Ooops, I forgot!

It suddenly dawned on me when working out the next organism to cover, that I had completely forgotten to mention how we got to molluscs! (Panic not, I have doubled my daily allowance of tea and cheese and biscuits to make sure this never happens again!)

Today is about how we ended up at molluscs….so, hold onto your hats, there may be some taxonomy coming up!

Cladogram (relationship tree) for molluscs. Image from UCMP Berkeley

The diagram above shows the relationships between different groups of molluscs.  Now, these diagrams are constantly being redrawn, as new information comes to light.  This does not mean that scientists do not either a) know what they are doing, or b) know whether these organisms are all molluscs.  The groups within molluscs, and the animals in each group are largely agreed.  The differences come when trying to find out where the groups separated from each other, and once in a while merging or unmerging smaller groups as new molecular or genetic information comes to light.

What the diagram above shows is that the polyplacophora (the Chitons we covered in the last two posts) and another group, the Aplacophora, are related more closely to each other than they are to the other mollusc groups, and that squids are more closely related to snails than they are to mussels.

It is thought that the Polyplacophora (meaning carriers of many plates) and the Aplacophora (meaning without plates) are the oldest living forms of molluscs.  Neither of these have shells in the way many molluscs do (we will cover squids and octopus later on), the Chiton, as we saw in the last post, have a series of plates which are held in place by a girdle, and Aplacophora have no shell at all, but instead have spicules, which are tiny spikes.  As an example of how groups can be moved around, the Aplacophora are split into two groups by some biologists, and some data appears to show that one group may be related to the Cephalopods.  (The details can get a bit techy, but it is to do with molecular data, and embryonic development, links as always, are in the Further Reading section)

A lot of the lack of information on these, and many smaller organisms is due to the fact that not many people study them.  Whilst research can take you into some very interesting, and unusual species, if you are not aware of them in the first place, then you will not consider them as research topics.

Back to the Aplacophora: The following images show some close ups of one species, from that favourite resource of mine, Encyclopedia of Life.  The first image is a zoomed out view.  The spicules are just visible as slightly shiny bubbly looking bits.

Chaetoderma elegans. Image from EOL

Now for a zoomed in image, using a dissection microscope.  The spicules are more easily visible in this image.

Chaetoderma elegans. Image from EOL

Finally, a view of some spicules.  These are so many different colours because they are taken using a cross polarizer, which means that you are better able to see the structures.

Spicules from Chaetoderma elegans. Image from EOL

 

Ok, one more picture…I did promise no more worms for a bit, and technically speaking, this is not a worm…here is a picture of the same species as above, but zoomed right out so we can see its shape:

Not a worm…honest! Image from EOL

Now, eagle eyed readers will recognise this shape as one we have bumped into before, when we were with the Nemertea, and indeed, Nemertea are grouped into a Superphyla, known as Lophotrochozoa, due to molecular and developmental evidence.

So, we have a worm-like creature which can secrete spikes from its skin, very handy as a defence mechanism (Think about hedgehogs!).  As we saw in earlier posts, some organisms headed underground for safety, whilst others secreted a shell round one end of their body, which then stuck out of their burrows (HERE)

Now, both of these are effective strategies, but not without their drawbacks.  Living below the surface limits the food that is available to you, and is not really very safe once predators find out you are in the sediment.  Living vertically in a burrow with a shell around your protruding end allows for you to access more food, but limits you to food which is passing by.  As a defence mechanism it is fairly good, keeping your head attached is very important!

Secreting small spikes from your body offers less protection than a shell around your head, but allows you to move about much better, meaning you can increase your range when looking for food, or proximity to mates (many of these species use external fertilization, which means they release sperm and eggs, which are then fertilized by others nearby).

If these spikes widened, and joined together in bands, then you have something which may look like the plates we see on Chitons (This part is purely hypothetical on my part, so feel free to yell if I have got it completely wrong).

I think that is enough wall of texting for today (and rambling!).  Next time is moving on from Chitons to the next animal in our journey, and I will make sure to try to explain the links between them in future!

Further Reading:

UCMP Berkeley Paleo site, always a very handy site for me: http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/aplacophora.php

Paleo-biology article about very early molluscs: http://palaeo-electronica.org/2011_2/221/index.html

Blog post outlining the above article (less techy version!) http://www.palaeo-electronica.org/blog/?p=416

Phylogenic discussion of one of the Aplacophora species: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2828982/

Article discussing predation and shell evolution: http://www.biology.ualberta.ca/palmer.hp/pubs/79Evol/Palmer’79.Evolution.hi.pdf

 

 

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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

Shells shells everywhere!

