When divers are stung by sea creatures - other than those microscopic jellyfish that take them unawares - it is usually because they have ignored the obvious. If a creature fails to retreat on your approach, suddenly exhibits bright colours or moves into a defensive posture, you can be fairly certain it has a defence mechanism that can harm you.
Generally, stinging mechanisms found around the mouth parts of marine creatures are used offensively, while stinging parts ranged along the back and tail are defensive in origin.
For the most part venom is used defensively, and different creatures use different toxins and potencies. Most cause localised effects such as numbness, irritation or paralysis; others kill nerves or blood cells, or attack muscles and affect internal organs.
The majority have a cumulative effect and cause several problems at the same time. In certain circumstances these toxins can even cause death in humans.
There are about a million reported cases of stings each year - most commonly from jellyfish, and particularly in tropical oceans where the Portuguese man-of-war and box jellyfish are prevalent. Around 50 of these stings lead to death.
Most jellyfish sting, though few are dangerous. They are primarily offensive stingers. The mechanism is usually a hooked barb, or nematocist, held inside a trap-door until released through touch or chemicals in the water, and fired by a hydraulic coiled spring. The hollow barbs are filled with toxins that are released as soon as the stinger penetrates its victim, the primary aim being to paralyse prior to ingestion.
When seasonal changes are in their favour you can encounter the lions mane jellyfish in British waters and the Portuguese man-of-war in tropical waters. These are highly toxic and continued exposure to the stinging cells could require hospital treatment.
Sea wasps and Chironex fleckeri can be found in shallow warm water at night and are attracted to light. These creatures often swarm and stings can be severe, causing muscle cramps, nausea and breathing difficulties. Chironex have trailing tentacles laced with microscopic, near-invisible stinging cells over 60 metres long. They have been known to kill a man in under five minutes.
Whenever the conditions are favourable for thimble jellyfish, be careful as that means they are favourable for micro-organisms too.
In the Caribbean recently I was stung by some unseen microscopic plankton around the exposed areas of my neck, ears and wrists. I had to resort to hydro-cortisone cream and wearing a protective hood and gloves along with my full wetsuit.
Local remedies are available for stings, but acetic acid - vinegar - is as good as anything. In cases of severe stinging medical attention is required.
Jellyfish belong to the same super-family as corals, hydroids and anemones, all of which have a surprisingly large number of harmful representatives. While most anemones will do no damage to the skin on your fingers, many can inflict painful burns on the softer skin on the inside of your arms or legs.
Species such as the berried anemone, found from Bermuda to the Red Sea and Indo-Pacific, have warty tubercles all over their stems, each berry being armed with lethal nematocists. The more common anemones use their barbs to hook and paralyse prey that swim within their sticky grasp.
The similar-looking corallimorphan covers large areas of dead coralline limestone boulders on Bermuda reefs. When disturbed, it produces sticky white filaments filled with nematocists.
Stinging hydroids such as Aglaophynia latecarinata in the Caribbean and Lytocarpus philippinus in the Indo-Pacific have harmless-looking, feather-like plumes that can inflict a rather nasty sting on the softer areas of your skin if you brush up against them. The most common stinger in this vast family is fire coral. This attractive yet painful coralline hydroid comes in a variety of guises, and although not a true coral exhibits many of the same characteristics.
The most common in the Caribbean is Millepora alcicornis, which tends to grow over and envelop various species of sea fans until it takes on the complete shape.
A closely related variety in the Indo-Pacific, Millepora dichotoma, is also branching in shape, with white tips to the golden-yellow body. This fire coral forms a hard structure similar to that of true corals and can be found in shallow waters, always aligned to the prevailing current.
Even the most innocuous-looking sea creatures can have a hidden battery of stingers just waiting for something to rub against them. A few vase sponges in the Caribbean have tiny calcium spicules that have an effect similar to that of glass fibre if rubbed against the softer parts of the skin. They can cause severe irritation, rashes and sores.
Fire worms might look cute but should never be handled. The clumps of white hairs along their sides display bristles when touched. These easily break off in the skin, causing a painful burning and intense irritation. Species of fire worm are found in all the tropical oceans and though not deadly their stings require treatment, principally with hot water and vinegar.
Species most associated with lethal stings are the members of the stonefish and scorpionfish families. Those found in the Indo-Pacific are considered to have the most active venom, in the modified hollow spines at the tips of their dorsal fins.
True stonefish are not found in the Caribbean but a variety of scorpionfish is. It is not considered dangerous but care should always be taken with the spines. Stinging can be treated by placing the affected area in very hot water.
Stonefish such as Synanceia verrucosa have been known to be fatal to man. As their name suggests, they inhabit shallow coastal waters and inflict damage on unwarily placed feet.
Lionfish, of which there are a number of representatives in the Indo-Pacific oceans, can cause a burn-like sting, again produced by modified fin rays.
Then there are stingrays. The ancient Greeks knew all about them: both Oppian and Pliny refer to their ability to kill a tree with their venom. If you are having an in-water experience with these creatures, such as at Stingray City off Grand Cayman Island, never attempt to grab hold of the tail or sit or stand on its back. The stinging mechanism is located in the tail and any undue force may cause it to spring forward in a reflex action, erecting the spine and causing serious damage.
In British waters, weavers prefer shallow coastal areas and lie partly buried in the sand. They have a venomous spine on top of the dorsal fin which, while not lethal, can cause extreme discomfort when stepped on.
A similar species is the stargazer, which has two venomous spines, one each side behind the gill covers.
Striped eel catfish (Plotosus lineatus) form large groupings or balls that move as one unit, and hide under coral reefs or wrecks in the Indian Ocean. They feed on small crustaceans, molluscs and fish that come out at night. At least three separate schools can be found on the twin barges wrecked in Beau Vallon Bay off MahÃ… in the Seychelles. Though easily approachable, care should be taken around these fish.
Not all stingers are large and obvious. Quite a few molluscs have stinging mechanisms, such as nudibranchs, which eat stinging hydroids and anemones and can store the nematocists of their prey in their own tentacles for defence against predators.
Cone shells in tropical waters have been known to inflict lethal jabs into unwary collectors who perhaps pick up the shell, thinking it empty, place it in a pocket and are later surprised when the creature wakes up.
Sea urchins and starfish are the last and most obvious group of marine creatures lurking to catch unwary and clumsy humans.
The spines of a number of sea urchins can be poisonous and, even if not, can puncture the skin through gloves, leaving painful wounds which can go septic. Though much rarer now in the Caribbean (because of an epidemic that almost wiped out the entire population), the long-spined urchin and a similar species in the Indo-Pacific, Diadema setosum, should both be avoided as the spines are brittle and easily broken off in the flesh.
As they are made of calcium, any such spines should dissolve after a few days, though deeply embedded ones can leave permanent scarring and patients may have to be treated for shock. Treatment includes rubbing with the juice and pulp of the paw paw fruit.
Another sea urchin to watch out for, particularly in the Red Sea, where it is very common on deep reefs and in the shaded areas of wrecks, is Asthenosoma varium. This soft-shelled urchin is bright red, with spines tipped with a whitish-yellow poison sac. These can cause severe pain and prolonged, infected wounds; fortunately they are hard to miss.
Crown-of-thorns starfish have venomous tips to the multitude of spines that cover the upper surfaces of their bodies. Divers should beware of them when diving inside wrecks, as exhaled bubbles can dislodge them. The starfish will then roll into a tight ball, spines on the outside, and if it lands on you, those spines can easily penetrate a wetsuit and dig into the skin. Wounds from crown-of-thorns have been known to go septic.
You have been warned!