Tongues, teeth and plates

First of all, apologies for the extended break…somehow one week turned into two before I noticed it.  I am back now, so on with the rambling posts!

Last post on evolution was about Chitons, and I said that I would write about the mouth this time, and about some other very cool features of this group of animals.

The mouth is common to most molluscs, and is called a radula (from Latin, meaning scraper), and it looks like this in action:

The radula is the part popping in and out of the right hand end.  I think of it as a tongue with teeth on it. These images are of the radula, at various magnifications, taken with scanning electron microscopes, which are really really really cool, and I really want to use one someday soon!

Radula of a slug, magnified 600 times. Image from PS Micrographs

Radula of Pomacea canaliculata (a freshwater snail). Image from Brazilian Journal of Biology

So, how exactly does this toothed conveyor belt work?

Diagram showing the radula in action. Image from wikipedia

When a mollusc eats something, the radula extends forward out of its mouth (sort of like when we lick an ice-cream), and as it comes into contact with the food, the teeth hook the food and as they are pointing backwards, carry it into the mouth.  The teeth lie flat when the radula is inside the mouth, but the action of extending it pushes them into an upright position.  The end of the radula scrapes at the food, breaking it into chunks that are carried back into the mouth.

The really really cool thing about radulas is that, obviously, all this licking and scraping at stuff wears teeth down, so, in order to allow the mollusc to keep feeding throughout its life, new teeth are continuously grown at the back of the radula (You may have heard about sharks doing a similar thing, with a conveyor belt of new teeth being constantly moved forwards.  If not, don’t worry, we will cover sharks a lot later!).  The numbers of teeth present, and the rate at which they regrow is astounding!  Some species may have up to 250,000 (two hundred and fifty thousand) teeth, and five rows a day may be regrown.

If this was not cool enough, there are adaptations of the radula which we will come to later, which include it being used as a harpoon to spear prey!

So, that was the introduction to tongue, and teeth, which leaves us with plates, and takes us back to Chiton, which is the group of molluscs which we are looking at these last two posts.

As it has been a while since my last post, here is a picture of a Chiton (Tonicella lineata) to remind you what they look like:

Tonicella lineata (Lineata meaning lined). Image from Alaska Fisheries Science Center

There are usually 8 plates on the back of a Chiton.  These are attached to a muscular band called a girdle (this is visible in the picture above as the pink and stripey bit along the bottom of the shell).  The girdle holds the plates in place, and can be decorated with a variety of coverings, from bristles, to hairs, to scales.  These decorations are not without function, as the spines and hairs and bristles serve a sensory purpose as they have nerves extending from them.

Diagram showing various ways in which the girdle may be decorated. Image from University College, Dublin

 Now, whilst all Chiton have these plates, not all have them on display as brightly as some of the ones we met last time.  There is one particular species I would like to show you, and it has its plates completely covered by the mantle (As mentioned last time, this is the part of the body which secretes the shell, and protects the internal cavity).  It has a very leathery appearance, and this gives it the common name of “The Gumboot Chiton”.  It is also known by its sciencey name of Cryptochiton stelleri. ( I don’t think it looks like a gumboot personally).  The “crypto” in the genus name means hidden, as the plates are hidden from view by the mantle.

The Gumboot Chiton, or Cryptochiton stelleri. Image from Encyclopedia of Life (EOL.org)

This is one of the bigger Chiton species, and individuals can be around 30cm long (most Chiton are 2-5cm long). Due to its large size, images of it make it easier to see certain parts. The next image is from underneath.  The mouth (where the radula pops out of) is at the left side.  The foot is in the middle, and the stripey looking things running along the body are its gills, which are contained in something called the pallial groove (pallial refers to the Latin word pallium, meaning cloak).

Underside of Cryptochiton stelleri. Image from wikipedia

When these animals are out of water during low tide, they usually close their mantle around them to prevent themselves drying out, but, they can also leave a little part of the mantle open to allow for very very limited air breathing.

One final cool thing…Some species have been observed to have homing behaviours ( a bit like pigeons).  The Gumboot Chiton is among those with this ability, although the mechanism of it is not yet fully understood.  Some species appear to retrace their steps using chemical signals, whereas others may use some form of magnetic sensing (See further reading).

Hopefully, these last two posts have shown you some interesting things, and when you hear about molluscs, you might not only think of snails, slugs and clams!

Further Reading:

EOL (Encyclopedia of Life) is an awesome resource, which I found out about whilst watching a documentary about the biologist E.O. Wilson:  http://eol.org/about

http://biology.fullerton.edu/deernisse/pubs/Eernisse_07_chitons_Tidepools.pdf

http://biology.fullerton.edu/deernisse/pubs/Ee_Clark_Draeger_Light_Smith_07.pdf

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7 thoughts on “Tongues, teeth and plates

  1. It’s great to see you back! Your enthusiasm is infectious. Who knew that molluscs could have pink girdles with decoration (ooh la la) and that they could be gumboots with homing behaviour like pigeons? A potential new sport for people to bet on. ‘My chiton gets home faster than your chiton!’

    As for those toothy tongues, you’ve just taught me why slug attack on my vegetable plants leaves distinctive rectangular holes.

    • Well, we could pick a beach in the UK, and race the Chiton across it!

      The radula is one of those organs which is both fascinating and repulsive at the same time, when you watch it in action, your brain is saying “That’s not right”. I have seen several videos of them, and I am still not used to it!

      I want to see one under a microscope, but, seeing as I jumped off my stool and squeaked a little bit whilst on my aquatic field course when some of the micro-organisms in my sample attacked another one, maybe I should not see these in close up yet! (My brain got confused and thought the action was happening at my scale!)

      • Yes it’s quite icky! In fact I’m gestating a new blog post entitled something like ‘You’re a plant. Look out! The herbivores are coming!’ If i get around to writing that, I’ll link to your post here because that electron micrograph of a radula is so graphic.

        Didn’t you say that you were about to start a Biological Science degree? If so I hope you’ll get past the squeamishness. Mind you, during my PhD the bloke next to me in the lab got upset because I had samples of ovine colostrum. He took milk in his coffee but somehow, colostrum was disgusting. I wasn’t even trying to use it as food.

        But who am I to talk? I was repulsed by the photo of pink worms on a beach, that you posted recently. And the first time I did a cranial dissection on a lamb, I had to sit down with my head between my knees until the shock wore off. It did wear off though and I still like eating lamb.

        Anyway, I’m in love with these gumboot chitons. Here in Yorkshire t’folk say ‘wellies’ and I can imagine a future in which pigeon lofts give way to chiton pools. Ey oop.

  2. I was not so much squeamish (I felt a bit funny like you did when I did a mouse dissection in molecular biology), as more thinking it was happening on my scale, instead of under a microscope, so it was “Quick, run, there is a many toothed monster coming towards you!”

    Pigeon lofts being replaced with chiton pools would make for some interesting landscape changes in Yorkshire! Could also be a new money spinner for Blackpool! Maybe we could start an inter-university chiton race!

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