Similar slimy fishing forms mix up worm, snail identity

How do you tell a tube worm from a worm snail? Got a few hours?

That’s how long I spent reading last week after I wondered about these different marine invertebrates while sorting my underwater photos. After I read three invertebrate zoology textbooks, checked out a dozen Internet sites and looked through two underwater guides, I finally found a sentence regarding my question: "Vermetid (worm snail) tubes can be distinguished by their shiny white lining from tube worms, which have a dull lining."

This made me laugh because I cannot see the insides of the tubes either in my pictures or on the reef. But I can see other parts of these unusual creatures, and after all that reading, the differences became clear.

A worm snail starts out life like most other snails, first as a drifting larva and later as a juvenile inside a tiny spiral shell. After settling down, a young worm snail glues its coiled shell to a rock, coral head or another shell. As the snail grows, the coils of its shell separate and spread out, often in a wavy tube or in spirals like a corkscrew.

Some worm snails settle together in a clump, their shells growing over and under one another in irregular shapes and directions like a mass of hard, hollow noodles. An unfurled shell gives the snail more surface area for attaching to its spot than if it remained in whorls.

The tubes of marine worms, however, nearly always remain hidden.

A worm snail feeds by casting from its opening a sticky mucus net that snares passing plankton. Periodically, the snail hauls in its net, eats the catch and sends out a new net.

It’s this fishing method that had me puzzled because some tube worms that build their tube homes inside coral heads, cracks and reef holes also make mucus nets to trap food.

My other photo is such a worm, casting out its net from its tube inside a living coral head.

Rather than discourage me, my long search for facts on tube worms and worm snails made me love marine biology more than ever. Where else could mucus be so much fun?

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Reach Susan Scott at www.susanscott.net.