Eight Eyes but no Brain
Surely an animal with eight eyes would also have a clever brain to sort out what it sees from all directions at once? Not necessarily so.
The Australian box-jellyfish has eight eyes: a compound pair of primitive eyes on each side of its cube-like bell; one looking up, one looking down; looking inwards through its own transparent tissue and out the other side. But it does not have a brain to organize or interpret the visual signals.
Box-jellyfish are known more for their lethal venom than their eyesight. False folklore has it that they 'attack' humans or hunt prey, and eight eyes would certainly help in that pursuit. However, the reason for its complex vision reveals a placid, defensive, but still lethal animal and a lesson for humans in how learning more about them helps us to live with them.
Martin Jones, Director of Reef HQ (formerly known as the Great Barrier Reef Aquarium), Townsville, says scientists have known for about a hundred years that a certain group of jellyfish have eyes and avoid dark shapes. In 1995, he and marine biologist William Hamner from the University of California set up experiments at Reef HQ to test how effective box-jellyfish vision was without a brain.
"There's no thought involved," Mr Jones says. "They have don't have a central nervous system but they do have a nerve net in their tissue. It's really a simple feedback loop, a reflex action. We believe that when a shadow goes across the eye, it changes the swimming behaviour so that the muscles on that side or the opposite side start contracting and the pulsating causes the animals to turn."
In the experiments, Jones and Hamner isolated groups of up to twelve box-jellyfish in a transparent raceway or in a rectangular tank. Under controlled lighting conditions they placed various sized black or white shapes at either end, outside the tank without disturbing the water or introducing a chemical response, then measured the reactions.
In every case the jellyfish quickly retreated from the dark shapes and ignored the white ones. The larger the dark shape, the further and faster they swam. In one experiment, black and white pipes were placed upright at random inside the tank. The jellyfish bumped and knocked over the white pipes trying to avoid the black ones.
An obvious interpretation for avoiding larger black shapes is to escape from predators such as turtles or fish, or to navigate around large obstructions. But the jellyfish even recoiled from a thin black strip one centimetre wide, with a few reacting to a half centimetre strip. Surely this is not a predator?
"No," Mr Jones explains. "But it is a mangrove root or other piece of floating debris like mangrove seeds that float vertically in the water. It's not in their interests to get trapped in a bunch of mangrove leaves or roots. Although jellyfish are transparent, they have tissue like any other animal."
Jellyfish tissue is delicate and easily damaged by abrasion and is then prone to bacterial infections like other animals. The most difficult aspect about keeping jellyfish in captivity is to stop them rubbing against the container. Their natural home is open water, without walls, where they rarely come into contact with anything other than prey.
Mr Jones says people who catch harmless types of jellyfish for home aquariums find them so hard to keep because people usually catch them in a bait net, then drag the net up the beach, lift them out, put them in a bucket and think they are going to live.
"It's like dragging somebody behind a car for 100 metres, picking them up and putting them back on their bike," he says.
Any aquatic animal so vulnerable to bumps and scrapes would not survive long without all-round vision, fast reactions, excellent swimming ability and a defensive instinct to flee from dark shapes in the water.
Box-jellyfish do not hunt prey. They glide through shallow inshore waters where their food supply lives, trailing dozens of long fragile tentacles, stinging whatever small fish or shrimp happens to drift into them. The near-instant death from the venom avoids a struggle which could also damage their tissue.
As for humans, Mr Jones says it is remarkable that more people do not get stung. "When we were collecting jellyfish at Cape Pallarenda one day, we caught about twenty, and in the same place there were fishermen standing in water up to their knees, line fishing and bait netting. So we assume the reason more people are not stung is that, most of the time, the jellyfish manage to avoid them."
Several reports of stings tell of people running or jumping wildly into the water and, unfortunately, straight into a box-jellyfish. On contact, millions of stinging cells like microscopic darts on the tentacles instantly fire venom into the skin. This is an automatic reaction to skin's chemical 'taste', not a deliberate attack. The cells react to the 'taste' and feel of human skin just as they do with animals stung for food.
A human sized body needs a large dose of venom to be fatal, but the more tentacle that comes into contact with skin, the more deadly venom is injected. Heart failure in the victim rapidly follows a severe sting. Immediate action can save lives. Vinegar flooded over wounds and tentacle fragments kills the stinging cells without releasing more venom.. Fragments should never be touched before the vinegar wash.. If the victim is resuscitated and rushed to a hospital, fast-acting antivenom will increase the chances of recovery without further complications.
This reaction to taste and feel of human skin also explains why nylon suits, panty hose, indeed any clothing cover, works as protection. Cloth does not taste like food, so the tentacles simply brush over it.
After a box-jellyfish fires off its stinging cells, it must grow new ones. "It takes a lot of energy to produce new stinging cells and that very potent venom," Mr Jones says. "The free swimming stage [of their life cycle] requires a significant amount of energy, so they're not going to waste it. They don't just madly sting anything."
Before the box-jellyfish was first described scientifically in 1956, people talked vaguely, fearfully, of 'stingers' and 'sea wasps' because they could not see the invisible, unknown creature that seemed to strike at random in warmer months, inflicting extreme pain, disfigurement or sudden death.
Inevitably, ignorance gave way to knowledge. As Mr Jones says, whenever humans do not know something, they are driven to discovery. Intensive research on the box-jellyfish began in the 1960s with Cairns-based doctor Jack Barnes. He was driven to discover its anatomy, where it lived, how it caught food, and to understand and thus predict its behaviour, so that beaches would be safer for humans. This knowledge was crucial to the discovery of the antivenom in the 1970s.
Mr Jones' says understanding the animal's behaviour teaches us to accept that box-jellyfish have a right to inhabit Australia's tropical coast just as humans do. He said the one of the payoffs of research is that wherever one travels now on North Queensland's coast there are stinger nets. Had scientists not done the research we would not have known that stinger suits and nets would protect swimmers against the sting.
Mr Jones says that scientists could not expect to change jellyfish behaviour, but could try to train humans to change their own behaviour. "I think there's always an early expectation that you can get rid of the pest or find a solution that allows humans to go on doing what they like to do. With box-jellyfish, it's ridiculous to assume that we are going to get rid of them. Although they are simple, primitive animals, they have done very well for themselves. They have been around probably two or three hundred million years, so we have to learn how to manage with them."
"Box-jellyfish don't have brains, we do. So it's up to us."