16 October 2008

Creeping with the tides: how do marine snails keep up?

To stay alive, small intertidal creatures need get with the tides. At low tide, a vast area is exposed for these animals to feed, find mates and do other stuff without worrying too much about underwater predators. But at high tide, they need to get to a safe place in good time.
Periwinkle snails (Family Littorinidae)
For a marine snail, this usually means getting above the waterline at high tide to avoid hungry fishes. Since snails are not the speediest of creatures, they need to get started in good time and get high enough. And tides don't rise to the same height every day, nor do they occur at the same time. And snails don't carry a watch or handphone.

So how do they do it?

Tiny Tide Travelers May Sense Gravity
Erin Espelie, LiveScience.com Yahoo News 15 Oct 08;
Life is no beach for tidal creatures that must migrate in sync with the waterline.

Imagine trying to gauge the tides that sweep through a Kenyan mangrove forest: how far the water rises up a given tree depends on the season, the phase of the moon, and the tree's position. Yet a pinkie-toe-size snail, Cerithidea decollata, seems to predict the height of the incoming tide. It ascends a trunk just high enough to escape inundation, then descends when it's safe to forage in the mud below.

To find out how, Marco Vannini of the University of Florence and colleagues observed the snails on plastic pipes - imitation mangrove trunks - that they stuck into the mud.

The scientists tried obscuring any chemical markers left behind by the tide line or the snails themselves, and still, the snails climbed to the right height. Nor do the predictive gastropods seem to be using visual cues from overhead foliage. They aren't even counting the "steps" they must creep to beat the tide: when the scientists tilted the pipes, the snails readily climbed the extra length.

When lead weights were glued to the snails' shells, however, they adjusted their ascents; the heavier the weight, the shorter the climb. So it seems that the snails' are sensitive to their own energy output. Perhaps, Vannini suggests, they actually perceive the variations in gravity that drive the tides: before a low tide, the snails feel heavier and therefore don't climb very high.

The research was detailed in the journal Animal Behaviour.


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