"If we and the rest of the backboned animals were to disappear overnight, the rest of the world would get on pretty well. But if the invertebrates were to disappear, the land's ecosystems would collapse."
David Attenborough

Sunday, December 12, 2010

Mollusc Giant: Gumboot Chiton- King of the slippery tide pool things.

Polyplacophorans make me feel uneasy, but in a good way. Whenever I find one of these armour-plated lumps stuck to a boulder in a rockpool I ask myself 'why don't I know anything about these?' Then I move on to watching the mussels feeding or annoying a whelk (which kind of answers the question- too many other distractions). I did finally get around to reading a little about them recently and I was pretty floored by what I found- it seem, against all the odds, polyplacophorans are amazing!
These beasts, better known as chitons, slide around most of our rocky coasts on their big molluscan foot, breathing with their long gills tucked into grooves in their underside scraping algae up with their remarkable radula (molluscan tongues). Why are their tongues remarkable? Well, the teeth lining them are tipped with magnetite- an iron-based material which is very tough and, importantly, magnetic. Magnetite has been found in various organs in bees and birds, creatures which need to navigate with precision and its speculated that the magnetite deposits they have help them detect the earths magnetic field and use it to help steer their journeys. Why would a little slow-moving mollusc need this biological GPS? Well, chitons have a habit of returning loyally to a the same spot to sit out their low tide exposure so actually finding their way home after a hard tides grazing is pretty vital to them actually. They have a compass for a tongue.
That is very cool, but its just one of many fascinatingly weird features of this group which I'd like to  explore in future Mollusc POW posts.
To get to the point of this post, another of the Mollusc Giant series, lets look at the biggest of all Polyplacophorans; the Gumboot Chiton Cryptochiton stelleri.
Its odd. It looks like the sole of a Wellieboot (AKA a 'gumboot' by you Aussies and Norte Americanos). Its armour plates (known as 'valves' in the trade) are covered in a funky leathery skin. They live in rocky intertidal zones in an arc around the northern Pacific from California to southern Japan. Its huge (for a chiton). It grows to over a foot long and can weight 2kg! They apparently neatly illustrate the difference between 'edible' and 'tasty' so they don't feature highly in human diets. In fact they have few predators- their main pursuer is Ocinebra lurida, a carnivorous marine gastropod which allegedly just nibbles its ample mantle. Being such a big thing it needs more grown-up food than algae so it occasionally makes holes in giant kelp and other seaweeds.
My only complaint about Gumboot Chitons is that the lack of quality photos of these monsters on the web doing chiton-y things. Oh well. Instead here is a good one of a handsome specimen being wrangled by a fearless rockpool-er.

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