, as well as the environmental factors that influence their behaviour. Spider crabs, scientifically known as ‘ Leptomithrax gaimardii ’, are a species of marine crab found in the southern oceans of Australia, New Zealand, and South America. They are known for their distinctive appearance, with long, spindly legs and a rounded carapace. These crabs typically inhabit rocky reefs and seagrass beds, where they feed on algae, small invertebrates, and detritus. One of the most remarkable aspects of spider crabs is their moulting process, which is crucial for their growth and development.

Like all arthropods, crabs have a hard exoskeleton that provides support and protection. However, as they grow, their exoskeleton becomes too small and must be shed in a process called ecdysis. Moulting is a complex process that requires careful preparation and coordination of physiological processes. The moulting process begins with the absorption of calcium carbonate from the old exoskeleton, which weakens and softens it. The crab then secretes enzymes that dissolve the inner layers of the exoskeleton, allowing it to break open and shed the old shell. Once free of the old exoskeleton, the crab rapidly absorbs water to expand its body, which has grown during the intermoult period. The new exoskeleton is initially soft but hardens over the course of a few days, during which time the crab is vulnerable to predation. The timing of moulting is influenced by a variety of

92

DIVE LOG Australasia #406 - June ‘24

www.divelog.net.au