Chordates
are defined as organisms that possess a notochord, a hollow dorsal
nerve cord, pharyngeal slits, an endostyle, and a post-anal tail for at
least part of their life cycle. We fall under the phylum Chordata, as do
all other vertebrates, but not all chordates are vertebrates. Tunicates
are part of the subphylum Urochordata, which is a sister subphylum to
ours (Vertebrata), but look nothing
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like
us or any other vertebrates. In fact, many tunicates look more like
simple sponges than anything else, but they might be an example of what
our first chordate ancestors would have looked like hundreds of millions
of years ago.
Most tunicates (or sea squirts) begin their life cycle as a free-swimming larva that develops all of the required chordate characteristics as well as a nervous system. Its primitive spinal cord sets it apart from its invertebrate cousins, but it is still more simple than the nervous system found in primitive fish. As the animal matures, it finds a rock or hard substrate and attaches itself head-first. The animal then transforms into a passive filter feeder, and its cerebral ganglion breaks down and is reused elsewhere while its adult ganglion is left intact, as it is needed for feeding and reproduction. In other words, the tunicate "eats" its own brain. Its body begins to morph into a simpler form and the tunicate remains this way for the rest of its life.
As simple as this animal looks, it is actually quite complex. It can stop its heartbeat and reverse the blood flow throughout its body, to replenish oxygen in parts of its body that might be starved. It can regenerate its entire body from tiny fragments of blood "vessels," called sinuses, which are similar to our own circulatory system. Tunicates are the only animal to have a high level of the element vanadium in their bodies, and they produce vanabins and vanadocytes in their blood to help collect and store the rare and toxic element. The reason for this is completely unknown. They also hold other useful chemical compounds, such as didemnims and aplidine, which are used to treat cancer.
It doesn't stop there; scientists have discovered predatory tunicates (pictured here) in Monterey Canyon off the coast of Northern California. Acting like the venus fly traps of the sea, they snap shut when a small meal happens to float inside. This is a much more efficient method of feeding in the deep sea, when algae is scarce and filter feeding is difficult.
Categorizing life is not easy. There are grey areas between taxonomic groups, and some organisms are simply too strange to fit in any specific group. Tunicates are not the only animal that fall between the vertebrate and invertebrates categories, and their fossil record is incomplete, although it is hypothesized that higher chordates (like us) evolved from an ancestral tunicate that retained its larval form. So next time you see a simple-looking animal in a tidepool, remember that it is still the product of millions of years of successful evolution, and it is never as simple as it seems.
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Most tunicates (or sea squirts) begin their life cycle as a free-swimming larva that develops all of the required chordate characteristics as well as a nervous system. Its primitive spinal cord sets it apart from its invertebrate cousins, but it is still more simple than the nervous system found in primitive fish. As the animal matures, it finds a rock or hard substrate and attaches itself head-first. The animal then transforms into a passive filter feeder, and its cerebral ganglion breaks down and is reused elsewhere while its adult ganglion is left intact, as it is needed for feeding and reproduction. In other words, the tunicate "eats" its own brain. Its body begins to morph into a simpler form and the tunicate remains this way for the rest of its life.
As simple as this animal looks, it is actually quite complex. It can stop its heartbeat and reverse the blood flow throughout its body, to replenish oxygen in parts of its body that might be starved. It can regenerate its entire body from tiny fragments of blood "vessels," called sinuses, which are similar to our own circulatory system. Tunicates are the only animal to have a high level of the element vanadium in their bodies, and they produce vanabins and vanadocytes in their blood to help collect and store the rare and toxic element. The reason for this is completely unknown. They also hold other useful chemical compounds, such as didemnims and aplidine, which are used to treat cancer.
It doesn't stop there; scientists have discovered predatory tunicates (pictured here) in Monterey Canyon off the coast of Northern California. Acting like the venus fly traps of the sea, they snap shut when a small meal happens to float inside. This is a much more efficient method of feeding in the deep sea, when algae is scarce and filter feeding is difficult.
Categorizing life is not easy. There are grey areas between taxonomic groups, and some organisms are simply too strange to fit in any specific group. Tunicates are not the only animal that fall between the vertebrate and invertebrates categories, and their fossil record is incomplete, although it is hypothesized that higher chordates (like us) evolved from an ancestral tunicate that retained its larval form. So next time you see a simple-looking animal in a tidepool, remember that it is still the product of millions of years of successful evolution, and it is never as simple as it seems.
http://www.earthlife.net/
http://
http://
http://www.youtube.com/
Image credit: io9.com
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