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Aquatic Animals

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D. Zannini*

* Adapted from 3rd edition, Encyclopaedia of Occupational Health and Safety.

Aquatic animals dangerous to humans are to be found among practically all of the divisions (phyla). Workers may come into contact with these animals in the course of various activities including surface and underwater fishing, the installation and handling of equipment in connection with the exploitation of petroleum under the sea, underwater construction, and scientific research, and thus be exposed to health risks. Most of the dangerous species inhabit warm or temperate waters.

Characteristics and Behaviour

Porifera. The common sponge belongs to this phylum. Fishermen who handle sponges, including helmet and scuba divers, and other underwater swimmers, may contract contact dermatitis with skin irritation, vesicles or blisters. The “sponge diver’s sickness” of the Mediterranean region is caused by the tentacles of a small coelenterate (Sagartia rosea) that is a parasite of the sponge. A form of dermatitis known as “red moss” is found among North American oyster fishers resulting from contact with a scarlet sponge found on the shell of the oysters. Cases of type 4 allergy have been reported. The poison secreted by the sponge Suberitus ficus contains histamine and antibiotic substances.

Coelenterata. These are represented by many families of the class known as Hydrozoa, which includes the Millepora or coral (stinging coral, fire coral), the Physalia (Physalia physalis, sea wasp, Portuguese man-of-war), the Scyphozoa (jellyfish) and the Actiniaria (stinging anemone), all of which are found in all parts of the ocean. Common to all these animals is their ability to produce an urticaria by the injection of a strong poison that is retained in a special cell (the cnidoblast) containing a hollow thread, which explodes outwards when the tentacle is touched, and penetrates the person’s skin. The various substances contained in this structure are responsible for such symptoms as severe itching, congestion of the liver, pain, and depression of the central nervous system; these substances have been identified as thalassium, congestine, equinotoxin (which contains 5-hydroxytryptamine and tetramine) and hypnotoxin, respectively. Effects on the individual depend upon the extent of the contact made with the tentacles and hence on the number of microscopic punctures, which may amount to many thousands, up to the point where they may cause the death of the victim within a few minutes. In view of the fact that these animals are dispersed so widely throughout the world, many incidents of this nature occur but the number of fatalities is relatively small. Effects on the skin are characterized by intense itching and the formation of papules having a bright red, mottled appearance, developing into pustules and ulceration. Intense pain similar to electric shock may be felt. Other symptoms include difficulty in breathing, generalized anxiety and cardiac upset, collapse, nausea and vomiting, loss of consciousness, and primary shock.

Echinoderma. This group includes the starfishes and sea urchins, both of which possess poisonous organs (pedicellariae), but are not dangerous to humans. The spine of the sea urchin can penetrate the skin, leaving a fragment deeply imbedded; this can give rise to a secondary infection followed by pustules and persistent granuloma, which can be very troublesome if the wounds are close to tendons or ligaments. Among the sea urchins, only the Acanthaster planci seems to have a poisonous spine, which can give rise to general disturbances such as vomiting, paralysis and numbness.

Mollusca. Among the animals belonging to this phylum are the cone shells, and these can be dangerous. They live on a sandy sea-bottom and appear to have a poisonous structure consisting of a radula with needle-like teeth, which can strike at the victim if the shell is handled incautiously with the bare hand. The poison acts on the neuromuscular and central nervous systems. Penetration of the skin by the point of a tooth is followed by temporary ischaemia, cyanosis, numbness, pain, and paraesthesia as the poison spreads gradually through the body. Subsequent effects include paralysis of the voluntary muscles, lack of coordination, double vision and general confusion. Death can follow as a result of respiratory paralysis and circulatory collapse. Some 30 cases have been reported, of which 8 were fatal.

Platyhelminthes. These include the Eirythoe complanata and the Hermodice caruncolata, known as “bristle worms”. They are covered with numerous bristle-like appendages, or setae, containing a poison (nereistotoxin) with a neurotoxic and local irritant effect.

Polyzoa (Bryozoa). These are made up of a group of animals which form plant-like colonies resembling gelatinous moss, which frequently encrust rocks or shells. One variety, known as Alcyonidium, can cause an urticarious dermatitis on the arms and face of fishermen who have to clean this moss off their nets. It can also give rise to an allergic eczema.

