Elasmobranchs . Also called plagiostomes or selacians, they are a subclass of fish from the class of chondrichthyans , with placoid scales , called dermal denticles.
Summary
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- 1 Features
- 2 Classification
- 3 Vision
- 4 Hearing
- 5 Electrosensitivity
- 6 Sources
characteristics
The head is prolonged in a face or cone previous to the mouth, and this one in ventral or ínfera position; gills housed in chambers that open independently by five or more indentations on either side of the pharynx , although sometimes ventrally; they are provided with a blowhole behind each eye, with a heterocluding tail fin and cloaca. The lack of operculums that control the entry and exit of the water forces these fish to remain in continuous movement to force the water to penetrate the gills.
The skin is covered with dermal denticles. They have a sensitive organ throughout the body called the lateral line that allows them to perceive the vibrations of the water and even the taste of their prey.
Classification
- The Seláceos: There are two types of sellaceae: the sharks or pleurotremes or sharks and the batoids or hypotremes or rays.
The sharks have a fusiform body and a pointed snout, highly developed fins with which they can move at high speed. This group includes sharks, who have powerful teeth. They are very voracious predators, most carnivores and some dangerous to swimmers. Rays have a flattened or rhombic or disc shaped body. The mouth, nostrils, and gill slits are located on the ventral side. Most of the tails are whip-shaped and swim due to the wave motion of the pectoral fins. They usually live quite deep. Some, like the torpedo, have electrical organs. Rays are rays, torpedoes, blankets, sea eagles. etc.
- The Holocephalus: The holocephalus are elasmobranch fish with a very long body with numerous spines, some of them with poisonous glands, dangerous for swimmers, since they are very aggressive.
They have a large head with large eyes and a small mouth, whose teeth are fused into dental plates, which form like a parrot’s beak. They have no blowhole. The seminal liquid is carried in the form of packages. The most typical is the chimera.
Vision
It is known worldwide that the vision of sharks is poor compared to other senses, especially smell; Sharks’ sense of smell is thought to be more important than vision and the other senses, to the point of being called “swimming noses,” although eye scientists acknowledge that the eye anatomy of elasmobranchs is highly developed. Research conducted in the 1960s and decades has altered our knowledge of the visual capabilities of sharks.
Hearing
Hearing in sharks is a subject of great interest since sound in water has the ability to spread over great distances and in all directions; in addition, because until now no shark is known to emit sounds, so listening skills must be shaped by environmental sounds.
Electrosensitivity
All elasmobranchs have an elaborate system of ampoules sensitive to low-frequency electrical stimuli (Bodznick & Boord, 1986 ; Montgomery, 1984 ; New, 1990 ). The electrosensitive ampoule system consists of groups of subdermal electroreceptive units known as “Lorenzini ampoules”, which can detect weak external electrical stimuli at intensities as low as 5 nV / cm (Kalmijn, 1982 ).
The Lorenzini blisters were recognized and initially described by Stenonis ( 1664 ) and Lorenzini ( 1678 ), but their physiological and behavioral functions were unknown for the next three centuries; Lorenzini ampoules were initially thought to be mechanoreceptors (Parker, 1909; Dotterweich, 1932), over time different hypotheses were raised about the function of these structures within which was the sensitivity to temperature (Sand, 1938; Hensel, 1955 ), or the detection of changes in salinity (Loewenstein & Ishiko, 1962 ), until in later years the idea of his task in electroreception was generally accepted.
The first authors to demonstrate this ability were Murray (1960b), Dijkgraaf and Kalmijn (1962); Brown (2003) who found evidences on the sensitivity to temperature by the extracellular gel contained in the channels of the ampoules, indicating that it can develop significant voltages in small temperature gradients.
It is now known that the extremely sensitive electro-receptor ampoule system of elasmobranchs is decisive for the orientation towards inanimate electric fields (Kalmijn, 1974, 1982; Pals et al., 1982b), in theory geomagnetic navigation (Kalmijn, 1974, 1988b, 2000; Paulin, 1995), and it is known that potential predators (Sisneros et al.) Are important for the detection of bioelectric fields produced by their prey (Kalmijn, 1971, 1982; Tricas, 1982; Blonder & Alevizon, 1988). , 1998), and their congeners during social interactions (Tricas et al., 1995).