On the face of it, Agaronia propatula is an unobtrusive sea snail. It’s smooth, brown and lozenge-shaped, and not even that big, maxing out at 6cm in length. Most are, however, a lot smaller. If you found one on the beach, you probably wouldn’t even trouble yourself to bend down and pick it up.
But don’t judge a snail by its shell. This particular one is full of surprises. Indeed, Agaronia is possibly about as exciting as a snail can get.
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It can be found on the sandy, wave-washed coast of the warm Pacific seaboard of America. Sandy shores might be good for a paddle but, for the life forms that call it home, life isn’t a beach (well, it is but… you know what I’m saying).
It’s tough, a constantly shifting, abrasive place with little in the way of shelter or places to hide. Anything that resides here is a specialist and has overcome the odds in some unusual ways. Agaronia is no exception.
That bullet-shaped shell is glossy, which helps it move through the wet sand with little resistance. But the shell is not the half of it.
Gastropod molluscs (such as garden snails) move by way of a muscular organ called the metapodium, or ‘foot’. This secretes mucus, over which the animal glides using bands of muscular contractions.
Agaronia does this, too, but is much faster. Its foot is huge and, when it is on the move, it closely resembles a lubricious magic carpet, with the more familiar shell riding plumb centre.
How does the surfing sea snail move?
This snail can ‘gallop’ – not a word you’d usually associate with snails. It does this by alternately and rapidly stretching the front and rear of the foot.
It is quite a sight, as the galloping gastropod puts on a last-minute sprint to close in on its fast-fleeing prey.
We’ve still not exhausted Agaronia’s locomotory repertoire. When it finds itself in deep water, it goes in for a bit of surfing or, to be more accurate, ‘swash-surfing’.
The swash is the water that runs up the beach after a wave has broken, and Agaronia utilises this phenomenon to get itself to where it needs to be – on the water line, where its main prey aggregate.
By expanding its foot and holding it rigid, it flips upside down. The foot can then catch the wave, and the snail’s shell acts as both ballast and keel.
Watching them zing along in the shallows, it’s hard to think of this as deliberate behaviour – it’s just not what you expect from a mollusc. When Agaronia gets to the place where the water kisses the beach, and where all the other snails are hanging out, it flips back over and resumes more orthodox means of locomotion.
How do surfing sea snails hunt their prey?
Agaronia is a short-range hunter, only able to detect prey from a few centimetres away, so watching one is like witnessing high-stakes bumper cars. It lacks the usual mollusc sensory front-end accessories, such as eyes or tentacles.
These are replaced by a battery of chemosensors and mechanosensors that are highly sensitive to smell and movement.
When it randomly bumps into prey or follows a mucous trail left by another mollusc, Agaronia sets off in pursuit.
But it’s not quite like a cheetah running down an impala because, when it closes in, its propodium (the front part of its body) rapidly rises and it can almost leap, coming down on and quickly smothering its unfortunate prey with a blanket of flesh.
The meal is then scooped backwards and subsumed by the metapodium, which closes around it like a drawstring bag.
Once dinner is secured, Agaronia burrows beneath the sand, taking its catch with it to liquify and digest. So efficient is Agaronia that, in these sandy, eulittoral habitats, it is considered the dominant predator – albeit an unlikely one.