Selachiis (Chondrichthyes). Animals belonging to this phylum include the sharks and sting-rays. The sharks live in fairly shallow water, where they search for prey and may attack people. Many varieties have one or two large, poisonous spines in front of the dorsal fin, which contain a weak poison that has not been identified; these can cause a wound giving rise to immediate and intense pain with reddening of the flesh, swelling and oedema. A far greater danger from these animals is their bite, which, because of several rows of sharp pointed teeth, causes severe laceration and tearing of the flesh leading to immediate shock, acute anaemia and drowning of the victim. The danger that sharks represent is a much-discussed subject, each variety seeming to be particularly aggressive. There seems no doubt that their behaviour is unpredictable, although it is said that they are attracted by movement and by the light colour of a swimmer, as well as by blood and by vibrations resulting from a fish or other prey that has just been caught. Sting-rays have large, flat bodies with a long tail having one or more strong spines or saws, which can be poisonous. The poison contains serotonine, 5-nucleotidase and phosphodiesterase, and can cause generalized vasoconstriction and cardio-respiratory arrest. Sting-rays live in the sandy regions of coastal waters, where they are well hidden, making it easy for bathers to step on one without seeing it. The ray reacts by bringing over its tail with the projecting spine, impaling the spike keep into the flesh of the victim. This may cause piercing wounds in a limb or even penetration of an internal organ such as the peritoneum, lung, heart or liver, particularly in the case of children. The wound can also give rise to great pain, swelling, lymphatic oedema and various general symptoms such as primary shock and cardio-circulatory collapse. Injury to an internal organ may lead to death in a few hours. Sting-ray incidents are among the most frequent, there being some 750 every year in the United States alone. They can also be dangerous for fishermen, who should immediately cut off the tail as soon as the fish is brought aboard. Various species of rays such as the torpedo and the narcine possess electric organs on their back, which, when stimulated by touch alone, can produce electric shocks ranging from 8 up to 220 volts; this may be enough to stun and temporarily disable the victim, but recovery is usually without complications.

Osteichthyes. Many fishes of this phylum have dorsal, pectoral, caudal and anal spines which are connected with a poison system and whose primary purpose is defence. If the fish is disturbed or stepped upon or handled by a fisherman, it will erect the spines, which can pierce the skin and inject the poison. Not infrequently they will attack a diver seeking fish, or if they are disturbed by accidental contact. Numerous incidents of this kind are reported because of the widespread distribution of fish of this phylum, which includes the catfish, which are also found in fresh water (South America, West Africa and the Great Lakes), the scorpion fish (Scorpaenidae), the weever fish (Trachinus), the toadfish, the surgeon fish and others. Wounds from these fishes are generally painful, particularly in the case of the catfish and the weever fish, causing reddening or pallor, swelling, cyanosis, numbness, lymphatic oedema and haemorrhagic suffusion in the surrounding flesh. There is a possibility of gangrene or phlegmonous infection and peripheral neuritis on the same side as the wound. Other symptoms include faintness, nausea, collapse, primary shock, asthma and loss of consciousness. They all represent a serious danger for underwater workers. A neurotoxic and haemotoxic poison has been identified in the catfish, and in the case of the weever fish a number of substances have been isolated such as 5-hydroxytryptamine, histamine and catecholamine. Some catfishes and stargazers that live in fresh water, as well as the electric eel (Electrophorus), have electric organs (see under Selachii above).

Hydrophiidae. This group (sea snakes) is to be found mostly in the seas around Indonesia and Malaysia; some 50 species have been reported, including Pelaniis platurus, Enhydrina schistosa and Hydrus platurus. The venom of these snakes is very similar to that of the cobra, but is 20 to 50 times as poisonous; it is made up of a basic protein of low molecular weight (erubotoxin) which affects the neuromuscular junction blocking the acetylcholine and provoking myolysis. Fortunately sea snakes are generally docile and bite only when stepped on, squeezed or dealt a hard blow; furthermore, they inject little or no venom from their teeth. Fishermen are among those most exposed to this hazard and account for 90% of all reported incidents, which result either from stepping on the snake on the sea bottom or from encountering them among their catch. Snakes are probably responsible for thousands of the occupational accidents attributed to aquatic animals, but few of these are serious, while only a small percentage of the serious accidents turn out to be fatal. Symptoms are mostly slight and not painful. Effects are usually felt within two hours, starting with muscular pain, difficulty with neck movement, lack of dexterity, and trismus, and sometimes including nausea and vomiting. Within a few hours myoglobinuria (the presence of complex proteins in urine) will be seen. Death can ensue from paralysis of the respiratory muscles, from renal insufficiency due to tubular necrosis, or from cardiac arrest due to hyperkalaemia.


Every effort should be made to avoid all contact with the spines of these animals when they are being handled, unless strong gloves are worn, and the greatest care should be taken when wading or walking on a sandy sea bottom. The wet suit worn by skin divers offers protection against the jellyfish and the various Coelenterata as well as against snakebite. The more dangerous and aggressive animals should not be molested, and zones where there are jellyfish should be avoided, as they are difficult to see. If a sea snake is caught on a line, the line should be cut and the snake allowed to go. If sharks are encountered, there are a number of principles that should be observed. People should keep their feet and legs out of the water, and the boat should be gently brought to shore and kept still; a swimmer should not stay in the water with a dying fish or with one that is bleeding; a shark’s attention should not be attracted by the use of bright colours, jewellery, or by making a noise or explosion, by showing a bright light, or by waving the hands towards it. A diver should never dive alone.



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