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Box Jellyfish: Not Dainty Delights

Box Jellyfish: Not Dainty Delights


Among living creatures, it does not take a specialist to realize that jellyfish are not dainty delights of the deep. Although they may be beautiful in appearance, jellyfish are successful predators.

Basically these creatures are composed of two layers of skin cells with a jellylike layer separating the inner and outer tissues. In the context of multitasking, some skin cells of the jellyfish double as muscle cells. Cooperative action with other similar cells enables the organism to move. In addition, some cells are shed from the outer skin layer into the inner layer of jellylike material. These cells then organize themselves loosely into a net of nerve cells. There is no brain.

Despite this apparently simple structure, these organisms are not simple blobs. Jellyfish consist mainly of a bell or umbrella shaped body with tentacles hanging down from the bell. On the underside there is an opening into an interior digestive cavity. The mouth serves both to take in food and to eject the waste. Most jellyfish catch their food by chance encounters with suitable specimens. It is a case of wrong place and wrong time for the victims.

All jellyfish are armed with unique stinging cells called nematocysts. Each such cell is armed with a potent poison, always a protein and usually a paralyzing nerve toxin. So strong and fast is the stinger release that it is able to penetrate such surfaces as crustacean exoskeletons, fish scales and human skin. Normally many nematocysts release at the same time.

Jellyfish have long received a bad press as far as humans are concerned, initially because of the Portuguese man-of-war. This fearsome creature has a float visible above water level. The tentacles however can extend up to 9 m from a comparatively tiny 12 cm long central body. After catching even a relatively large fish, the tentacles quickly contract to one hundredth of their original length. This extreme contraction of muscle tissue is most remarkable. Other organisms cannot even begin to match it. Obviously the mechanism by which this happens is not as yet understood.

In Australia, since the late 1800s, records have been kept of human deaths from jellyfish. Until 1943, all were attributed to the Portuguese man-of-war. It was then, that biologists began to pay attention to cubozoans or box jellyfish.

The first confirmed death from a cubozoan was in 1955 near Cairns on the Queensland coast. During the next 45 years, 59 more deaths were attributed to these jellyfish. Often victims die within five minutes. Such notoriety has certainly focused attention on these organisms.

Cubozoans or box jellyfish derive their name from their unusual shape. Unlike most jellyfish which have a round bell shape, cubozoans are square when viewed from above. There are about twenty species to be found in tropical seas around the world, but the sea wasp Chironex fleckeri of northern Australian waters is among the deadliest creatures known. A large adult may reach 35 cm in diameter with as many as 15 tentacles, each 3 m long, extending from each corner of the bell.

The shape is not the only noticeable thing about cubozoans. Unlike most jellyfish which merely happen upon their victims, cubozoans actively pursue their prey. Naturally the experts have wondered how these creatures are able to exhibit such complex behaviour.

Box jellyfish are somewhat more complicated than other jellyfish but the pursuit of their victims is their really exciting feature. By contracting muscles in the bell, as well as by contracting the rim of the bell to force water out faster, Chironex has been observed to cruise as fast as 2 m per second! More typically they move at 1 m every five to ten seconds.

These creatures avoid crashing into objects like mangrove roots in their favourite habitat. They can avoid capture and they pursue their victims much the way predatory fish do. It is apparent to everyone that these jellyfish must be able to see. Recent studies have shown that this is indeed the case, but these new insights have led to even more puzzling questions.

Even ordinary jellyfish have special sense organs near the margins of the bell. Each organ contains two sensory pits to perceive chemical stimuli (like our sense of smell), one spot sensitive to light, two structures sensitive to touch, and one body which indicates orientation (upside down, sideways etc.) Biologists have long known that ordinary jellyfish only swim when they have at least one of these organs functioning. If all are destroyed, the jellyfish stops swimming and dies.

Similarly box jellyfish have sensory organs, but theirs are much fancier. Box jellyfish have four sensory organs in the form of small clubs attached by a stalk to the bell proper. Inside the club there are four pigment filled pits to perceive light. In addition there are two much larger eyes. The interesting thing about the two larger eyes, one of which faces upward and the other outward, is that these are camera style eyes. The questions that arise from this fact are: why do these jelly fish have eyes like animals with backbones (vertebrates), what do these jellyfish see, and how are they able to respond without a brain?

The camera style eyes of box jellyfish have all the appropriate components: cornea, lens, retina, a pigment layer and an iris. All parts function too. In the lower eye, even the pupil contracts in response to bright light. This choice of eye design is indeed unexpected. The surprise is that supposedly extremely simple organisms possess such fancy eyes.

Most animals without backbones (invertebrates) have compound eyes (as in insects, for example). However there are a few invertebrates such as squid and octopus, famous for their intelligence, which enjoy camera style eyes. One expert, Simon Conway Morris declares that “camera eyes are not only different from compound eyes; they are better.” (Life’s Solution p. 160)

So we have a supposedly primitive organism with the best type of eye. The wonder of it is that cubozoans have no obvious brain to interpret messages from their eyes.

The question still arises as to precisely what the box jellyfish eyes perceive. The scientific journal Nature (May 12, 2005 issue), published a report on such a study. A team of scientists studied the optics of the lens from the cubozoan eye. They already knew that this protein is unique, unlike that from any other eye. Thus the jellyfish eye had a separate origin from other organisms with eyes of similar design. On this issue the scientists muse: ” Making good lenses seems to be a demanding task, because only a few animal phyla have accomplished it.” (Nature p. 202).

How is it then, that the jellyfish managed it? That issue however was just the beginning of the paradoxes. What the team discovered is that the jellyfish lenses are so well constructed that the focus is very sharp (p. 202). A commentator in the same issue of Nature, points out that the lens design involves highly sophisticated optics, conforming exactly to optical theory. It is amazing how clever those jellyfish must be.

There is however another remarkable feature of the cubozoan eye. The retina, which receives the image from the lens, is much too close to receive a sharp focus. These jellyfish are like humans who need glasses. The commentator points out that this situation leads to “a loss of fine visual detail that the lens is able to provide.” (Nature p. 159) Now the questions become really pressing. The commentator asks “But what are the jellyfish’s eyes designed for?”

The scientists conclude that the jellyfish may navigate better with a blurry focus of large objects rather than precise images of irrelevant small objects in the environment. From an evolutionary point of view, it seems incredible that so fancy a lens would arise when something less precise would be adequate. How would natural selection select for a sharp focus when that wasn’t being used or needed?

Surely in this case it is obvious that we are dealing with intelligent choice rather than evolutionary processes. This is evidently an instance where the Designer selected features on the basis of artistic interest rather than mere utility. Since these creatures lack a brain, scientists wonder how they see.

All we know is that cubozoans live as they were designed to live, with talents and design features beyond our understanding. Indeed we should feel humble at the creative finesse, which we can never hope to duplicate, that we observe in nature.

Margaret Helder
July 2005

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