The means of defence in fishes are extremely varied. Some species (torpedoes or electric rays, electric eels) destroy their enemies by electric discharges; others are provided with true poison-glands and inoculatory organs, usually represented by opercular spines or by the fin-rays. The species of the genus Muræna, however, possess a poison-apparatus connected with the buccal teeth, as in the case of snakes.
It has been clearly established by Bottard[138] that at least three very distinct types of venomous fishes exist, according as the venom-apparatus is:—
(1) Entirely closed (Synanceia type); (2) half closed (Thalassophryne type); (3) in more or less direct communication with the exterior (Trachinus and Scorpæna type).
The greater part of the following statements has been borrowed from the excellent work of the author referred to, from the writings of A. Corre,[139] the fellowship thesis of Henry Coutière,[140] and the magnificent atlas published at St. Petersburg in 1886 by P. Savtschenko, of the Russian Imperial Navy.
Except in the case of the species of Muræna, the venom of fishes is generally found in one or more special glands, situate at the base of the dorsal or caudal fins, or beneath the opercular spines. When the animal defends itself it inflicts wounds with these rays, and ejects from its poison-glands a toxic or irritant liquid, which enters the sores.
The flesh of these fishes is not usually poisonous, whereas a fairly large number of other species, which do not inflict wounds, cause intoxicating effects when eaten. These latter do not come within the scope of this work; but the reader who may desire to obtain information with regard to them will find them well described in J. Pellegrin’s memoir,[141] in that by Dupont, and especially in the papers of A. Corre.
Venomous fishes almost all belong to sedentary species, as in the case of the genera Trachinus, Cottus, Scorpæna, and Synanceia. This fact suggested to Dissard and Noë[142] a very hazardous theory in order to explain the existence of a poison-apparatus in these animals. The venomous fishes being sedentary, say these authors, have no need of a poison-apparatus; their prey offers itself to them without effort on their part, and, on the other hand, they escape destruction by their enemies. If, therefore, they possess a poison-apparatus it is because the conditions under which they live entail the lowest value for the co-efficient of respiration, diminish the quantity of the ambient radiations and the oxygenation of the medium, and lead to diminished hæmatosis. For these reasons the activity of anaerobic life becomes greater, and the formation of venoms takes place.
This theory, derived from the conceptions of A. Gautier with regard to the formation of toxic leucomaines, appears scarcely tenable, for it is evident that the weever, for example, erects its first dorsal spine as soon as it is seized, and that Scorpæna and Synanceia likewise protrude their venomous spines when conscious of danger. The poison-apparatus of these fishes is therefore of an eminently defensive character.
According to Bottard, the spawning season increases the activity of the poison-glands and at the same time the toxicity of the secreted product. Several species, such as those of the genus Cottus and the perch, possess no apparent secreting cells except at this period. Certain toxicophorous or poisonous fishes, such as the species of Tetrodon, are particularly noxious at the time when their genital glands are at their maximum activity.