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Cast-Iron Foot: Undersea snail has mineral armor

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10:36am, November 5, 2003
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An as-yet-unnamed species of snail living around hydrothermal vents deep beneath the Indian Ocean bears an unusual suit of armor forged from the dissolved minerals spewing into its seafloor habitat.

The sides of the snail's foot are covered with scales that range up to 8 millimeters in length and overlap like roof tiles, says Anders Warén, a marine biologist at the Swedish Museum of Natural History in Stockholm. The core of those structures is made of a protein called conchiolin, a common component of many mollusk shells. What makes these flaps unique is their 100-micrometer-thick coating of iron sulfide, a biological armor that's made of mineral particles just 1 m in diameter.

Bacteria living on the surfaces of the scales may contribute to the formation of the mineral particles there. However, because the tiny iron sulfide spheres also show up throughout the conchiolin core of each scale, the snail itself probably controls the overall growth and placement of the particles, says Warén.

As snails are wont to do, these sulfide-armored creatures live sedentary lives. This species doesn't even bother to eat. Instead, the animals gain energy from symbiotic bacteria that live within the cells of a gland in their esophagus, says Warén. Most mollusks have such tissue, but in this armored species, the gland is about 100 times the size of that found in related species. The bacteria harbored in the gland oxidize dissolved sulfides that are absorbed through the snail's gills, says Shana K. Goffredi, a marine biologist at the Monterey Bay Aquarium Research Institute in Moss Landing, Calif. She, Warén, and their colleagues describe the armored snail in the Nov. 7 Science.

The species is one of several creatures that make up the unique ecosystems surrounding hydrothermal vents in the western reaches of the Indian Ocean (SN: 9/15/01, p. 165: Available to subscribers at Scientists spy sixth undersea-vent ecology). Although the researchers aren't sure about the function of the snail's sulfide armor, it may provide protection from predatory snails. Those killers inject their prey with poison, but their barbs aren't long enough or tough enough to penetrate the mineral sheaths on the newfound snail species, says Warén.

David R. Lindberg of the University of California, Berkeley rates the snail as "really interesting," but he doesn't buy the idea that the mineral coating serves only as armor. There have to be other advantages to the sulfide-coated scales, he says, because other species of snails–ones that are just as common and presumably just as meaty and delicious as their armored kin–thrive around the vents despite the presence of predatory snails.

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