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Defence and Deception on the Reef

Updated: Aug 30, 2019



Defence and deception on the reef

by Brian Maudsley

It's a cut-throat world for the creatures of the deceptively tranquil Indian Ocean. have each had to evolve different defensive strategies in order to survive.

A coral reef is deceptive in its tranquillity. Every clump of seaweed, every crevice can hide a multitude of dangers; predators can dart in, without warning, from the clear, blue sea. To minimise these dangers the animals in the community have to adopt a variety of survival strategies to make them as unattractive as possible to predators or to make them nearly invisible. Spines, poisons and armour accomplish the former; camouflage and stealth the latter. Every colour, every shade and pattern on each living animal is not an accident of nature but the result of millions of years of evolution. What we see now is a community of animals perfectly adapted to their particular environment. The specialisations which provide safety to some of these creatures are often bizarre but always effective.


Many marine animals use poison. They may have poisonous flesh or may be able to inject venom into the prey through barbs resembling the fangs of a snake. The stone-fish, Synanceichthys verrucosus, is the most infamous of the venomous fish as it is able to deliver a lethal dose of poison into the foot of even a casual reef walker. This ability is combined with immobility and a camouflage so perfect as to make it very nearly impossible to see, even in an aquarium. After a long examination of one at Shelly Beach, in about 5cm of water, it was only when I was moving away that I noticed a second stonefish a few centimetres from my foot. Since then I have been made to wear tennis shoes lined with galvanised iron inserts when walking in shallow water! Extreme measures maybe but reports of the results of stonefish stings are extremely unpleasant. The immediate result is excruciating pain; if the victim survives, paralysis and weakness last for many months afterwards. The poison comes from a line of 13 short, hard spines, protruding slightly from wart-like growths running along the back. Each spine is grooved, the channel being used to transfer poison from the venom sacs at the base. When trodden on, the spines deliver poison in an all or nothing manner. The stonefish does not use its venom to kill its prey. As the smaller fish drift overhead, quite unaware of its presence, the enormous mouth opens, the stonefish lunges upwards and the prey is sucked in and devoured. The need to produce such a powerful poison illustrates the dangers of reef life. Its effectiveness is undoubted: stonefish are widespread and successful animals and very highly specialised for their particular mode of life.


There are other fish with dangerous spines such as the surgeon fish,Acanthurussp. It has a scalpel-like blade on either side of its tail, which, with a twist of the tail, can be brought into action faster than any flick knife. These are not poisonous, however, although they can inflict a nasty gash. But the scorpion (or lion) fish, Pterois volitans, are venomous. Unlike the quick and timid herbivorous surgeons, they are slow and carnivorous. Rather than trying to run their prey down, the small fish and prawns are gently herded into a corner and eaten at leisure. Sometimes small fish mistake the waving fins for shelter and actually swim unknowingly towards their demise.


The scorpion fish rely on deception. Their fins are elongated into a mass of delicate, creamy-brown filaments and the fish drift gently around the coral pools looking pretty and harmless. Such a slow creature would be easy prey for other carnivores if it were not for its poison glands and vicious spines. Though reputed to be not as lethal as the stonefish, a brush against the spines by a swimmer's hand or foot could end tragically. The small barbel eels, which often occur in schools in the lagoons, also have poison spines on the back; to touch one can cause great pain.


Every organism must have some means of defence, thus the slower and more conspicuous the creature the more likely it is to be poisonous. Many of the brightly coloured little sea slugs, the nudibranchs, are poisonous or have a very unpleasant taste. Many sea cucumbers have toxic insides which they throw out—guts, respiratory apparatus and all—at a moment's notice. This may seem a rather drastic response, but regeneration occurs quite quickly, and the animal resumes a normal life of shovelling sand in one end and out the other.

One very interesting case of a poison defence has been seen in a little brown-speckled, whitish flatfish with distinctive golden markings on the upper side, the Moses sole, Pardachirus marmoratus, which is quite commonly found in sandy areas of the lagoons. When placed in aquaria with other fish, the rest quickly expired. When it was dangled in front of voracious moray eels, they took half-hearted lunges then retreated rapidly into their holes. Most interesting, from a human point of view, was the reaction by sharks. In tank and open-sea experiments, sharks would not touch them although they readily ate anything else offered to them. One researcher described a tank shark which took a live sole into its mouth but could not bite on it and tore round the tank in a frenzy; the poison seemed to paralyse the jaw muscles. Yet these fish are often eaten by people in Red Sea areas, with no ill effects; it seems that cooking destroys the poison.


In all these examples, poison is only used for defence, but some marine animals do use poison to catch their prey. The cone shells, Conus sp., have a poison barb which can be shot out at a great speed to penetrate the prey. Poison is injected and the paralysed animal eaten. Some cone shells are potentially lethal to man, these include the textile cone, Conus textileand the geography cone, Conus geographus. People have been killed by them, so it is a wise precaution never to pick one up. Sea snakes attack like their land cousins but it would be a rare occurrence to encounter one along the Kenya coast.

The coelenterates, the corals, sea anemones and jellyfish, also poison their prey. The tentacles of these creatures bear microscopic stinging cells each with a tiny needle-like hair. These are fired when stimulated and inject 'toxin into the prey. Most are harmless to humans but the fire corals, Millepora sp., and several jellyfish can deliver a very painful sting. No lethal types are found off the Kenya coast though




the Portuguese Man o' War, Physalia physalia, is potentially dangerous. These stinging cells can be used to deter predators and even the crown-of-thorns starfish, Acanthaster planci, avoids eating the corals which have more effective stinging cells, such as most of the massive brain corals.


