Saturday, 24 March 2012

The Snail that eats Crabs.

Nactid Gastropods (Moonsnails) are predatory snails with a long fossil record. They typically predate smaller Gastropods and Bivalves, drilling through their prey's shells, then injecting digestive juices and sucking out the dissolved soft tissues if their victims. This leaves a distinctive boring, which is often used by palaeontologists to identify shells that have been attacked by Nactids.

A recent edition of the journal Molluscan Research contains a paper by Thomas Huelsken of the School of Biological Sciences at the the University of Queensland, describing how one species of Moonsnail, Conuber sordidus, native to Queensland, Australia, regularly attacks and eats Soldier Crabs, Mictyris longicarpus and Mictyris platycheles.

The snails attacked the crabs by fastening onto the back of their shells, the quickly enveloping and immobilizing them with they muscular feet. Once this was done the snails secreted mucus to cover the crab, then dragged it beneath the surface of the sediment to drill through its shell at leisure.

An attack on a Soldier Crab by a Moonsnail. (A) The snail seizes the crab from behind. (B) It envelopes the crab with its muscular foot, immobilizing it. (C) It secretes mucus, covering he crab, and making it harder for the crab to escape. (D) The crab is drawn beneath the sediment to be finished off at leisure. Huelsken (2012). A video of this feeding behavior has also been placed online.

The crabs, which feed when the tide is out, usually hide beneath the sediment when the tide is in. The were apparently induced to flee by the snails passing over them, when they were captured by the snails. Larger crabs were sometimes able to fight off the snails, but crabs 60% of the size of the snails or smaller were almost inevitably consumed. The snails were also seen to attack hermit crabs, though as they eat other snails it was unclear if they were deliberately targeting these.

Hermit Crab being consumed by a Moonsnail. Huelsken (2012).

Huelsken theorizes that this behavior may not be restricted to this species of Moonsnail, but may be widespread within the group. He also suggests the behavior may be precent in the fossil record. Crustacean shells with bore-holes are not uncommon fossil finds, but these are generally assumed to be the result of octopus attacks, as palaeontologists do not tend assume snails are capable of this sort of attack. Huelsken suggests that a review of drilled crustacean fossils in university and museum collections might reveal the presence of Nactid drillings previously identified as octopus drillings.

The Pleistocene Moonsnail Polonices heros (left), with clams showing signs of drilling attributed to a Nactid Gastropod from the same deposits. Maine Geological Survey.

Modern crab showing octopus drilling (top). Drilling is bottom left near the three large barnacles. Close up of drilling (bottom). Walla Walla University.

Soldier Crab remains with Nactid drilling. Scale bar is 5 mm. Huelsken (2012).

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