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