The crown-of-thorns starfish is well able to protect itself. It is covered by a mass of hard, sharp spines which are coated with a toxic substance. In the Pacific, if the islanders tread on one and the spines penetrate the skin, they pick up the starfish and apply the underside to the wound. Small, sucker-like tube feet reach out and stick onto the flesh and, so the theory goes, suck out the poison. Despite having these spines this starfish has enemies. The triton shells, Charoniasp. attack and eat them, as do puffer and trigger fish.


The crown-of-thorns are infamous for their great variation in numbers, occasionally causing plagues which devastate whole reefs. The reasons are not very clear, but these population explosions could well be caused by humans reducing the numbers of their enemies. Triton shells are very attractive and easily marketed. Because of the shell trade they have all but vanished from the Kenya coast.


A final, common poisoner is the sea urchin Toxopneustes pileolus. It has no common name but Toxopneustes describes its abilities well: it certainly is toxic. Its body is covered by short spines, but these are nearly hidden by pretty, pinkish discs about 2mm in diameter. Each of these is supported by three sharp pincers known as pedicellariae and are connected up to large poison glands. They truly 'look like the innocent flower but be the serpent unerr't' (Macbeth, Act 1, Sc. V). Should an unwary hand get too close the result can be lethal. Japanese divers fear this urchin, as a sting while in the water could easily be fatal. Again, it seems that the poison is purely defensive, although suggestions have been made that small animals settling on the surface could be stung and the bodies then passed to the mouth. As they feed by rasping on alga-covered pebbles this idea seems unlikely. Spines need not be poisonous to be effective. The majority of sea urchins have sharp spines, but the best local examples are the Diadema urchins. The name refers to the jewel-like iridescent blue and green lines which form a delicate tracery over the deep black body surface. Their hollow, barbed spines may be 30cm or more in length and are very brittle. It only needs a slight touch for the tip to become irretrievably embedded in the skin. They are not poisonous but can be painful. The pieces soon dissolve without trace. Diadema is very common around the reefs and feeds on plant material during the night. In the daytime they tend to cluster together in large groups, often wedged into crevices. The mass of vicious spines is very sensitive and when a shadow passes over them, or if they are touched, they wave about wildly, pointing toward the disturbance. Some fish, such as a black-and-blue-striped cardinal fish and a small striped shrimp, actually seek shelter in the spines. Diademais not immune to attack. Trigger fish have been seen blowing isolated urchins over with a jet of water and attacking the unprotected lower side.


A rather bizarre fish, the porcupine fish, Diodon hystrix, is able to inflate its body with water, causing the spines which cover its skin to stick out in all directions. There have been cases of sharks found dead with a Diodon lodged in the throat.


Armour need not be spiky. The aptly-named box fish Ostracion tuberculatus, for example, swim slowly around the reefs at all hours of the day, seemingly without a care in the world. Their defence is hinted at by their angular shape. Under the skin is a very strong, rigid box of bone firmly welded together. If a predator should ever manage to penetrate the armour it would have little satisfaction because the flesh is poisonous. This may not be much help to the individual victim but is a great help to the species as a whole; predators soon learn to avoid them. Other groups of animals which use armour plating are the crustacea, as anyone who has eaten crab can testify, and the shelled molluscs, whose soft bodies would be easy meat otherwise.


Camouflage is used extensively by predators and prey. Many marine animals are adept at looking like pieces of rock or seaweed. Their capacity for changing colour is often astounding, leaving the humble chameleon looking like a fumbling amateur. One of the most dramatic of the colour change magicians is the octopus; colour serves as a camouflage and also changes with `mood'. They can change to greens, browns and greys, plain or mottled, and even alter the texture of their skin to match that of the rock they sit upon. When 'angry' or attacking, a red flush can pulsate across the body and they can go 'pale with fear'! Unfortunately, this is inadequate against the steel hooks of the fishermen, and hundreds are taken daily from the coast.


Many reef fish fade at night. They often settle into a crevice and remain there until daytime. The parrot fish secrete a poisonous mucous cocoon around themselves as well. Some fish change colour with their background, such, as the small shark repelling sole, Pardachirus. Cave-dwellers and nocturnal animals are often red. This is especially effective in deeper water as red appears black due to the fact that red light is absorbed by the upper layers of water. Some shrimps are very nearly transparent. The colours of marine animals are so varied and serve so many functions that it is impossible to do them justice in a short article. A few creatures do not rely on their own resources for protection but borrow from others. Crabs can be seen adorned with living sponges and seaweeds. Some hermit crabs place sea anemones on their shells, carefully transferring the anemones when they change to a new shell. The anemones provide for the crab with protection; both animals share the crab's meals. These varied specialisations for survival are just a small sample of the immense variety which can be found. The fact that they exist underlines what a cut-throat world it is in the warm Indian Ocean. The impression that one gets from even a cursory survey is that there is no room for mistakes, the animals that made them no longer exist. An animal out of its habitat has little chance. If you turn over a rock near the reef there is a rapid attack, by all sorts of fish, on the unfortunate animals which are exposed. A black brittle-star under the rock is safe and sound; in the light of day it is merely a tasty morsel. An injured or sick fish it rapidly eaten. Truly, the survival of the fittest.


Poisons, spines, armour and colour are all different facets of survival strategies. Some organisms have multiple defences, others are more sparing, relying on speed, stealth or size like the barracuda and the shark. Although all the interrelationships have not been worked out, and probably never will be, the results are there to be observed and enjoyed by any snorkeller or reef-walker. A coast holiday can take on a new dimension and provide some memorable moments.




1. Stonefish 2. Blue-spotted lagoon ray 3. Scorpion fish 4. Barbel eel 5. Barracuda 6. Moray eel 7. Shark 8. Portuguese Man o' War 9. Stinging corals 10. Diadema urchin 11. Crown-of-thorns starfish 12. Textile cone

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