An introduction to the study of fishes

by Albert C. L. G. Günther

Fig. 107.—Mugil septentrionalis.

Heads of Grey Mullets, fishes of Brackish water.

CHAPTER XIX.
THE DISTRIBUTION OF MARINE FISHES.

Marine fishes fall, with regard to their mode of life and distribution, into three distinct categories:—

1. Shore Fishes—That is, fishes which inhabit chiefly parts of the sea in the immediate neighbourhood of land either actually raised above, or at least but little submerged below, the surface of the water. They do not descend to any great depth,—very few to 300 fathoms, and the majority live close to the surface. The distribution of these fishes is determined not only by the temperature of the surface water but also by the nature of the adjacent land, and its animal and vegetable products; some of these fishes being confined to flat coasts with soft or sandy bottoms, others to rocky and fissured coasts, others to living coral formations. If it were not for the frequent mechanical and involuntary removals to which these fishes are exposed, their distribution within certain limits, as it no doubt originally existed, would resemble still more that of freshwater fishes than we find it actually does at the present period.

2. Pelagic Fishes—that is, fishes which inhabit the surface and uppermost strata of the open ocean, which approach the shores only accidentally, or occasionally (in search of prey), or periodically (for the purpose of spawning). The majority spawn in the open sea, their ova and young being always found at great distance from the shore. With regard to their distribution, they are still subject to the influences of light and the temperature of the surface water; but they are independent of the variable local conditions which tie the shore fish to its original home, and therefore roam freely over a space which would take a freshwater or shore fish thousands of years to cover in its gradual dispersal. Such as are devoid of rapidity of motion are dispersed over similarly large areas by the oceanic currents, more slowly than, but as surely as, the strong swimmers. Therefore, an accurate definition of their distribution within certain areas equivalent to the terrestrial regions is much less feasible than in the case of shore fishes.

3. Deep-sea Fishes—that is, fishes which inhabit such depths of the ocean as to be but little or not influenced by light or the surface temperature; and which, by their organisation are prevented from reaching the surface stratum in a healthy condition. Living almost under identical tellurian conditions, the same type, the same species, may inhabit an abyssal depth under the equator as well as one near the arctic or antarctic circle; and all we know of these fishes points to the conclusion that no separate horizontal regions can be distinguished in the abyssal fauna, and that no division into bathymetrical strata can be attempted on the base of generic much less of family characters.

It must not be imagined that these three categories are more sharply defined than Freshwater and Marine Fishes. They gradually pass into each other, and there are numerous fishes about which uncertainty exists whether they should be placed in the Shore or Pelagic series, or in the Pelagic or Deep-sea series; nay, many facts favour the view that changes in the mode of life and distribution of fishes are still in progress.

The change in habitat of numerous fishes is regulated by the distribution of their favourite food. At certain seasons the surface of the sea in the vicinity of land swarms with mollusks, larval Crustaceans, Medusæ, attracting shoals of fishes from the open ocean to the shores; and these are again pursued by fishes of larger size and predacious habits, so that all these fishes might be included, with equal propriety, in the littoral or pelagic series. However, species which are known to normally spawn in the open ocean must be always referred to the latter division.

Chondropterygii, Acanthopterygii, Anacanths, Myxinoids, and Pharyngobranchii furnish the principal contingents to the Marine Fauna; whilst the majority of Malacopterygians, the Ganoids, and Cyclostomes are Freshwater Fishes.