Keep swimming!

The somewhat fuzzy photograph above, was taken, in a rush, at the end of a rockpool survey.  Looking at it back at home it wasn’t at all obvious to me what it was, or even that its subject was organic. More geological perhaps? Some sort of crystal growth maybe?  A cluster of stars on a piece of rock at the bottom of a rockpool. 

I would normally have reached for reference books or the internet to identify a new find, but this time I didn’t know where to start, so I took the easy route and appealed to Twitter, hoping one of the several expert rockpoolers I know to be online would get back with an identification. And, within a few minutes they did1..  Not geological but biological. Animal in fact. A specie of sea-squirt commonly known as Star Ascidian.  Hence the stars, of course.

I was pleased. A new entry for my growing rollcall of species found in the pools of this little north Pembrokeshire Bay2..  I looked up Star Ascidian online to get the proper scientific name and its scientific classification:

Common name: Star Ascidian

Scientific Name: Botryllus schlosseri

Kingdom: Animalia

Phylum: Chordata

Subphylum: Tunicata

Class: Ascidiacea

Order: Stolidobranchia

Family: Styelidae

Genus: Botryllus

I like these scientific names and classifications. I enjoy the feeling of learning and arcane knowledge they project. They conjure images of ranks 18th and 19th century amateur scientists grappling to understand the world around them, at a time when the balance between knowledge and mystery left room for such intellectual curiosity and simple enthusiasm. The problem is they pique my interest.  Like tips of icebergs, they hint at more below; hidden research, information and knowledge.  I find I need to delve further, and that one thing leads to another, so that what might have started as a chance finding of something curious in a rockpool, all too easily mushrooms into a much larger, sometimes even burdensome, investigation.

Whenever I sense this process starting, I am reminded of J.R.R. Tolkien’s beautiful short story ‘Leaf, by Niggle’, in which an artist, Niggle, becomes obsessively absorbed in a particular painting which seems to take on a life of its own.

“There was one picture in particular which bothered him [Niggle]. It had begun with a leaf caught in the wind, and it became a tree; and the tree grew, sending out innumerable branches, and thrusting out the most fantastic roots. Strange birds came and settled on the twigs and had to be attended to. Then all round the Tree, and behind it, through the gaps in the leaves and boughs, a country began to open out; and there were glimpses of a forest marching over the land, and of mountains tipped with snow. Niggle lost interest in his other pictures; or else he took them and tacked them on to the edges of his great picture. Soon the canvas became so large that he had to get a ladder; and he ran up and down it, putting in a touch here, and rubbing out a patch there. When people came to call, he seemed polite enough, though he fiddled a little with the pencils on his desk. He listened to what they said, but underneath he was thinking all the time about his big canvas, in the tall shed that had been built for it out in his garden (on a plot where once he had grown potatoes). “

I particularly that last line ‘on a plot where once he had grown potatoes’, presumably illustrating how the obsessive work on the painting displaced more practically fruitful activity!

Anyway, my little sea-squirt acted, for a while, a little like Niggle’s Leaf. I have sketched one of the main branches of its tree here and will add one or two more in a second blog shortly.   

The first thing I noticed was that the sea-squirt belongs to the Phylum Chordata. The ‘chord’ in Chordata refers to a primitive nerve chord that may run along a creature and from which the spinal column and backbone evolved. Chordata is therefore the phylum to which all vertebrates, including ourselves belong. 

Two things followed this; the first was I realised that although I had recorded many different species since I started delving into these rockpools, this was the first that belonged to this group.  How different this is to the land-based animal life we are all familiar with, where Chordata seem so dominant.  For example, all the mammals, including ourselves, amphibians, reptiles, birds, fish, snakes, lizards are all Chordata. I started to wonder why this difference is so marked, which led to reading read about disparate subjects such as why insects are small and how the average density of many animal bodies, including our own is almost the same as that of sea-water, which means that marine life doesn’t feel gravity in the way land animals do enabling the wonderful strange and delicate forms of rockpool life I had been finding.

Having learned that belonging to chordata implies a primitive spinal cord or backbone, I looked again at the photograph.  It was not at all obvious how this thing, which appeared little more than a smear of patterned jelly, could have a backbone or indeed would have any use for one. Turning to the formal definition of Chordata I read,

“Chordata: A phylum of the animal kingdom comprising all the animals that have, at some stage in their life, a notochord (a hollow dorsal nerve cord), pharyngeal slits, and a muscular tail extending past the anus. Includes the subphyla Cephalochordata, Urochordata, and Vertebrata (vertebrates).3.

The clue is in the second line; “at some stage in their life”.  It turns out that the sea-squirt starts out as a free-swimming larva, somewhat like a little tadpole, complete with a primitive, eye, tail and notochord and importantly, a very rudimentary brain, designated to controlling it’s motion.

After a brief free swimming (pelagic) period the tadpole attaches itself to a suitable substrate and metamorphoses into the attached (sessile) jelly form I found. 

This life-cycle, of pelagic larva to sessile sea-squirt, turned out to be more significant than it at first seemed. 

Evolutionarily speaking the story goes like this; Once Upon a Time (roughly 550 million years ago) there was a specie of tiny tadpole like creatures that spent their days swimming in the sea. Some of these tadpoles found that it took much less effort to cling to the rocks and wait for the food to come to them than to actively seek it out by swimming, while the others stuck with the original swimming about plan.  Each group became somewhat set in their ways and slowly evolved to become more suited to their adopted lifestyles.   

The group that stuck with the swimming, we could call them the pelagic group, went on to great things, or, even if we eschew this somewhat subject value judgement, at least went on to great variety and experimentation, as it is now believed, that these little free swimming tadpoles evolved to give rise to all the vertebrate life on earth, including, of course, ourselves. 

Things did not go as well for the sessile group. Nature discovered (through the usual method of ruthless experimentation no doubt) that once attached to its rock the organism no longer needed its tail, eye, embryonic spinal cord and backbone, or, for that matter its brain. These organs the sea-squirt evolved, in a process known as regressive evolution, to digest for the sake their protean content!

The element of black humour in this coupling of an organism ‘settling down’ with the loss of its mental faculties, has not been missed. The philosopher of mind, Daniel Dennett, draws a parallel to the process of a university professor obtaining tenure!

“ The juvenile sea squirt wanders through the sea searching for a suitable rock or hunk of coral to cling to and make its home for life. For this task, it has a rudimentary nervous system. When it finds its spot and takes root, it doesn’t need its brain anymore so it eats it! (It’s rather like getting tenure.) ” 4.

Professor Sea Squirt5

Different lines of investigation, including modern genetic analysis, indicate that the sea-squirt is the immediate ancestor of all vertebrates and therefore is considerably closer to humans than many other forms of equally unlikely looking life.  For this reason, among others, the sea-squirt has become the model organism for studying the evolution of vertebrates, including the origin and development of human hormone, nervous and immune systems.  The latter, among other things, providing a new range of chemo-therapy drugs6..

The above is a tiny sample of what investigations of the humble sea-squirt have revealed and are still revealing, but already I somehow felt the need to shake its hand, if only it had one.

Evolution of chordates showing the sea-squirt (Tunicates) as the ancestor of the vertibrates.7.

  1. Thanks to:
  3. For example:
  4. MLA (7th ed.) Dennett, D C. Consciousness Explained. Boston: Little, Brown and Co, 1991.
  5. Drawing by Paul Jackson. ©Randel McCraw Helms, 2018

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