"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 of jellyfish.

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 jet-propelled!

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 cells!

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".

Acknowledgements

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.

Originally published in modified forms in Océanorama (France) Dec. 1993, Divestyle Nov. 1995 and Earthyear June 1999. 


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