Orange army on the sea floor

by Kate C
Publish date
23 May 2012
Comments (2)

Every year, thousands of Giant Spider Crabs (Leptomithrax gaimardii) congregate in Port Phillip Bay ahead of their annual winter moult.

When solitary, these crabs are often hard to spot; algae, sponges and sea squirts set up shop on their shells and provide excellent camouflage. However when the crabs aggregate and march, this hungry army is easy to spot. They scavenge whatever food they can find, including the wildlife on the shells of one another. The spectacle of hundreds of large orange crabs against the bare, sandy sea floor is an amazing sight.

It’s still a bit of a mystery what the aggregations are all about but senior curator Dr Julian Finn has some ideas from several years of observation.

Like many crustaceans, Giant Spider Crabs are protected by their hard body shell, rather like a suit of armour. The trouble is that a hard shell doesn’t allow room for growth. Crabs must shed their old skin to get bigger; they can expand their size in the brief window before the new skin hardens. The process of moulting takes up to an hour and all the crabs in an aggregation moult almost simultaneously.

Spider crab emerging Spider crab emerging from its old shell. The new shell is a vivid orange colour.
Image: Julian Finn
Source: Museum Victoria

Newly-moulted Giant Spider Crab Newly-moulted Giant Spider Crab in its fresh orange shell.
Image: Julian Finn
Source: Museum Victoria

A soft, freshly-moulted crab is irresistible to predators such as rays, seals and birds. By aggregating in the thousands an individual crab reduces its chance of being eaten, much the same way as mammals in herds find protection in numbers. Movement into shallow waters may help the crabs, usually dispersed throughout Port Phillip Bay, aggregate in a single mass and gain refuge from the strong tidal currents that scour the deep channels.

An earlier explanation that the annual aggregations were related to mating has thus far proved unlikely, as following the moulting of tens of thousands of crabs, only the odd couple has been observed to mate. We still don’t know however what happens when they disperse back into deep water. Julian believes this sudden influx of tender crab meat is an important part of the Port Phillip Bay food chain.

Spider crab moults Hundreds of cast-off spider crab moults on the sandy seabed.
Image: Julian Finn
Source: Museum Victoria

If you'd like to see some Giant Spider Crabs without the need for SCUBA gear, have a look at the entrance of the Marine Life exhibition at Melbourne Museum.

Comments (2)

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Johnny 24 May, 2012 09:34
Outstanding photographs and video Julian! I'd like to see the Museum produce a prime time natural history program on the marine life of Port Phillip Bay. Keep up the excellent work!
George white 14 June, 2016 18:38
I would like to know the dates these crabs have moulted every year for as long as you have data for Thanks George
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