Tuesday, 1 August 2006

Multicoloured mangrove molluscs

Littoraria is a genus of snails living in mangrove forests around the tropical Indo-Pacific. If you're searching for them (and they're worth the hunt), look up rather than down. Unlike many other mangrove molluscs, species of Littoraria spend most of their time among foliage. Mud is not their thing.

Littoraria comes in a variety of shell colours from yellow to brick red to mottled, as in this specimen from near Yeppoon in mid-east Queensland. These colour forms were once thought to belong to a single, highly variable species—Littoraria scabra. If they were all different morphs of one species, what caused such diversity?

David Reid studied the snails on Magnetic Island in North Queensland. It soon became obvious to him that they did not all belong to Littoraria scabra. In fact, at least six closely-related species occurred together in the mangroves, all with similar shells but different anatomy.

So was colour a clue to identity? No. Each of those species (including the real Littoraria scabra) exhibited the same range of variation. Every one had yellow to brick red to mottled individuals. What was maintaining these parallel patterns of coloration?

Evidence suggests that selection by predators is keeping the diversity going among these species. The shells are camouflaged against the different backgrounds, especially leaves and bark. But it's not that simple. Shell strength also varies between different colour morphs. Darker shells are stronger than paler ones. Is this a mechanism for resisting predation by crabs?

The Littoraria story has a tangled plot.

[Thanks to Dark Orange for the snail photo. And the mangrove photo is of a beach near Shute Harbour not on Magnetic Island. But you get the idea.]

Read more

Reid, D.G. (1986). The littorinid molluscs of mangrove forests in the Indo-Pacific region. British Museum (Natural History).

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