Fig. 286.—Astronesthes niger. The white spots in front of the eye are phosphorescent organs.
Some have two dorsal fins, the posterior of which is adipose; they belong to the genus Astronesthes, are the smallest of the family, and frequently met with in the Atlantic.
The others—viz. Stomias, Echiostoma, Malacosteus, and Bathyophis, lack the adipose fin, the rayed dorsal being opposite to the anal. Of these the one longest known is
Stomias.—Body elongate, compressed, covered with exceedingly fine and deciduous scales, which are scarcely imbricate, lying in subhexagonal impressions; vent situated at no great distance from the caudal fin. Head compressed, with the snout very short, and with the cleft of the mouth very wide. Teeth pointed, unequal in size, those of the intermaxillaries and of the mandible being the longest; maxillary finely denticulated; vomer with a pair of fangs; palatine bones and tongue with smaller pointed teeth. Eye of moderate size. Opercular portion of the head narrow. A fleshy barbel in the centre of the hyoid region. Dorsal opposite the anal, close to the caudal; pectoral and ventral fins feeble, the latter inserted behind the middle of the length of the body. Series of phosphorescent dots run along the lower side of the head, body, and tail. Gill-opening very wide. Pyloric appendages none.
Three species are known; beside specimens which were found floating on the surface, others have been dredged from depths varying between 450 and 1800 fathoms.
Fifteenth Family—Salmonidæ.
Body generally covered with scales; head naked; barbels none. Margin of the upper jaw formed by the intermaxillaries mesially, and by the maxillaries laterally. Belly rounded. A small adipose fin behind the dorsal. Pyloric appendages generally numerous, rarely absent. Air-bladder large, simple; pseudobranchiæ present. The ova fall into the cavity of the abdomen before exclusion.
Inhabitants of the sea and freshwater; the majority of the marine genera are deep-sea forms. The freshwater forms are peculiar to the temperate and arctic zones of the Northern Hemisphere, one occurring in New Zealand; many freshwater species periodically or occasionally descending to the sea. One of the most valuable families of the class of fishes. No fossils of the freshwater forms are known; but of the marine genera, Osmerus occurs in the greensand of Ibbenbusen, and in the schists of Glaris and Licata; a species of Mallotus, indistinguishable from the living M. villosus, occurs abundantly in nodules of clay of unknown geological age in Greenland. Other genera, as Osmeroides, Acrognathus, and Aulolepis, from the chalk of Lewes, belong to the same fauna as species of Beryx, and were probably deep-sea Salmonoids.
Salmo.—Body covered with small scales. Cleft of the mouth wide, the maxillary extending to below or beyond the eye. Dentition well developed; conical teeth in the jaw bones, on the vomer and palatines, and on the tongue, none on the pterygoid bones. Anal short, with less than fourteen rays. Pyloric appendages numerous; ova large. Young specimens with dark cross-bands (Parr-marks).
We know of no other group of fishes which offers so many difficulties to the ichthyologist with regard to the distinction of the species as well as to certain points in their life-history, as this genus, although this may be partly due to the unusual attention which has been given to their study, and which has revealed an almost greater amount of unexplained facts than of satisfactory solutions of the questions raised. The almost infinite variations of these fishes are dependent on age, sex and sexual development, food, and the properties of the water. Some of the species interbreed, and the hybrids mix again with one of the parent species, thus producing an offspring more or less similar to the pure breed. The coloration is, first of all, subject to variation; and consequently this character but rarely assists in distinguishing a species, there being not one which would show in all stages of development the same kind of coloration. The young of all the species are barred; and this is so constantly the case that it may be used as a generic or even as a family character, not being peculiar to Salmo alone, but also to Thymallus and probably to Coregonus. The number of bars is not quite constant, but the migratory Trout have two (and even three) more than the River-Trout. In some waters River-trout remain small, and frequently retain the Parr-marks all their lifetime; at certain seasons a new coat of scales overlays the Parr-marks, rendering them invisible for a time; but they reappear in time, or are distinct as soon as the scales are removed. When the Salmones have passed this “Parr” state, the coloration becomes much diversified. The males, especially during and immediately after the spawning time, are more intensely coloured and variegated than the females; specimens which have not attained to maturity retaining a brighter silvery colour, and being more similar to the female fish. Food appears to have less influence on the coloration of the outer parts than on that of the flesh; thus the more variegated specimens are frequently out of condition, whilst well-fed individuals with pinkish flesh are of a more uniform though bright coloration. Chemistry has not supplied us yet with an analysis of the substance which gives the pink colour to the flesh of many Salmonoids; but there is little doubt that it is identical with, and produced by, the red pigments of many salt- and freshwater Crustaceans, which form a favourite food of these fishes. The water has a marked influence on the colours; Trout with intense ocellated spots are generally found in clear rapid rivers, and in small open Alpine pools; in the large lakes with pebbly bottom the fish are bright silvery, and the ocellated spots are mixed with or replaced by X-shaped black spots; in pools or parts of lakes with muddy or peaty bottom, the trout are of a darker colour generally, and when enclosed in caves or holes, they may assume an almost uniform blackish coloration.