Specimen of the Week: Week 143
By Rowan J J Tinker, on 7 July 2014
For this week, it’s my turn to step up to the ravenous hoard of knowledge-hungry blog followers (that’s you fantastic lot). But first, before I am ripped apart in a gladiator-esque fashion, I’d like to take this opportunity to introduce myself; Hi all, I am Rowan. I am currently acting as Visitor Services Assistant on a temporary basis, so my time with you shall be unfortunately short yet sweet. So do drop in and you can see me at the front desk fumbling around in childlike wonder at all the amazingly weird thingies the Grant Museum has to offer.
I’ve decided to choose a specimen who will always hold a special place in my heart, having been paired with this sullen looking creature during one of my zoological assignments this year (I’ve just finished the second year of my UCL Natural Sciences degree). One of us was tasked to identify the other, yet I’m still unsure as to who (between me and this fine critter) actually did any effective identification as I spent most of my time confusedly prodding and pestering this specimen; a scientific method which I can only professionally describe as “faffing around”.
Sadly, this specimen is a little lonely having been blessed with an underwhelming greyish-brown and mistakenly ugly appearance. Unfortunately, being tucked away in a quiet corner along with the rather garish cephalopods, annelids and tapeworms (I’m sure they make wonderful neighbours) doesn’t quite help their romantic situation either.
Without further ado, this specimen of the week is….
**The giant chiton**
1) Chitons live in coastal shoreline regions, most spending a mixture of their time both submerged and exposed with the movement of the tide. These shy little molluscs spend their day hiding under rocks and within crevices; only deciding to emerge during the night, when they begin grazing on algae growing on the rock’s surface and munching down on smaller crustaceans and other molluscs.
2) These molluscs have a set of eight serially overlapping calcareous plates called valves which are surrounded by an oval shaped leathery lip-like structure known as the girdle. As molluscs, their underside is occupied by a large muscle called a foot; instead of strapping on boots and going for a stroll, however, this foot structure secretes a mucous layer onto the attaching surface with which the chiton uses as grip to move about. Together, all three of these structures help the chiton to carry out its favourite pastime; generally being grumpy and shy. By pressing its girdle to the surface and raising its foot, a chiton acts as a suction cup and strongly sticks to whatever may be lucky enough to lie under it (be it a rock, or someone’s face; a chiton does not discriminate). This method of refusing to budge acts as a defence against predators and as a buffer from strong tidal forces. Even when detached, a chiton will roll into a ball to protect its soft underside (we can assume they are not partial to tummy rubs and would probably make the least effective dog substitute).
3) My most favourite thing by far about chitons is their teeth. Their mouth opens out onto their underside and from it, a rasping ribbon-like tongue called the radula is used to scrape surfaces for food. The radula is lined with many teeth which are constantly replenished, rather like a conveyor belt. Since chitons seem to enjoy licking rocks and other hard substances, their teeth are biomineralised. Both iron compounds such as magnetite and other crazy molecules like calcium phosphate make up the teeth’s structure; a chiton’s teeth are harder than stainless steel!
4) Chitons are dioecous, meaning they have both male and female individuals (unlike many other molluscs, which are hermaphrodites). During the spring and summer, chiton colonies release huge clouds of gametes into the water which are externally fertilised. To avoid the waste of potential offspring, the mass release of gametes is synchronised by a series of chemical signals.
5) The girdle structure of chitons is highly variable; using their girdle in different ways, many species can achieve some especially amazing feats. In particular, the girdle of the veiled chiton (Placiphorella velata) is significantly longer at the front and resembles a tiny wedding veil. When an unsuspecting worm (or other tiny being) wanders within range of this deviant chiton’s grasp, this veil is slammed down and a slow and raspy death awaits.
Rowan Tinker is Visitor Services Assistant at the Grant Museum of Zoology, and a UCL Natural Sciences student