Classification

Fish ("Pisces") constitute the largest group of vertebrates, with approximately 28,000 species. This is roughly the same number of species as found in the amphibians, reptiles, birds and mammals together. They live in nearly all types of water, but over 90% of species live in the marine environment. In a wider sense fish represent a superclass, composed of the 3 classes Cyclostomata (= Agnatha, cyclostomes or jawless fishes, approx. 100 species), Chondrichthyes (cartilaginous fishes, sharks, rays, skates and chimaeras, approx. 1,000 species) and Osteichthyes (bony fishes, over 26,000 species).

Fish with venomous spines are found amongst the cartilaginous fishes (in particular rays and skates, less so sharks and chimaeras) and the bony fishes. Virtually all the venomous individuals amongst the bony fishes belong to the superorder Teleostei (true bony fishes). This is the best known and largest group of fish, to which most edible fish belong. Venomous teleosts that frequently cause accidents are found in particular among the Perciformes (perch-like fishes, e.g. weeverfishes, Trachinus sp.), Siluriformes (catfishes) and Scorpaeniformes (scorpionfishes, lionfishes and stonefishes, among others).

Morphological characteristics

The presence of gills is one of the most important characteristics of fish. Water is sucked in via the mouth and channelled through the branched gills. The oxygen dissolved in the water is absorbed via the fine capillary network in the gills and the "used" water is then pumped out. In bony fishes, the gills on both sides of the head are covered by a gill cover, unlike in sharks and rays, where the water is expelled through a row of 5–7 gill slits. The latter do not have a bony skeleton as do the bony fishes, but rather a cartilaginous skeleton that can be strengthened by storage of calcium carbonate. In addition, the skin of sharks and rays is not covered in true scales, but rather in tiny, dentin-containing dermal denticles.

Most bony fishes possess a swim bladder. Through variation of the degree to which it is filled with gas, the fish can easily swim at different water depths. The swim bladder is generally deflated in bottom-dwelling species.

Fish fins are composed of either flesh or skin and are supported and extended by bony or cartilaginous spines that protrude from the backbone, the pectoral girdle and the pelvic girdle. Starting from a basic form (Fig. 4.19), the fins of some fish have undergone modifications and taken on various new functions. In this context the development of venomous fin rays is of particular interest.

Fig. 4.19 Schematic representation of a fish.
1 Anterior dorsal fin
2 Posterior dorsal fin
3 Caudal fin
4 Gill cover (operculum)
5 Pectoral fins (paired)
6 Pelvic fins (paired)
7 Anal fin

Fig. 4.20 Venom apparatus in various venomous fishes (adapted from Halstead 1988).

a Tail spine of a stingray. On the ventral side there are 2 venom glands arranged in pairs. The spine is serrated and can cause deep wounds.
b Dorsal spine of a lionfish (Pterois sp.). There is a narrow venom gland on both sides of the slender spine. The spine and glands are surrounded by a fine integument.
c Dorsal spine of a stonefish (Synanceia sp.). A massive bony ray with a large venom gland on both sides that is surrounded by a sturdy integument.
1 Venom gland
2 Integument (enveloping sheath)
3 Calcified cartilaginous spine (a) or bony spine (b and c)

Venom apparatus

The most common type of venom apparatus is the fin ray with grooves on both sides. A venom gland is located in each groove, and these glands range from poorly to well evolved (Fig. 4.20b and c). The spine and glands are surrounded by an enveloping sheath, known as the integument. These venomous spines, which are particularly efficient and numerous in the broader family to which the scorpionfishes belong (Scorpaeniformes), are mostly used passively, in that they are erected when the fish feels threatened. If the spine penetrates the flesh of the attacker or of a careless person, the integument is pushed downwards, and under the resulting pressure the venom glands discharge their contents along the grooves and into the wound. Such venomous spines are found in dorsal, pectoral, pelvic or anal fins of various venomous fishes. The tail spines of rays represent modified spines arising from the dorsal fin but located in a very posterior position. As is also often the case in catfishes, they are highly serrated or barbed, and cause gaping wounds (Fig. 4.20a).

Some species have venomous spine processes on the gill cover (e.g. Weeverfishes, Trachinus sp.). Surgeonfishes (Acanthuridae) possess erectable razor-sharp spines at the base of the tail, which can be actively employed while swimming past an attacker. Crinotoxic fishes are a borderline case amongst the venomous fish. Their epidermal gland secretions can enter wounds made by normal fin rays and thus lead to local effects.

Range of venom effects

See the Clinical flowchart for Venomous fish.

Way of life

Most medically relevant species of venomous fish are marine bottom-dwellers that live in waters close to the coast, with the greatest variety occurring in tropical waters. Many have camouflage colouring and can also dig themselves into the loose substrate of the ocean floor. They are often found in shallow water and can also occasionally be seen in pools in intertidal zones. Members of this group include rays, weeverfishes, scorpionfishes and stonefishes, among others. Lionfishes move freely through the water and have a distinctive warning colouring.

In freshwater, the only fish that cause accidents of any consequence are the cosmopolitan catfishes or the freshwater rays of South America (Potamotrygonidae). They mostly live on the bottom of murky and muddy rivers or lakes.

Epidemiology

Accidents caused by venomous fish are not infrequent, but only rarely do they cause life-threatening systemic effects. Fatalities are very rare and have generally been caused by stonefishes (Synanceia sp.) or stingrays. In the case of the latter, the reported fatalities are generally more often associated with large puncture wounds in vital organs than with the actual effect of the venom.

Victims are commonly fishermen who are stung while emptying their nets or removing fish from a fishhook. Bathers and divers are another group at risk, in particular in tropical regions, but also in the Mediterranean, for example. These victims are usually stung in the foot by bottom-dwelling venomous fish while wading in water. However, accidents do also occur when victims needlessly touch or bother venomous fish. On the other hand, accidents may also occur outside of the water, namely in the kitchen, during the preparation of edible fish. Weeverfishes are a common ingredient in the French fish soup known as "Bouillabaisse". However, in France it is required by law that the venomous spines be removed from these fish before they are sold.

Prevention

Inexperienced fishermen should take care when handling venomous fish and use thick, sturdy gloves. When preparing venomous fish for cooking, the fins should be carefully removed.

The following principles apply for ocean bathers and divers:

- It is better to swim in shallow water rather than wade. In addition, shoes with sturdy soles should be worn.

- A diving mask allows clear visibility under water.

- Do not swim or dive in murky water or in stormy weather.

- Do not touch venomous or unknown fish.

General reviews:

Bone and Marshall 1985, Grüter 1990, Halstead 1988, Helfman et al. 2009, Maretic 1988, Mebs 1992, Russell 1965, Smith and Heemstra 1986

Classification: Catalogue of life