David Winter: Sea Squirts
Dave Winter over at Science and Sensibility has sent me the following post on Sea Squirts. Thanks David!
Take a close look at the picture above this text. If you haven’t
been introduced to a sea squirt before you may be surprised to learn
the blue blob depicted above is an animal. If that surprised you then
you’ll be flabbergasted to learn you are more closely related to that
animal than 97% of all the species so far described on earth and very
much more than 99% of the individual organisms here.
The animal
in question is a sea squirt (or Urochordate or Tunicate) and it fits
into the Phylum Chordata - the same one as you, me and all the
vertebrates. On the face of it that seems a puzzling designation, the
animal in the picture doesn’t look much like respectable chordates
like mammals or birds or fish, or even like visually similar
invertebrates like the lancelet. That is, until you realise that what
you’re looking at is one half of a lifecycle split into two such
disparate parts that it would do a butterfly proud. See, sea squirts
start out life as larvae that look more or less like tadpoles:
Now that’s a bit more like it. Sea squirt larvae contain notochords, nicely defined muscle segments and a
post-anal tail . Just like you. When you where an embryo. The tiny
tadpoles swim in the plankton before they metamorphose into their
adult form. When the time comes they plant themselves head first onto
some hard surface (rocks, reefs and piers being among their
favourites) and proceed in dispensing with all the stuff they needed
as a free swimming tadpole - including their tail and the head
ganglion which served the role of a brain From here the sedentary,
adult form develops. Essentially the adult sea squirt is a sack fill
of sea water with two syphons - one “sucking” water in and the other
“spitting” it out. These syphons ensure water flows over the
pharyngeal basket which filters out food.
The sea squirts are
also of interest to evolutionary biologists for a number of reasons.
Firstly they may be able to tell us something about the evolution of
the vertebrates. All the vertebrates share a common ancestor with the
sea squirts, there are two main schools of thought as to how each
lineage (vertebrates and sea squirts) got to where they are
today:
- The common ancestor shared by sea squirts and
other chordates was something like a sea squirt larva. After
that point the sea squirts took up a sedentary “adult stage” while the
vertebrates went on to form bones and limbs and all those great
things. - The common ancestor shared by sea squirts and other chordates was
something like a modern sea squirt. At some stage a sea squirt larva
became sexually mature before it took on a filter feeding “adult”
stage. In this scheme the individual larva that reached sexual
maturity as a ‘tadpole’ would be the progenitor of all vertebrates.
The development of sexual maturity in an otherwise ‘juvenile’ stage is
called neoteny and is epitomized by the Axolotl which is a sexually
mature salamander larva.
Richard Dawkins reports
in The Ancestor’s Tale that molecular data tends to support the
first of those schemes. Coincidentally, that scheme was proposed by
that most prescient of biologists, Charles Darwin.
The other
reason sea squirts interest evolutionary biologists is that, seen
through our vertebrate minds, they seem to be bristling with potential
for great things. Recent studies have shown that the sea squirts have
genes containing all the motifs needed to get a blood clotting system
going. Additionally they have some proteins that look a lot like
Toll receptors - proteins associated with identifying pathogens in our
system. In fact a glance through the genome of a sea
squirt revels the seeds of nervous systems, eyesight, the immune
system, and even the cardiovascular system.
Of course we
shouldn’t get carried away on thinking that the sea squirts are some
sort of museum piece displaying the seeds from which we sprung. The
sea squirts have kept what genes they have for a reason - living
organism can be primitive but they can’e be ancestral. It’s likely
that genes we share with sea squirts have been co-opted into
completely different roles and many that where in our common ancestor
500 million years ago have probably been lost. Still, not too bad for
a blue blob huh?
