About 50 species, referred to the following genera:—Scomber, Auxis, Thunnus, Sarda, Cybium, Acanthocybium, Gastrochisma (Lepidothynnus). Numerous fossil representatives in Tertiary beds, belonging to Scomber, Auxis, Thunnus, Cybium, and to the extinct genera Eothynnus, Isurichthys, Palimphyes, Scombrinus, Sphyraenodus, Scombramphodon.
These fishes, elegant in form and often in colour, are among the swiftest of the inhabitants of the sea. Some are migratory, like the Mackerel (Scomber scombrus) of the North Atlantic, whilst others are remarkable for their wide distribution. The Tunny (Thunnus thynnus), for instance, the largest member of the family, reaching a length of 10 feet, inhabits the Atlantic, Pacific, and Indian Oceans, extending as far north as the British seas, Newfoundland, California, and Japan. It supplies important fisheries in France and Italy. The Tunnies are the only fish known to be warm-blooded.
Fam. 4. Trichiuridae.—Praemaxillaries not protractile. Vertebrae 32 to 160, without transverse processes; ribs sessile, on the centra or on the haemal arches when these are present; epipleurals, if well developed, on the centra. Scales very small or absent. Spinous portion of dorsal fin much longer than the soft, the spines more or less feeble. Pectoral fins inserted low down the sides. Pseudobranchiae present.
The members of this family show a great variation in the shape of the body, which, although always strongly compressed, is not unlike that of a Mackerel in the more normal types, such as Thyrsites and Ruvettus, whilst, through a chain of genera, it generally assumes an extremely elongate form; concurrently with this elongation of the body, the dorsal fin loses its differentiation into two portions, the ventrals become reduced and disappear, as in the Scabbard- or Frost-Fish (Lepidopus caudatus), while the caudal fin decreases in size, loses its fork-shape, and is finally lost in Trichiurus, in which the body is ribbon-shaped and tapers to a point.
About 25 species are known, pelagic and widely distributed, many descending to great depths.
Principal living genera: Ruvettus, Thyrsites, Epinnula, Nesiarchus, Nealotus, Promethichthys, Dicrotus, Gempylus, Aphanopus, Lepidopus, Euoxymetopon, Benthodesmus, Eupleurogrammus, Trichiurus.
Remains of several species referred to Thyrsites, Lepidopus, and to the extinct genera Thyrsitocephalus, Hemithyrsites, and Trichiurichthys, have been found in the Oligocene and Miocene of Europe.
Fam. 5. Histiophoridae.—Praemaxillaries not protractile; snout produced into a spear-shaped rostrum; a praedentary bone; teeth minute. Body elongate, covered with small or rudimentary scales. Vertebrae 24 or 25, without transverse processes; ribs sessile; no epipleurals. One or two dorsal fins, without a distinctly spinous portion. Pectoral fin low down the side. Pseudobranchiae present.
The Sail-Fishes are large oceanic fishes, endowed with great strength and swiftness, occurring in the tropical and sub-tropical seas. Four or five species are distinguishable, and are referable to two genera: Histiophorus, with a single dorsal fin and 2 or 3 ventral rays, and Tetrapturus, with the dorsal divided into two parts and a single ventral ray.[[723]]
Fossil Histiophoridae are known from the Eocene and later beds in Europe and America. Dr. A. S. Woodward observes that the known fossils are too imperfect to be referred with certainty to their respective genera. Most of them probably belong to Histiophorus, but at least one genus from the Eocene (Xiphiorhynchus) appears to be well distinguished.