Thursday, September 25, 2008

Having the Slime of my Life: defensive reflexes in Myxiniformes

Some animals have unfortunate names.

The dik dik, for example.

Or the nudibranch.

Other animals not only have a funny name, but remind us that nature is not always beautiful. The naked mole rat springs to mind.

But my favourite animal in the aforementioned category is the hagfish. Hagfish resemble the earliest fish in that they have no lower jaw. Instead, they tear pieces off polychaete worms and dead fish. I am not sure whether it's their gummy, left-my false-teeth-out style of eating that gives them the name 'hags', but it seems appropriate.

Hagfish are also known, charmingly, as slime hags, due to their numerous mucus glands. This is what my textbook has to say about hagfish:

"A disturbed hagfish can produce enormous volumes of protective mucus; once released from the body, the mucus expands very rapidly and can completely fill a bucket containing the hagfish within minutes."

Disappointingly, it seems that there is no such creature as a fagfish, with whom these slimy, disturbed hags would presumably form a symbiotic relationship.

Monday, September 8, 2008

Fish fingers: Acanthostega and tetrapod evolution

This handsome devil is Acanthostega. One of my lecturers refers to him as 'friend Acanthostega', and, for reasons I will explain, I have come to think of Acanthostega as a friend.
There are no photos to show you, because Acanthostega has been dead for approximately 365 million years.

A cultural theory enthusiast might say that Acanthostega inhabited the liminal zone. A normal person might say that he was a half-fish, half reptile who lived in a swamp. When you hear about creatures crawling out of the primeval soup, it's Acanthostega who did the crawling.

As you can see, Acanthostega's limbs sit a little awkwardly on his body. This is because limbs have only just been invented. Acanthostega's predecessors were fish with fins. He was probably not mobile on land, but could use his legs to brace himself against aquatic plants, and push up out of the mud.

Early limb-bearing creatures like Acanthostega usually had six to eight digits on each hand. It was only later in evolutionary history that most animals settled on five digits as the optimal number.

Of all the species that have ever existed, it is estimated that 99% are extinct. It's sad to think of the last Acanthostega sinking lifelessly into the Upper Devonian mud. However, there is a happy ending. Limbs proved to be a very successful evolutionary strategy. Acanthostega is not just a friend, but a relative. It seems (my textbook is not explicit on this) that his descendants include ourselves!