As it has been a while since we have been on the evolutionary journey, here is a link to the last post in the series.  So far, we have got to wormey things with shells, and today we take a step into a world full of shells, that of the molluscs.  I am not doing land based molluscs yet, as they do not turn up in our little journey for a while yet, and the sea has more than enough cool animals for now!

Molluscs are a HUGE phylum, with 90,000 species living, and 70,000 fossil species, so I am going to break this section down into segments.  I will start with Polyplacophores (Chitons), then move onto Bivalves, then Gastropods (marine only), before finishing with Cephalopods.  I will do two posts on each group, one outlining their habitat and morphology (body shape and structure) and one on particularly interesting features of each group.  I could write a load of posts on each group, but I will be coming back to them when I do future series on sea-life, so this will just be an intro to whet your appetite, and maybe show you some cool stuff you did not know.

So…lets get started by meeting the Polyplacophores (Meaning “carrier of many plates”):

Tonicella lineata. Found from Alaska to California in the USA, and on the Pacific coast of Russia and Japan. Image from wikipedia

Mopalia muscosa, a Chiton found on the coasts of California, but also up into Canada, and on the Mexican Pacific coast.  Image from University California Santa Cruz

Lepidozona cooperi. Found from Alaska to Mexico. Image from Seanet, Standford University.

These beautiful creatures are found in shallow waters globally, generally up to 90m deep , and the bright colours of the ones I have shown above is to blend in with their surroundings (There are several species with more “boring” brown and green exteriors, but I thought I would show the pretty ones here!)

So, what do they look like on the inside?

Polyplacophore anatomy. Image from UCMP Berkeley

By now in our journey through organisms, we have run into many of the terms in the diagram above, so there are fewer of them which are scary looking.  (My functional biology professor would be impressed at me remembering these terms, I truly sucked at it in lectures!)

We have already run into “pedal” several times, meaning foot, “Nephridiopore” is the same as when we had nephridia, which we encountered when we first met bilateral animals, and relates to excretion. A nephrologist is a doctor who specialises in kidneys, our excretory organ.  Gonad and Gonopore relate to sexual organs (the name is not specific to either male or female).  The dorsal artery is the “top artery”, remember sharks have dorsal fins on their top side.

One which may seem new, but is not, is haemocoel (or hemocoel). Remember how we had acoelomate animals here? That meant that they were without body cavities, and coel relates to a body cavity. Hemo relates to blood, so this is a cavity through which blood travels.  These animals do not have a full network of  capillaries, veins and arteries like we do, and the blood is pumped out from the heart (at the back end of the animal, labelled as Pericardial cavity, ventricle and left atrium), and travels through arteries into spaces in the body where it is distributed to the tissues before going back to the heart via a vein.  This is called open circulation, and is very common in smaller animals. As organisms become larger, this becomes less efficient for transporting oxygen to tissues, and so networks of capillaries develop to better transport the blood around the body.

The radula is a new term, and it is a specialised feeding organ that molluscs have, and we will be covering that in detail in the next post, as it is really awesome.

The mantle is the part which the shell is secreted from, and is actually like a layer of skin which hangs down over the sides of the animal, protecting the internal organs.

As you have noticed in the pictures above, these animals do not appear to have one continuous shell, but rather resemble woodlice (albeit brightly coloured ones).  This is not because woodlice are in the same phylum, or even class as these, as woodlice are crustaceans (yes, the same group (subphylum) as crabs and lobsters).  Rather, it is because the shell on these animals and on woodlice serves the same purpose, to allow maximum movement while giving protection.

I think most people have seen a woodlouse curl up when it is in danger (or when we try to pick it up to move it).  These do the same, and they could not do that if they had a single, solid shell as other molluscs do.  There are many ways in which a shell can protect an animal, and the each way of using a shell occurs in several distinct species. Some species of Armadillo have plates which allow them to roll up in this manner.  As it has evolved in several different organisms, it is clearly an effective defense strategy!

I will leave you with a video of one of these animals moving around, and next post will cover the radula, and some other features which I find fascinating about these creatures.