Page 320 - (DK) Ocean - The Definitive Visual Guide

318 ANIMAL LIFE

Tunicates and Lancelets Anatomy

When tunicate larvae become adults, they lose the
TUNICATES HAVE A LONG, baglike body often
DOMAIN Eucarya supporting notochord, but lancelets keep it during
attached to the sea floor; lancelets resemble small, stiff the adult stage. Tunicates are covered by a tough
KINGDOM Animalia
worms and live buried in sediment. Despite their simple protective bag made out of cellulose called a tunic,
PHYLUM Chordata
appearance, these animals are included not with the which sticks to the sea floor by means of rootlike
SUBPHYLA Tunicata world’s other invertebrates but in the same group as projections. Inside is a big sievelike structure, the
Cephalochordata pharynx, which connects the mouth and gut. This
backboned animals such as fish and mammals. This is
CLASSES 5 because, uniquely among invertebrates, tunicates and has a sticky mucus coating to trap plankton from
the seawater passing through it. Lancelets also filter
SPECIES About 3,056 lancelets possess an internal skeletal rod, or notochord.
water through a pharynx, expelling it through an
The best-known tunicates are sea squirts, some of which
opening near the anus. A ring of stiff hairs (cirri)
form colonies, whereas lancelets are all solitary. surrounding their mouth prevents sand getting in.
Lifestyle LANCELET ADULT pharynx dorsal nerve cord
Sea squirts live attached to hard surfaces such as rocks, reefs, BOTTOM-LIVING SQUIRTS A lancelet’s muscular body notochord
is flattened from side to
and shipwrecks. They spend their time filtering seawater, Sea squirts sometimes grow side and is supported
drawing in food-rich water through one siphon (inhalent) together in clumps with by a stiff notochord.
and releasing waste water through another (exhalent). Most cnidarians, and sea sponges, swimming
and they can be very colorful. cirri surrounding muscles anus
sea squirts occur in shallow coastal waters where there is mouth
plenty of plankton, but there are also a few deep-water
species. In sheltered sea lochs, they can cover TUNICATE LARVA
hundreds of square yards of sea bed. Some The tadpole-shaped tunicate larva‘s nerve
cord and notochord are reabsorbed when it
tunicates, including salps and pyrosomes, changes into the adult form.
drift along on ocean currents with the nerve cord
plankton, often forming giant swarms. siphon notochord
Lancelets are strong swimmers due pharynx
to their flexible, muscular bodies, attachment
but they usually just burrow organ inhalent
in sediment with only their heart siphon
head sticking out. exhalent
siphon
tunic
pharynx
water
current
digestive
gland
ovary
heart
gut

SWIMMING SQUIRT TUNICATE ADULT
Floating salps swim by jet propulsion, Most of the space inside a tunicate is taken
taking in water at one end and up by the huge pharynx, visible through the
squirting it out of the other. tunic of this translucent species.

siphons and uses the larger of these, sea squirt lives up to its name and is
SUBPHYLUM TUNICATA
the inhalent siphon, to draw in water; found attached to a wide variety of CLEANING UP
Common Sea Squirt the smaller, exhalent siphon is used rocks, reefs, seaweeds, and, in particular,
to expel water, and its opening has man-made structures. The legs of oil In sheltered sea lochs and harbors,
six lobes, while that of the exhalent platforms and jetties, for example, are
Ciona intestinalis the common sea squirt often
HEIGHT Up to 6 in siphon has eight. The common often festooned with this sea squirt. covers large areas of rock or wall.
(15 cm) In spite of its small size, it is able
DEPTH 0–1,600 ft to filter several quarts of water
(0–500 m) per hour, filtering out plankton
HABITAT Any hard and other organic particles and
substrate leaving the water much clearer
DISTRIBUTION Atlantic, Pacific, Indian, Arctic oceans; than it might otherwise be.
possibly Southern Ocean
The common sea squirt has no
supporting structures in its adult
form, so when it is seen out
OCEAN LIFE a jet of water when prodded.
of water, it resembles a blob
of jelly that may squirt out
It is a typical solitary tunicate,
whose internal structures are visible
through its pale, greenish yellow,
gelatinous outer covering, called a
test or tunic, which is smooth and
translucent. It has two yellow-edged