Sunday, August 19, 2001		Nature

NEMATOCYSTS or stinging cells are the weapons of all sea-anemones, jellyfishes, and coral polyps. These deadly cells contain a coiled, barbed thread, resembling the finger of a rubber glove pushed in. The cell is capped with a lid and has a trigger on the outside. Whenever a prey blunders into the tentacles, pushes against the triggers of hundreds of nematocysts, numerous miniature harpoons are automatically fired. The lid goes off and the coiled barbs shoot out, entering the flesh of the careless victim. The discharge is so fast that it is impossible to escape after touching these venomous cells.

Barbs shoot out at the speed of 2 metres per second with an acceleration of 40,000 times the force of gravity, about 10,000 times that experienced by astronauts at take-off. The harpooning is over in three-thousandths of a second, making it the fastest cellular process in nature.

Even though they are microscopic in size, the stinging cells of some jellyfish and Portuguese men-of-war are so strong that they can penetrate a rubber glove and human skin. A single jelly might produce several different poisons. One poison stops the heart of the victim, another disrupts the blood system by destroying red blood cells, and a third attacks the brain. The poison is a powerful neurotoxin related to that produced by the king cobra, but it takes about 25,000 nematocysts to make a milligram.

Although the Portuguese man-of-war is considered dangerous to man, few deaths have occurred directly from its sting. Usually, a bather is stung over the body and drowns accidentally. This creature is colonial hydroid and consists of several different types of polyps, known as ‘persons’, that join together for the greater good of the cooperative. Some specialise in digestion, others in reproduction, floating or catching food. The 10-30-cm-long float, which is frilled along the top, is filled with gas secreted by a special gland.

Periodically, the float is collapsed first to one side and then the other to moisten it and to protect the delicate membrane from drying out. When inflated, it sits on the surface of the sea, where it is blown about the ocean by the wind. Under certain conditions hundreds of men-of-war are blown together in huge rafts.

The flattened, frilled part of the bladder is angled at 45 degrees, which means that the animal actually sails. Men-of-war in the northern hemisphere have their sails set differently from those in the southern hemisphere, some being right-handed and others left-handed. The vast array of tentacles below act as a drogue or sea anchor, helping to steer a course at a set angle to the wind. If caught in westerlies, the sail aligns south-west to north-east, the tentacles trail to the north-west, and the creature makes progress towards the south-east. By angling the sail, the Portuguese man-of-war avoids being blown into areas of doldrums like the Sargasso Sea.

The pink-and-blue curtain of tentacles can reach down 30 feet or more below the float and they can catch fish as large as mackerel. Once caught and paralysed by the ‘prey-catching persons’, the prey is hoisted up to the mouth, where the ‘digestion persons’deal with it and distribute the nutrients to the entire colony.

A relative of the Portuguese man-of-war is the by-the-wind sailor, and it too sails in the wind. Instead of a single, large, gas-filled bladder, the by-the-wind sailor has a 10 cm diameter, a circular disc of gas-filled chambers which keeps afloat. Like the man-of-war, the creature is a colonial one with reproductive persons surrounding the central mouth below the disc, and stinging persons fringing the disc, but its most interesting feature is the very obvious ‘sail’ on top of the disc. It is also angled so that the colony sails like a yacht, and flotillas of by-the-wind sailors all travel together in the same direction.

Floating along with them are often purple sea-snails. These voracious snails float on a raft of bubbles and feed on these animals. They eat all soft parts, leaving the circular disc to which they sometimes attach their eggs.

The sea-wasp, however, is deadly. It is one of the box jellies with a small bell about 10 inches across and 66-feet-long tentacles. It is the most dangerous sea creature known, for it can kill a person in just 3-8 minutes.

Some creatures have come to terms with the toxic tentacles of the jellyfish and sea-anemones and use them as weapons. The blanket octopus, for instance, snips off manageable pieces of Portuguese man-of-war tentacles and uses them to sting prey such as shrimps. Similarly, the boxer crab grabs a chunk of sea-anemone in each claw and brandishes them like boxing gloves.