Tuesday, September 25, 2012

Species species of the Week week # 6 OR Goodyear Mollusks

Previously on Species species of the Week week: The tautonomical diet of the Blue Dragon!  Up first on the Menu menu

Janthina janthina
Figure 1. The Violet or Purple Snail, a prey item of the non-tautonomical
Blue Dragon nudibranch. I propose the new common name: Baggins Snail

If we accept the premise that snails are like Hobbits - you know, pretty much homebodies (insert rimshot here for the best snail pun ever!), and not overly predisposed to doing anything too hastily - Janthina janthina (Figure 1) is the Bilbo Baggins of snails. J. janthina is not content to trail unambitiously along the ground, or leaves, or rocks, or mud, or whatever. This snail FLIES! And it does this by making its own blimp (Figure 2). True they do so underwater, but to a regular old humdrum sea snail confined haul their shells across the substrate, this must look like magic.

Figure 2. Witness: the Goodyear snail. J. janthina (Baggins Snail)
constructing its own dirigible. Credit where credit is due.
The snail uses mucus to trap air from the surface into bubbles, making a raft that keeps them afloat. The raft is driven around by wind and surface currents, and the snail goes on undirected, upside-down walkabouts on the surface of the ocean - so, blimp might not be as apt as hot-air balloon, really.

How did the snail learn to fly then? It turns out this is kind of a cool story (1). The bubble raft is an adapted form of the egg mass. Lots of snails lay their eggs in mucus-y masses. So the theory goes that some ancestor of this snail ended making egg masses with air trapped in them, which was good for the baby snails since they got dispersed, and ultimately the snails themselves started floating and now both sexes have the bubble-making-skillz, which are no longer directly related to making eggs. There are other adaptations that make them good at a sea-faring life, too (Figure 3). The shell is flat and thin(2) - which keeps it from sinking fast, and it's counter-shaded (dark on top, light on bottom) to make it harder to for predators to see.

Figure 3. Janthina janthina is well adapted to a flying lifestyle.  

Why is it good to be in the pleuston(3)? Because it's delicious! J. janthina float along waiting to bump into delicious tautonomical prey: Velella velella


And that topic will be covered ... Next time, on Species species of the Week week.

References and miscellany:

(2) Most land snails have super thin shells, because shells are heavy to carry on land. But marine snails often have thick shells, because it's easier to carry thick shells under water, and there's some nasty crabs out there that would like nothing better than escargot.

(3) Wait, what's the pleuston, again?  Well it's like the neuston, only for the big guys.

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