SPECIAL ICHTHYOLOGY - R. S. Pentyliuk - 2011

2. CLASS CARTILAGINOUS FISHES (CHONDRICHTHYES)

2.1 Subclass Elasmobranchii

The subclass Elasmobranchii encompasses the vast majority of modern cartilaginous Fishes, comprising about 750 species of sharks and rays. Bone tissue is entirely absent, and the Skeleton is exclusively cartilaginous. The Skin is typically covered with placoid scales. Tooth Morphology is highly diverse—flat and triangular, pointed and conical, knob-like, awl-shaped, smooth, serrated, unicuspid, or multi-cuspid. Teeth are arranged on the jaws in straight and oblique rows, with each row containing teeth of several generations. Usually, only the front row is functional, while the rear ones are tilted inward and replace the anterior teeth as they wear down. Elasmobranchs lack a gill cover (operculum), leaving 5 to 7 pairs of gill slits open to the exterior on each side of the body. In sharks, the gill lamellae are plate-like and attached to the arches along their entire length (hence the name "elasmobranchs", meaning plate-gilled). Many elasmobranchs possess a small paired opening known as the spiracle, which is a rudiment of an additional gill slit. In holocephalans, there are 4 pairs of gill slits, and the spiracle is absent.

A swim bladder is absent in all cartilaginous fishes, and the intestinal surface area is significantly increased by a spiral valve. The Heart features an conus arteriosus, which enhances the efficiency of Blood Circulation and METABOLISM associated with an active lifestyle. The presence of a spiral valve in the intestine and a conus arteriosus in the heart are key Anatomical Features of elasmobranchs. Elasmobranchs possess a cloaca, which is absent in holocephalans.

The Osmotic Pressure of the internal environment is maintained largely by urea dissolved in the blood.

The reproductive process has specific features: Fertilization is internal, and males possess a pair of copulatory Organs called pterygopodia (claspers) for this purpose. Reproduction occurs via oviposition (egg-laying), ovoviviparity, or viviparity. In some spiny dogfish sharks (Squalus) and electric rays (Torpedo), a unique form of embryonic feeding takes place: the walls of the "Uterus" form trophotenial projections that penetrate the embryo's Oral Cavity and secrete a milk-like nutrient fluid. Viviparous sharks, whose embryonic development also takes place within the uterus, possess a Placenta-like Structure that nourishes the embryo via the mother's blood.

The body shape of elasmobranch fishes varies greatly. Some species feature a torpedo-shaped body adapted for rapid swimming and are strong swimmers, whereas others are dorsoventrally flattened and typically spend their lives resting on the bottom. Their sizes differ drastically: the smallest species do not exceed 15-30 cm in length, whereas giant sharks and rays can reach 15-20 m in length and weigh up to 14 tons. Cartilaginous fishes are highly specialized animals, with many acting as apex predators.

The taxonomic affiliation of certain elasmobranchs can only be determined through internal anatomical features. For instance, angel sharks (Squatiniformes) bear a closer resemblance to rays, whereas sawfishes (Pristiformes), conversely, closely resemble sawsharks (Pristiophoridae) in appearance.