In this article we will discuss about:- 1. Origin of Dogfish 2. Systematic Position 3. Distribution 4. Habits and Habitat 5. Economic Importance.
Origin of Dogfish:
Pisces (Fishes) are aquatic, cold blooded vertebrates that breathe by means of gills and live in water. There are about 30,000 to 40,000 species of fishes differing widely from each other in shape, size, habits and habitat.
They live in all the seas, rivers, canals, lakes, dams, ponds and in almost every place where there is water. Fishes usually have a streamlined body but some are elongated snake-like and few are dorso-ventrally flattened.
They have paired and unpaired fins supported by soft or spiny fin rays. Dorsal, anal and caudal fins are unpaired, while the pectorals and pelvics are paired. There is always an exoskeleton of dermal scales. Respiratory organs are chiefly gills. Besides the olfactory, photic and auditory organs they have a lateral-line system or receptors. Sexes are always separate.
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Economically the fishes constitute a very important group of animals. Besides being used as food, fish liver is an important source of oil. Fishes also provide fish-meal, fish- manure, isinglass and several other products of commerce. It is, therefore, natural that man has paid a considerable attention to the study of habits and habitat, anatomy and physiology of fishes.
Living fishes with jaws fall into two well marked classes- Chondrichthyes, (Gr., chondros = cartilage; ichthys = fish), the cartilaginous fishes such as sharks and rays and Osteichthyes (Gr., osteon = bone; ichthys = fish), the bony fishes such as ray-finned fishes and lung fishes.
The Chondrichthyes, also called elasmobranchs, comprise sharks, rays, skates, chimaeras, etc. Dogfish sharks are extensively studied nearly all over the world. The description included here belongs to the common Indian dogfish shark, Scoliodon sorrakowah, worked out by E.M. Thillayampallam and first published in 1928 in the series of Indian Zoological Memoirs.
Systematic Position:
Distribution:
Scoliodon has wide geographical distribution. It has been recorded from Zanzibar to Sri Lanka, and Sri Lanka to Malay Archipelago in the Indian Ocean, from the Bay of Bengal, the East Indies and the Philippine Islands, from Mexico to Panama in the Eastern Pacific, from Labrador to Brazil in the Atlantic, from Cuba in West Indies, and also off the Eastern coasts of South America.
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Four species of Scoliodon are found in Indian waters and these are S. sorrakowah, S. dumerilii, S. palasorrah and S. walbeehmi. The common Indian dogfish is Scoliodon sorrakowah which means ‘black shark’ in Tamil (sorra = a shark + kowah = black). Fossil records of Scoliodon have been in the geological strata from the lower Eocene to later periods.
Habits and Habitat:
Scoliodon is marine like most sharks. It is found in all parts of the open sea, i.e., Indian, Pacific and Atlantic Oceans. It is carnivorous feeding on crabs, lobsters, worms and fishes, etc. It is a fast swimmer as the body is perfectly stream-lined without having rigid projections. Sexes are separate. Fertilisation is internal and development is direct. It is viviparous, i.e., producing living young ones that develop inside the uteri.
Economic Importance:
Sharks are predaceous carnivores feed on lobsters, crabs and fishes. Poor people living along the sea coasts eat dogfishes. Its flesh is not delicious but it is highly nutritious. Its dried skin called shagreen is used as an abrasive for polishing furniture, etc. It is used for dissection in laboratories of the colleges and universities.