Page 263 - (DK) Ocean - The Definitive Visual Guide

CNIDARIANS 261

Locomotion Reproduction

The most mobile cnidarians are free-living jellyfish and medusae, which Members of the class Anthozoa, such as corals and anemones, reproduce
mainly drift in water currents but also swim actively using a form of jet by asexual budding. A genetically identical copy of the adult grows on
propulsion. Most colonial cnidarians, such as corals and sea fans, cannot the polyp’s body wall. This budding juvenile drops off or stays attached
move from place to place. However, they can expand and contract their to form a colony. Anthozoans also reproduce sexually, producing eggs
polyps to feed or avoid danger, and some sea pens can withdraw the and sperm within the polyps. Fertilized eggs develop into hairy, oval
whole colony below the surface of the sediment in which they live. larvae (planulae), which either swim free or are brooded internally and
Unattached mushroom corals may move then released. Hydrozoans have a two-stage life cycle. Their polyps release
slowly, or even right themselves if JELLYFISH SWIMMING tiny free-swimming medusae into the water which, when mature, shed
A jellyfish swims by using muscles
overturned. Anemones can creep slowly eggs and sperm. The resulting fertilized eggs develop into planulae that
to contract its bell, forcing water out
over the seabed on their muscular basal and pushing it along. The muscles then settle in a new area to grow into
disk, and a few species swim if attacked. relax and the bell opens again. polyps. In contrast, the medusa
form of jellyfish is usually much
larger than the fixed polyp form
and the polyps bud asexually.

BUDDING JELLYFISH POLYPS
Jellyfish polyps are minuscule, and their
sole function is to reproduce asexually
bell fully contracted, by budding off baby jellyfish.
with little water
bell begins to remaining inside
bell relaxed and contract and
flattened, ready to force water out
propel forward Zooxanthellae
The massive skeletons secreted by reef-building corals
require energy for their construction. Corals cannot catch
enough plankton in clear tropical waters to provide this
energy. Instead, they rely on tiny, symbiotic single-celled
algae, called zooxanthellae, living in their cells. These algae
manufacture organic matter by photosynthesis, and make
more food than they need, so the excess is used by the
coral. The algae benefit from a safe place to live
and obtain “fertilizer” from the coral by using
its nitrogenous waste products. If stressed by ZOOXANTHELLAE
disease or high temperatures, corals expel In this image of coral polyps, the green
patches are zooxanthellae living in the
their zooxanthellae, in a process called corals’ tissues. The zooxanthellae give
coral bleaching, and may die of starvation. color to the otherwise colorless polyps.

CNIDARIAN CLASSIFICATION

Cnidarians are divided into five classes and a large number of orders
and families. This phylum used to be called the Coelenterata, a name
still used by some authorities. Many species remain undescribed.

BOX JELLYFISH ANTHOZOANS
Order Cubozoa Order Anthozoa
41 species 7,095 species
These jellyfish have a cube-shaped bell with These colonial or solitary polyps are
four flattened sides and a domed top. There diverse in shape and have no medusa
are four tentacles or clusters of tentacles, phase. Octocorals (soft corals, sea fans,
one at each corner. Most are virulent stingers. and sea pens) have polyps with eight
feathery tentacles; hexacorals (including
hard corals and anemones) have polyps
HYDROIDS
Order Hydrozoa with multiples of six simple tentacles;
ceriantipatharians have polyps with
unbranched tentacles.
3,516 species
These colonial cnidarians mostly resemble
plant growths attached to the sea bed. A few
JELLYFISH
have hard skeletons and resemble corals, and Order Scyphozoa
some colonies float at the surface like jellyfish.
Most species have a free-living medusa stage. 186 species
These mostly free-swimming medusae
are shaped like a bell or saucer with a
STALKED JELLYFISH
Order Staurozoa fringe of stinging tentacles. The edges
of the mouth, located on the underside,
are drawn out to form trailing mouth
48 species
Less than an inch high, stalked jellyfish have tentacles or oral arms.
a bell-shaped body with eight clusters of short, OCEAN LIFE
knobbed tentacles around the rim of the bell.
They attach to seaweeds by a stalk that
extends from the “top” of the bell.