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[tim the toolman]

Vermetid Snail
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Vermetidae This group of snails have forgone their mobility by attaching their "shell" to the substrate and converting it into a calcerous tube. They release a mucus web out into the water, which they then reel back in and consume, eating the food particles caught in it. Their presence is a sign of a healthy system and don't typically cause any problems. The exception is if the mucus net irritates a neighbouring coral, which can lead to death of the polyps that it is irritating.
Fromhttp://www.masa.asn.au/masawiki/index.php/Hitchhikers_Guide_to_the_Reef_Tank