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The
Diversity of The World of Life
Chordates
(Chordata)
Tunicates,
or Sea Squirts
(Urochordata)
Representatives
Tunicates
Biology
APPROXIMATE NUMBER OF KNOWN SPECIES WORLDWIDE
1,600
ENVIRONMENTS
Tunicates
are marine species, typically "benthic"
(bottom-dwelling) and "sessile" (attached to the bottom) as adults
but free-swimming as "tadpole" larvae (and,
in some species, as adults) -- this free-swimming form is presumably similar to the
ancestor of fishes and, thus, of all vertebrates.
DESCRIPTION
Unlike other chordates, urochordates
lost their "coelom" during
evolution -- their body cavity is not lined by a "peritoneum"
membrane.
The microscopic "tadpoles" are bilaterally
symmetrical (that is, the left and right sides, only, are mirror images of
each other). Their bulbous body has a finlike tail, flattened from
side-to-side, and adhesive attachment points at the front end, which
anchor the larva upside-down on the seafloor for its dramatic development:
The tail is "resorbed", back into the bulbous body; and
the gut grows up around into a "U" shape, the mouth ultimately pointing upward. The bulb-like, microscopic to macroscopic adult secretes
around itself a gelatinous or leathery "tunic", made largely of
a material chemically related to "cellulose" (a "polysaccharide"
typically made only by plants). About
the only notable external features of the adult are an "incurrent
siphon" (a water-intake duct, extending upward from the mouth) and an "excurrent siphon"
(a
water-output duct, also atop the body). Sea-squirts
do indeed squirt water, out of their excurrent siphon, as when molested.
Some tunicates are colonial. In some species, the
individual animals of the colony form from buds on branching
"stolons"; and in other species, the individuals are fused within a common tunic,
having a
common excurrent siphon.
FEEDING HABITS
Tunicates are filter-feeders.
MOTION
The mobile larvae have well-developed tail muscles.
Like various life stages of other chordates, the
"tadpoles" of urochordates have a springy (strong,
yet flexible) "notochord" in their tail, which converts the muscle contractions
alternating on either side of the tail into a waving, swimming
action. Upon settling down,
the developing larva resorbs the notochord, with the rest of the tail.
Some of the colonial tunicates that are fused together within
a common tunic use their common excurrent siphon to "jet propel"
through the water!
DIGESTION
Typical of chordates, urochordates have "pharyngeal
gill clefts" (slits). As
the larva develops into the adult, the pharynx (throat)
develops into a large, perforated "basket", used in
filter-feeding.
Typically, water drawn in through the incurrent siphon moves
past
the small tentacles surrounding the mouth and into the basket; water then
passes through the many perforations in the basket and collects in the
surrounding "atrium", before exiting the body through the excurrent siphon.
Mucous in the basket catches food particles in the water current, which is
driven by the action of hair-like "cilia" in the basket; and
still other cilia in the basket move the food particles down to
the entrance of the esophagus.
The gut is "complete" -- it has both a mouth and
an anus, the anus opening into the atrium, flushed clean by the current of
water. Between the basket-like pharynx and the anus, the gut is
differentiated not only into an esophagus (with an associated digestive
gland) but also into a stomach and an intestine.
RESPIRATION
Gases are exchanged across the extensive surface area of
the thin, perforated pharyngeal basket, within whose
interwoven "ribs" circulates blood (described below).
CIRCULATION
Dissolved gases and other materials are carried by the
"open" circulatory system -- the blood is pumped from the
heart (sometimes in one direction, sometimes in the other) out through short
arteries and into the rest of the body cavity (like the "hemocoel" of
mollusks). Some of the
blood circulates into the ribs of the pharyngeal basket. The blood
eventually returns to the heart through the body cavity (not through membrane-bound
veins, as in a "closed" circulatory system).
EXCRETION
There are no kidneys:
Water-soluble wastes rich in nitrogen simply diffuse out of the body and/or are
flushed out
by the water current; other nitrogen-rich wastes are stored within the
body as insoluble crystals of "uric acid".
COORDINATION
The growth, development, and activities of tunicates are
under genetic and hormonal control, influenced by the environment.
Like
other chordates, urochordates have a tubular "nerve cord"; it
is located dorsal to (that is, just above) the supportive notochord in the
tail of the free-swimming larva. In
its "metamorphosis" to the sessile (attached) adult, the
tunicate loses its nerve cord and is left with just a small
"ganglion" (nerve knot), derived from the ganglion within the head
of the larva.
REPRODUCTION
Most tunicates can reproduce asexually, by a complex
process of "budding".
Tunicates can reproduce sexually and are typically
"hermaphroditic" (one individual having the gonads of both
sexes). The gonads deliver the gametes (eggs and sperms) into the
atrium, which is thus a "cloaca" (an exit chamber for both the
reproductive and digestive systems). The
eggs are fertilized and the embryos develop either in the atrium or out at
sea. As described above, the free-swimming,
"tadpole" larvae typically anchor themselves to the bottom and
transform into the sessile, "basket-feeding" adults.
Chordates
(Chordata)
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