Speedy snail is surprise crab killer

By Judy Leung 17 August 2011
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Snails are known for their slow movement, but the Australian moon snail is quick to attack soldier crabs.

VIDEO: A moon snail attacks a soldier crab

IN THE ANIMAL KINGDOM, bold predators have been known to bite off more than they can chew, and for one slow-moving native snail, this means attacking dangerous crabs.

Scientists have discovered the Australian native moon snail, (Conuber sordidus) – so-called because of its polished shell – can rise out of the sand to attack soldier crabs, which are typically bigger than itself.

The discovery changes what was understood about the predatory behaviour of moon snails, and more broadly, it may influence the conservation of species within the sandy ecosystem, says lead researcher Dr Thomas Huelsken from the University of Queensland.

Moon snail a vicious predator?

The discovery came about by chance. In April 2010, while out collecting snails for another project at Amity Point on North Stradbroke Island, Thomas observed a moon snail holding a much larger soldier crab. It piqued his curiosity into testing whether or not his observation was the snail’s natural behaviour or a freak occurrence.

Moon snails were thought to survive mainly by drilling into the shells of other molluscs and eating them alive through the hole, leaving clear evidence of their attack on the remaining shells, says Thomas. In their experiment, scientists examined the success of the moon snail hunting tactics in the laboratory and the field, where the size of the prey, number and timings of the attacks was observed.

The findings supported earlier studies that showed some marine snails were adaptable – they were able to switch prey to non-mollusc species when usual food sources weren’t available.

Marine snail ecosystem a complicated puzzle

Dr Winston Ponder, a malacologist (mollusc expert) at the Australian Museum, Sydney, believes the findings are significant because virtually all of the considerable literature on moon snails has assumed they are specialist predators on molluscs. Winston says there is only one previous and unconfirmed record of the moon snail eating crabs.

The results are also important for palaeontologists, who will also need to take into account the fact that ancient marine snails may have had an alternative source of food, says Winston. Ancient moon snails were probably eating crabs too, but have not left a fossil record for that part of their diet, Thomas says. This means the fossil record of moon snail predatory behaviour may not be as comprehensive as previously believed.

Thomas says his findings, recently published in the journal Molluscan Research, are only a small piece in the complicated puzzle of structures in this ecosystem. Thomas plans to expand his research to find out whether moon snails have a broader diet than expected – and if this behaviour is restricted to particular ecosystems or availability of prey.