During your strolls in the sands of the Illawarra’s beaches, have you ever come across a small butterfly shell? Two chitinous wings flaring, the colours and striations giving the illusion of a moth fallen in flight to rest at the water's edge. The coloured scales of the wings appear somehow untouched by the wash of salt water.
These winged shells are actually one section of the eight-part shell of a chiton.
Most sea snails (molluscs) have only one or two parts to their outer shell. Think a whelk (single) or a mussel shell (two-parts). Chiton (pronounced "kai·tn") have eight shell plates that fit together and are held in place by a muscular girdle or band that runs around the outside of the animal.
Although there are species differences in the shape of the shell plates, in general the head plate is crescent shaped, the six internal pieces are winged and resemble butterfly wings, and the shape of the anal plate is like a rising sun and reminds me of the ANZAC emblem. The individual plates can be beautifully coloured and shaped and when put together they form a functional protective shell.
The combination of the girdle and the shape of the shell plates allows the animal to articulate its protective armour to manoeuvre over uneven rock surfaces and evade predators. Much like limpets, chitons stick to the hard surfaces like rocks and can be very difficult to dislodge. If they are picked off, chitons can roll into a ball to protect their soft inner body. They remind me of a sessile, legless version of the grey slater bugs that scurry away after you dislodge the stone roof from their soil homes.
Although legless, they do have a foot, like all snails. That muscular foot provides grip and locomotion and contains a mouth and anus. The mouth features a hard radula, which is a modified tongue that they use to scrape algae from the rock surface.
Nearly all species of chitons are grazing herbivores. The girdle holds two primitive eye spots that detect light and dark. It’s been suggested that chitons have homing abilities that allow them to move about to forage and graze at night, then return to their ‘home base’ in the light of day.
Chitons live all over the world and although most live in the intertidal zone, some species occupy deeper ocean zones up to 6000 metres. Out of the roughly 1000 species found worldwide, Australia claims 150 of those. More impressively, 90% of those Australian species are endemic, found nowhere else.
Chitons belong in the class Polyplacophora, which comes from Greek origins meaning "bearing many tablets/plates". Their unusual shell structure has yielded some very literal common names as well: sea cradles, coat-of-mail shells and, my personal favourite, suck-rocks.
Chitons breed externally, meaning that the males release sperm and the females release eggs into the water column simultaneously so fertilisation can occur.
After the chiton dies, the musculature that holds the plates in place breaks down and the eight plates come apart where they wash up on the beach. And that’s how we find them: as deconstructed remnants of a mollusc reminiscent of a Lepidopteran.
