Vertebrate Zoology - V. M. Konstantinov 2011

Chordates
Gnathostomes
Class Bony Fishes

General characteristics

The Class Osteichthyes (bony Fishes) comprises the overwhelming majority (over 20,000) of species within the superclass Pisces. Bony fishes inhabit a remarkably wide range of aquatic environments. This diversity of habitats accounts for the group's extraordinary species richness and morphological variety. The principal diagnostic Features of the class are outlined below.

The Skeleton is always ossified to a greater or lesser degree. Bone Formation occurs via two distinct pathways. The phylogenetically earliest type of ossification produces the so-called dermal, or membrane, bones. Embryologically, these develop within the Connective Tissue layer of the Skin independently of the cartilaginous skeletal elements, to which they merely lie adjacent. Due to these developmental characteristics, dermal bones typically take the form of flat plates.

In addition to dermal bones, the fish skeleton contains chondral, or endochondral, bones. Embryologically, these are formed through the progressive replacement of Cartilage by bony tissue secreted by osteoblasts. Histologically, mature chondral bones are indistinguishable from dermal bones. While chondral ossifications do not introduce major modifications to the overall skeletal architecture, The formation of dermal ossifications gives rise to novel skeletal elements, thereby increasing overall structural complexity.

The interbranchial septa in the Respiratory system are reduced, so that the gill filaments are situated directly on the bony gill arches. A bony operculum is invariably present, covering the gill apparatus externally. The vast majority of species possess a swim bladder. Embryologically, this originates as an outpocketing of the dorsal wall of the gut. In primitive groups, it retains its connection to the digestive tract throughout life. The swim bladder serves as a vital hydrostatic organ: altering the volume of gas within it changes the fish's body density, which in turn plays an adaptive role when fish move between different Water depths.

In the vast majority of bony fishes, Fertilization is external, and the eggs are small and devoid of horny envelopes. Viviparity occurs in only a negligible number of species.

The Classification of bony fishes is exceptionally challenging, and currently, several differing views on the Taxonomy of this group coexist. The authors adopt one of these systems as a basis, distinguishing two subclasses: Actinopterygii (ray-finned fishes) and Sarcopterygii (lobe-finned fishes). It should be noted that the latter retain many primitive traits that link them to cartilaginous fishes, and in most textbooks, they are discussed prior to the ray-finned fishes. Nevertheless, ray-finned fishes have traditionally served as the primary model organisms in ichthyological studies.