Although for the vast majority of fishes humans are the significant source of danger, a number of fish species cause concern for humans as well. Probably foremost are the perceived aggressors, like sharks and barracudas. Others are renowned for their needle-like spines, some of which have associated painful and occasionally deadly venoms, like the stonefishes and allied scorpion fishes. A few, such as the electric rays, have biological batteries capable of producing jolting shocks to ward off attackers. In the 'serves you right' category are fishes that cause discomfort and even death when eaten by humans, whether the source of the poison is inherent in the biology of the fish or a result of the accumulation of a toxin through the food chain. The vast majority of fishes having these dangerous attributes have acquired them as a means of survival.
Sharks, like the White Shark Carcharodon carcharias, are invariably the first to come to mind when the subject of fish attack arises, but many a diver has been bitten by species a tiny fraction of their size, like the diminutive damselfishes of tropical and temperate reefs. For sharks, barracudas and even the razor-toothed Tailor Pomatomus saltatrix that are sought by many anglers, attacks occur in the process of feeding, with humans regarded as a legitimate food source or mistaken for food that is normally consumed. Other aggression may be part of simple territorial behaviour, like the protection of a food source or the guarding of a nest. Potential harm in these cases is determined by the size of the aggressor. Attacks by fishes, like morays and conger eels are invariably defensive by fishes that have the teeth that can produce a formidable bite.
A surprisingly large number of fishes, including most sharks and rays, have the ability to detect electrical impulses to help them escape capture and locate prey. Some have developed the ability to also produce significant electrical discharges that act like radar in sensing objects in surrounding water and even repel predators. Numbfishes, coffinfishes and electric or torpedo rays (families Hypnidae, Narcinidae and Torpedinidae) are particularly well known to produce stunning shocks.
A great many fishes, from primitive sharks to modern bony fishes, have acquired pungent spines as protection from predatory species. Many of these have gone one step further in augmenting their spines with venom produced from surrounding tissue or a special gland at the spine base. The potency of toxin varies between species from mildly painful to lethal, with the Stonefishes of the genus Synanceia infamous as villains in the deaths of tropical reef waders. The venomous spine or spines on the tail of Stingrays and their relatives can be directed effectively at perceived attackers.
Little known among the venomous fishes are the Sabre-toothed Blennies (Blenniidae, genus Meiacanthus) with venomous canines that emulate the fangs of land based vipers, although at a tiny fraction of the serpents' size.
Poisons in fishes have been acquired in the course of evolution and are therefore a characteristic of the species or, as with ciguatera, are produced by a micro-organism and accumulated through the food chain. Probably best known among poisonous fishes are the puffers or toadfishes, which have their own tetradotoxin, a powerful neurotoxin that regularly causes fatalities in countries where these fishes are consumed. In other groups, like the Dragonettes or Stinkfishes (Callionymidae), the poison contained in the flesh is less potent and acts more as a poor tasting deterrent. In the Roughies (Trachichthyidae), the poison is in the skin or mucous so the fish is not seriously harmed before the deterrent has had its effect. Still others, like the Prowfishes (Pataecidae) and Boxfishes (Ostraciidae), can release a poison to the water to ward off attackers.
Ciguatera, also known as tropical fish poisoning, is first produced by the dinoflagellate Gambierdiscus toxicus. The toxin is then concentrated as fishes feeding on the single celled organisms are eaten and they are in turn eaten by ever larger predators, right up the food chain. Only when the amount of poison in a fish reaches a critical concentration does it have the devastating effect on humans who may consume affected fishes.
The practice of disposing of industrial waste into the aquatic environment has reached such a point that in recent years aquatic organisms have been discovered to have absorbed dangerous levels of toxic chemicals such as heavy metals. Like ciguatera, these poisons are concentrated through the food chain, and the top level predators - the large sharks, mackerels and tuna - are the fishes of most concern.
Author: Martin F. Gomon