"The magnificent jellyfish"
by Bevan Pank
One day last summer, many visitors to a popular Cape
beach had their holiday ruined by the intimidating presence of a huge washed-up
jellyfish. It was the largest known species - a Root-mouthed Jellyfish - which
averages 0,30 m, but can reach 1,5 m in diameter. The bathers did not realize
that this species lacks stinging tentacles and poses no threat!
To prevent the spread of false rumours
overseas, people in tourism need to assure visitors that South African waters
are virtually free of lethal marine invertebrates. Some give a painful sting,
but it can usually be relieved with a topical treatment of common household
vinegar. A notable exception is the bluebottle’s sting, which should rather be
treated with ice and cold water. Beach constables and life savers should also be
able to reassure the general public, especially on the often exaggerated danger
Although our box jellyfish inflicts a
painful sting, it is not as dangerous as its Australian relative - the sea wasp.
Nevertheless, according to the Great Barrier Reef Marine Park Authority in
Queensland, "the use of vinegar on box jellyfish stings disables the
nematocysts thus preventing any further injection of toxin." The term
"nematocysts" refers to stinging cells, that can fire harpoon-like
coiled hollow threads full of poison at a velocity of two metres per second with
an acceleration of 40,000 times gravity!
Despite the awesome firepower of some
jellyfish species, it is very important that children be taught not to hate
them. Their weapons are for defence and to immobilize prey. They obviously do
not deliberately target bathers, who foolishly ignore their presence and touch
the hair-triggered firing mechanisms. Furthermore, it is not their fault for
being driven inshore by wind and currents. On the positive side, they are a food
source for ghost crabs, plough snails and certain turtle species. There are also
those that shelter reef dwelling fish, crabs and shrimps. Perhaps we should
therefore get to understand these often misjudged animals.
Most large jellyfish are zoologically
classified as the Scyphozoa, of which some 200 species have been described. This
name is derived from the Greek - skyphos for "cup" and zoon
for "animal". Together with corals, sea anemones and their kin, they
belong to the major group or "phylum" Cnidaria. This is also derived
from the Greek - knide for "nettle". Incidentally, a rich
fossil record of the phylum dates from over 650 million years ago!
Developing from a fertilized egg, the
jellyfish typically becomes a free-swimming larva before it settles as a little
polyp. Without the mouth’s fringe of long tentacles, this resembles a classic
Greek urn. It then grows into a tall and slender cylindrical shape ringed with
saucer-like buds. These develop stubby forked arms as they move up the column
and launch themselves one at a time as young medusae. This budding process is
skipped by at least one species of box jellyfish, which changes directly from a
polyp to a medusa.
Although jellyfish comprise mainly sea
water, they are more complex than is generally believed. Most belong to the
order Semaeostomeae, in which their disc-shaped bell has a scalloped edge
bearing numerous tentacles with stinging and sensory cells. The mouth is drawn
out to form four frilly lips beneath the bell and opens into a central stomach
with four partitioned gastric pouches. Each has an opening for water circulation
and a muscular lobe with often colourful and clearly visible reproductive
organs. The inner margin bears filaments containing stinging and gland cells,
which respectively subdue still active prey and produce enzymes to break it
down. The processed food is then distributed through a system of many radial and
often branched canals.
The body wall of jellyfish has three
layers. These consist of an outer epidermis, an inner cellular lining and a
thick intermediate gelatinous layer with powerful circular muscles. Since the
muscle fibres have elastic recoil for producing swimming pulsations, the body
wall reforms with minimal energy loss. Some species - especially among box
jellyfish - are fast swimmers, with a flap on their outer margin folding inwards
to form a nozzle. On ejecting water from their muscular cavity, they then become
In the case of our harmless root-mouthed
jellyfish, minute plankton trapped in mucus under its huge bell are driven by
whip-like structures to the edge. The prey is then sucked into a mouth formed
into intricate folds with numerous tiny ducts, where it is digested to extract
the juices. Another species accommodates an alga in a symbiotic relationship and
survives entirely on the products of photosynthesis.
To control its pulsations, the box
jellyfish has a nerve ring. The common scyphozoans have paired finger-like nerve
nets on each scallop. Between some of these nets are club- shaped structures in
multiples of four with hooded sensory areas and pigment spots. One area is
dedicated to numerous balls of lime and their movement when the bell tips
signals muscles on the drooping side to contract more vigorously. Another area
is dedicated to photo-receptors that gauge the intensity of available light. Box
jellyfish even have eyes with lenses and a retina-like arrangement of sensory
Whereas twilight and cloudy weather draws
jellyfish to the surface, bright sunlight keeps them fully submerged. This
phototaxic aspect is very important, since their planktonic prey follow a
similar migratory pattern. They are actually striking a balance between surface
and opposing sub-surface currents. In effect, they maintain horizontal positions
in the most viable feeding situation with minimal energy loss.
Of course, this is a very simplified
explanation of how the jellyfish "tick". It will hopefully indicate
that although a nuisance to fishing fleets, they play very useful roles as
shelter for some animals and food for others. They are also so complex as to
warrant respect. More important, not all of them are dangerous and those that
sting should never be labelled with the same aggressive instincts as mankind.
On a lighter note, some zoology students
at the University of Cape Town once missed an important lecture to loaf on a
popular beach. Bearing in mind that jellyfish reproduce by releasing eggs or
sperm into surrounding water, the loafers later explained that they had gone to
watch jellyfish mate! In commemoration of this lame excuse, the entire campus
has since been decorated with paper jellyfish and jelly sweets are handed out
every anniversary of "Jellyfish Mating Day".
My thanks to the Immelman Science &
Engineering Library at the University of Cape Town for access to research
papers. I also thank Dr. Mark Gibbons of the Zoology Department at the
University of the Western Cape for his technical input.