The living forms of cartilaginous fish can be divided into two groups: those with only one gill opening on each side of the head and those with many openings on each side. The clade with an opening is the Holocephali (Greek holo = whole and cephalo = head), named thus due to the undivided appearance of the head, which results in only a gill opening. The common name for this group – chimera – comes from its bizarre forms: a long and flexible tail, a fish-shaped body and a head with large eyes and protruding teeth that resemble a caricature of a rabbit. The Elasmobranchii (Greek elasmo = plate and branch = gill) have multiple gill openings on each side of the head and include sharks (usually cylindrical, five to seven gill openings on each side of the head), as well as streaks (flattened forms with gill apertures on the ventral face of the head ).
The Elasmobranchii, with about 1,100 species, are divided into three main subgroups (Squalomorphii and Galeomorphii – sharks and rods, and Batoidea rays), and a much lower number of Holocephali (chimeras, with about 40 species). All are predators, using large pelagic prey (fish, turtles, aquatic mammals) to small prey, such as invertebrates with hard exoskeleton, that crush with flat teeth (case of chimeras and most of the rays), and even plankton (filter – whale shark, blankets). Cartilaginous fish inhabit marine environments (most) and may be sweet as well, such as the sting rays of the genera Plesiotrygon , Potamotrygon and Paratrygon.
The oldest fossils of Chondrichthyes date from the lower Devonian, and the group survived successfully to this day. The Chondrichthyes are characterized by:
1. Extreme reduction of the dermal armature, which does not cover the animal’s body, being restricted to some structures, such as spines of the dorsal fin;
2. Endoskeleton basically cartilaginous, often calcified, but never ossified; the skull is a single piece, without sutures, as well as the jaws;
3. Scallops of the placoid type, exclusive of this group – are scales similar to small teeth, formed by dentin and a type of enamel (enamel is a highly mineralized tissue, very hard, of epidermal origin, as opposed to dentin and bone, of origin mesodermic), immersed in the body of the animal (the tip of the scale breaks the skin), giving a rough, “sandpaper” texture.
The teeth of these animals are homologous to their placoid scales which, in the mouth, grow well, supporting themselves in the jaws by ligaments (and not with roots in jaw depressions as in mammals); however, are not homologous to the teeth of the Osteichthyes, representing parallel evolution from the denticles of the armor of the ostracoderms. One problem is these teeth, not being properly fixed, are lost by wear and tear. See the post about replacing the teeth of sharks .
This problem was overcome by the substitution capacity, a mechanism also observed in some bony fish: the teeth are arranged in rows along a spiral, so that in the outer line of the jaws are the rows with functional teeth, while internally, covered by the buccal mucosa, are the rows of developing teeth.
4. Presence of clasper in males, a paired copulation organ developed from the medial rays (facing the ventral midline of the body) of the pelvic fins.
Reproduction occurs in such a way that the male touches the clasper in the female cloaca and the sperm are released by flowing into the female. Thus, sperm migrate to the ovum through the oviduct where fertilization occurs. The cloaca is an opening that assists in reproduction and also in the excreta. Female sharks and rays are larger than males. Therefore, fertilization is internal, opening up the possibility for vivacity and viviparity (see below) in this group.
The condricties do not have a swimming bladder. In cartilaginous fish, the specific weight decrease (body density) is due to the accumulation of oil (lighter than water) in the liver. So it is not true that sharks need to swim all the time not to sink. It is also not true that sharks swim all the time also because of the gas exchange, since they would not have mechanisms for ventilation of the gills. These animals have muscular mechanisms of active entrance of water in the mouth and exit through the pharyngeal crevices, bathing the gills in order to provide the gas exchanges. Therefore, there are elasmobranchs standing in the background, without problems to breathe.
These animals have a ventral mouth with dentin-coated teeth, 10 pairs of cranial nerves, no swimming bladder, rigid skin covered by placfoid scales, pelvic and pectoral girdle, heart with two chambers (atrium and centriole), mesonephric kidneys of the animal), separated genders, breathing by gills of 5 to 7 pairs, reproductive and excretory ducts in which they open in the cloaca, indirect development (they develop inside the mother), eggs megalécitos with large amount of calf. The circulation of these animals is closed, where the different blood (rich and weak in oxygen) are mixed, because there is no separation of the cardiac chambers.
Reproductive modes are equally varied. The most frequent is external fertilization, followed or not by parental care, but there are cases of viviparity, of which the best known are the lebistes and guarus. In general, they are dioecious (male and female), but there are cases of hermaphroditism and even sex exchange throughout life.