Wednesday, October 19, 2005

Urochordates

Well, now that the introduction is out of the way, time to bring out the first (of I hope, many) creatures I'll be highlighting on this blog. And what better organism to start with than the one dear to my heart, Oikopleura dioica? But first, I need to throw in a bit of general background on the larger group of organisms to which Oikopleura belongs.... the urochordates.

So what is a urochordate? Probably the most familiar member of this group is the sea squirt... a marine animal that sits attached to rocks, spending its life sucking in water through one hole, filtering out the food with a big bag of mucus, and then sending the waste water out another hole. Sea squirts lead a sedentary existance, quietly filtering away until old age or predators get them. However, for a very brief period in their life sea squirts can be very active. Newly hatched sea squirts come out of the egg looking like little tadpoles... a lump of a body with a tail attached to the rear. They swim about for several hours looking for a solid surface to land on, then glue their heads to ground, absorb their tails, and metamorphosize into a sac-like creature with an intake and an outflow hole.

Its that tail that makes sea squirt larva so interesting... it has several features that indicate urochordates branched off early from the same evolutionary line that led to us. Running down the center of a sea squirts tail is a rod-like structure known as a notochord, which gives support to the tail and helps provide something for the muscles to act upon. Running the length of the tail just above the notochord is the dorsal nerve cord, transmitting signals from the sea squirts brain to the tail muscles.

If this sounds a bit familiar, its because we have a similar system in our bodies... a dorsal set of vertebrae to which our skeleton and muscles are attached and a dorsal nerve cord running above the main axis of the vertebral column (although also surrounded and protected by vertebrae). Early in our development, we also have a notochord, but as we grow it becomes replaced by our bony vertebrae. The reason for this replacement is obvious... what can provide structural support for a 2mm long aquatic larva would barely hold together a 6 foot tall terrestrial biped. In fact, only two other groups retain their notochords... the lancelet fishes (not an actual fish, but similar in appearance) and the hagfish. In all other chordates, the notochord is replaced by either bone or cartilage.

Although all urochrodates have a notochord, a dorsal nerve chord, and thrive by filter feeding plankton through a mucus bag, not all have become sedentary animals like the sea squirts. Oikopleura belongs to a group of urochordates called the Appendicularians.... these fellows hatch out of the egg looking like sea squirt tadpoles, but instead of attaching to a rock and losing their tails, they keep their tails into adulthood. Whats unusual about the Appendicularians is that to help themselves get food out of the surrounding water, they build for themselves houses of protein and cellulose that contain an elaborate system of filters to concentrate the food, remove stuff they can't eat, and bring the rest into their mouths. The beating motion of their tails powers this massive food concentrator, drawing in the food laden water to the filters.

I'll have more to say about the Appendicularia in my next post... hopefully with some pictures included.