Know your jellyfish: some sting, some don't by Dr J Floor Anthoni www.seafriends.org.nz/indepth/jellyfish.htm
Jellyfish paintings by Sir Alister C Hardy, 1956 (1896-1985)
The
jellyfish that we both admire and hate today, could have stepped out of
a lost world dating back four hundred million years before the dinosaurs.
They are old, primitive creatures, yet so effective that they hold their
own in our modern world. Scientists would be interested to see what they
actually looked like when they first appeared on Earth, but their soft
bodies left few fossil traces. In this article we'll examine the few large
species found in New Zealand waters. But jellyfish are also harbingers
of pollution that kills other marine organisms.
.
The jellyfish of the world are such effective ocean wanderers that they
haven't diversified very much. With about 200 species world-wide, one could
easily learn to identify them. But in New Zealand waters, jellyfish are
not predictable either in numbers or species.
Like the other members of the cnidaria or stinging animals, jellyfish
are radially symmetric, resembling flowers. Their structure is simply a
bag with cells on both the inside and outside with a mouth in the middle.
Most have two life cycles (like butterflies and caterpillars), alternating
between free swimming medusa (bell) and attached polyp, with a sexual and
a budding type of reproduction. Although primitive, they nonetheless represent
a pinnacle in elegant design and locomotion. Watching a jellyfish move
through the water, lit by the flickering light broken through the water's
surface, is always a breathtaking experience.
All cnidaria have stinging cells adapted to stun and catch prey (see
box). As jellyfish swim, they are aware of how they are swimming by means
of a balancing organ, and they are also somewhat aware of their environment.
One can observe them swimming almost horizontally upward until deciding
to swim downward again.
As their name suggests, jellyfish store large amounts of jelly. It makes
them rather inedible because jelly contains almost exclusively water with
no substance. Yet several creatures have learned to prey on them, targeting
the good bits like gonads, which on a large jellyfish, can be a rewarding
meal. Some jellyfish can live for two years or more.
The stinging common blue jellyfish Cyanea lamarcki
varies in colour from deep blue to pale white. It can reach almost 2m across.
Its long tentacles hanging down from the bell's rim, can reach five metres
while being almost invisible. It fishes the sea by swimming actively, letting
its tentacles out and drawing them in again to eat the catch.
The slightly stinging purple jellyfish, Pelagia noctiluca
has only few catch tentacles trailing from its bell and five feeding tentacles
surrounding its mouth. It can grow 40 cm across. When first discovered,
it glowed at night, presumably because it was feeding on sea spark dinoflagellates
(Noctiluca
scintillans).
The sea nettle or brown jellyfish, Chrysaora hyoscella
can grow to 30cm across, trailing its 24 brown tentacles to 2 metres behind.
Its bridal train of four tentacles, often looks frilly. It can deliver
potent stings and can be present in large numbers.
The harmless common jelly or moon jelly or earlobe jelly,
Aurelia
aurita becomes only 20 cm across and its sting is not able to
penetrate our skins. Initially a fragile disc, it may acquire copious amounts
of jelly to become hard and soapy. Many small tentacles hang from its rim,
lashing small suspended particles onto its four feeding tentacles surrounding
its mouth underneath the bell. The four earlobes on top are its gonads,
which are eagerly sought out by leatherjackets and other fish.
The harmless sea mushroom jelly Rhizostoma octopus
does not have trailing tentacles to catch food. Its eight 'legs' are covered
with a network of mouths that catch particles fanned down onto them by
the swimming motion of the bell.
The portuguese man-of-war Physalia physalis and left
the by-the-wind sailor Velella velella are siphonophores, which
are colonies of specialised polyps that no longer cycle their lives between
medusa and polyp. The colony has specialised polyps for catching and stinging,
feeding, reproducing and inflating a bladder with carbon monoxide gas.
Physalia
delivers a burning sting but Velella is entirely harmless.
f030804: In 1999 this siphonophore (Praya dubia),
not seen here before, delivered painful stings to bathers. Its polyps look
like a long, woolly barbed wire, up to 2m long. When one end stings the
skin, it attaches, and in trying to rid oneself, more and more parts of
the string attach to one's skin. Eventually the whole string clings to
one's body, delivering powerful and painful stings.
f029609: one of the many beautiful jellyfish visiting New
Zealand's shores. This species remains rather small.
f029503: a minefield of jellyfish often chases swimmers out
of the water, since it has become impossible to avoid them. This photo
shows a plague of Pelagia noctiluca.
f029605: close-up of a Pelagia noctiluca jellyfish.
f002804: a plague of sea gooseberries, but no fear of being
stung. These firm jelly balloons, no larger than 3 cm, have ribs of vibrating
hairs (cilia), by which they swim and steer.
f016510: some jellyfish consist of joined-together seasquirts
like these rosettes, each consisting of some 10 seasquirts. The orange
bits are the seasquirt's gonads, a target for food.
stinging cells All cnidaria have stinging cells (cnidocytes), capable
of inflicting rashes, stings or even burns. The animals can make these
amazing cells at such rate that they can be used as dispensible tools.
They can have different types, for stinging, sticking and gripping. The
one shown in the diagram is of most concern to us since it is what causes
the feared sting. The skin of a jellyfish tentacle is studded with these
stinging cells. Each has a trigger hair (cnidocil) that activates a nerve
circuit to unlatch the lid (operculum) and at the same time compress the
cell. Several adjacent cells can be triggered simultaneously. Inside the
capsule, the harpoon and its stinging thread (nematocyst) are ejected under
pressure. The barbed harpoon is filled with poisons and everts itself
like turning a finger of a rubber glove inside out, punching a hole in
one's skin, releasing the poisons and pushing the thread into the punctured
skin, to release even more poison. Once used, a stinging cell is discarded
and replaced within two days. It is so amazing that such a complicated
double-walled structure is made by a single cell!
The
jellyfish that we both admire and hate today, could have stepped out of
a lost world dating back four hundred million years before the dinosaurs.
They are old, primitive creatures, yet so effective that they hold their
own in our modern world. Scientists would be interested to see what they
actually looked like when they first appeared on Earth, but their soft
bodies left few fossil traces. In this article we'll examine the few large
species found in New Zealand waters. But jellyfish are also harbingers
of pollution that kills other marine organisms.