Abalone

J
ellyfish are members of the phylum Cnidaria
which include corals, anemones and
bluebottles. Jellyfish are typically bell-shaped,
fringed with stinging tentacles. The adult stage is
called a medusa after the unfortunate woman of
Greek mythology who was loved by the god of the
sea and as a result had her hair changed into snakes
by a jealous goddess. Jellyfish are transparent and
constructed of two layers of cells separated by a
jelly-like mesogloea that gives the organism a
degree of rigidity so that it can withstand the
buffeting of the sea. The jelly layer is slightly lighter
than seawater enabling the jellyfish to float in the
sea. Jellyfish can swim by convulsive contraction
of their bells but are largely at the mercy of ocean
currents and frequently wash ashore after storms.
They are an important food source for leatherback
turtles, sunfish and beach dwelling scavengers
such as plough snails and ghost crabs.
Feeding and swimming
Jellyfish swim by contracting and relaxing sets of muscles at the
margin of the bell. Contraction of the muscles tightens the
bottom of the bell, like pulling the drawstrings on a bag. This
forces water out through the bottom of the bell, and jets the
jellyfish forward. Relaxing the muscles opens the bell again,
adult medusa phase
balancing organ
tentacle
frilly ‘arms’
around mouth
C O A S TA L A N D M A R I N E L I F E – A N I M A L S : I N V E RT E B R A T E S – C N I D A R I A N S
Jellyfish 3B
preparing for another contraction. The pulsing of the bell is
controlled by a simple nervous system, and eyespots (or
complex eyes in the case of box jellyfish) allow the animal to
respond to light. Many jellyfish display distinct phototaxis: they
come to the surface of the water during cloudy weather and at
twilight, and move downward in bright sunlight and at night.
Some feed on the organic particles that float around them as
they drift through the sea. Others capture small fish that blunder
into them. Since food may come from any direction jellyfish are
radially symmetrical with a ring of tentacles to intercept and
convey the food to the central mouth. Most jellyfish belong to
the order Semaeostomae and have bell-shaped medusae with
a scalloped edge bearing numerous tentacles that catch the
prey. The mouth is divided into four frilly, oral arms that open
into a stomach with radial canals: a common example is the
red-banded jellyfish, C h rysaora hysoscella. Jellyfish of the ord e r
Rhizostomae do not have tentacles, and instead use their
deeply-folded oral arms to capture tiny zooplankton. Rather
than a single mouth, there are a number of small secondary
mouths that open
into the stomach by
way of canals in the
oral arms. A
common
example of this
order is the root-
mouthed j e l l y f i s h
R h i z o s t o m a.
Giants up to one
Life cycle of red-banded jellyfish
gonad
egg
stomach pouch
metre in diameter often wash ashore.
planula larva
(2 mm)
ephyra larva
(3 mm)
Root-mouthed jellyfish
sedentary
polyp-stage
schystostoma
larva
buds off
small mouths