Fishes are the earliest and largest group of all the back-boned animals and are found throughout the oceans from the tropics through the polar regions and from surface habitats to the deepest oceanic trenches 10 or more kilometers deep. They range in size from 1 centimeter gobies to 12 meter whale sharks (Paxton and Eschmeyer 1995). Fishes are the most diverse vertebrates with over 24,000 species in 4,200 genera, 483 families and 56 orders. The jawless fishes such as the hagfish and lampreys and the cartilaginous fishes, including the sharks and rays, are the most primitive groups and the bony fishes are more advanced. Of this large group of bony fishes, the fleshy-finned fishes (Sarcopterygii) such as the lungfishes and coelacanths are among the oldest and the ray-finned fishes (Actinopterygii) ranging from sturgeons and paddlefish to salmon and perch are among the most recent.
CLASS AGNATHA (Jawless fishes - lampreys and allies)
Lampreys and hagfishes (Agnatha) are not true fishes because they lack paired fins and jaws. However they are usually included with the fishes because they are frequently collected with fishes (Moyle 1993). Lampreys (Order Petromyzonti-formes) are long and eel-like as adults that are readily recognizable by the sucking disc on the mouth that is covered with teeth, the line of gill slits along each side of the head and the large eyes. The lamprey latches on to the side of its victim, holding on with its sucking disc and rasping a hole with its tongue. It then sucks out blood and other bodily fluids. Lampreys are found in the temperate waters of the northern and southern hemispheres, and in cool deep waters in some parts of the tropics.
CLASS CHONDRICHTHYES (Cartilaginous fishes - sharks, rays, skates, chimaeras)
Fishes in the Class Chondrichthyes have a skeleton that is almost entirely cartilage and thus they are typically referred to as the cartilaginous fishes. They differ from the Agnatha in that they have well-developed jaws, paired nostrils and paired pectoral and pelvic fins. Longevity records include species in the following orders (descriptions of orders taken from Paxton and Eschmeyer 1995; Moyle 1993): (1) Carcharhiniformes (ground sharks) - this order contains 8 families and over 200 species. They include "typical" sharks such as whaler or requiem sharks, cat sharks and hammerhead sharks; (2) Heterodontiformes (horn sharks) - this order contains one family of 8 species; most species live mainly in shallow-water and are bottom living with restricted distributions in warm temperate and tropical areas; (3) Lamniformes (basking sharks) - (4) Orectoblobiformes (carpet sharks) - a diverse group of 7 families and around 33 species; includes the gigantic whale shark, the carpet shark and the zebra shark. Habitats range from specialist feeders on reefs to open water filter feeders; (5) Rajiformes (skates) - the skates are a diverse group of rays with approximately 200 species worldwide. They live near the bottom in all oceans from shallow estuarine to deepsea habitats; (6) Squaliformes (dogfish sharks) - this is a large group of mainly deepwater sharks which occur in tropical, temperature and even artic waters. There are 3 families and over 90 species; (7) Myliobatiformes (develrays and allies) - this order consists of three families and around 150 species, most of which live on the continental shelf in tropical regions. Species include the plankton-feeding manta which can grow up to 6.5 meters in width and the stingray.
CLASS OSTEICHTHYES (Bony fishes)
This group contains the vast majority of extant species with over 20,000. Longevity records were obtained for species in the following orders (descriptions of orders taken from Paxton and Eschmeyer 1995; Moyle 1993): (1) Acipenseriformes (sturgeons and paddlefish) - confined to the northern hemisphere in both fresh waters and coastal marine waters; sturgeons are bottom feeders and paddlefish are filter-feeders; (2) Amiformes (mudfish; bowfins) - typical lowland species common in backwaters and oxbow lakes preferring clear waters; (3) Anguilliformes (eels) - this group consists of 15 families with about 650 species; includes the tropical and subtropical moray and conger eels and the freshwater eels that live in the Americas and Europe; (4) Atheriniformes (silverside family) - this group consists of 7 families with approximately 275 species; includes California grunion and jacksmelt; (5) Ceratodonitiformes (lungfish) - species in this group are relicts of ancient fish groups that were near the evolutionary stem of amphibians. They possess a swim bladder that developed into a highly vascularized lung that enables them to breathe air; (6) Characiformes (Charcoid) - this group consists of 12 families and around 1,400 species found in Africa and Central and South America; best known species is the South American piranha; (7) Clupeiformes (shad; herring) - (8) Cyprinidonti-formes (killifishes and ricefishes) - this group consists of 9 families and 7000-800 species; members in this group are among the most popular fishes in hobby aquariums including the guppy, the platy and Gambusia spp; (9) Cypriniformes (carps; loaches; shiners) - this mostly freshwater group consists of 6 families and over 2,000 species; found on all continents capable of supporting fish life; (10) Gadiformes (cod; haddock) - wide-ranging group found around the world inhabiting deep to shallow seas and ranging from tropics to both polar regions; includes rattails, hakes, and a variety of cod species; (11) Syngnathiformes (seahorses) - seahorses are contained in a single genus, Hippocampus; possess a prehensile tail and a head positioned more or less at a right angle to the body - hence the term "seahorses"; (12) Gobiesociformes (dragonettes); (13) Perciformes (perches and allies) - found in almost all kinds of aquiatic habitats from mountain streams to deep oceans; over 9,000 species in 160 different families including drums, croakers, snappers, butterfly fish, freshwater perch, sunfishes, freshwater basses, tunas, mackerals and billfishes; (14) Pleuronectiformes (flatfishes; soles; flounders) - this group consists of 7 families and 540 species; found mainly on the continental slopes of oceans and include many of the seafood varieties including sole, halibut, turbot, and brill; (15) Salmoniformes (pikes; smelts; salmons and allies) - most salmoniform families occur in fresh water though some species spend part of their cycle in the sea and migrate to rivers to spawn; smelts resemble salmons and trout but are smaller and primarily marine; pikes are mostly freshwater, large, toothed predators; (16) Siluriformes (catfishes) - more than 2,200 species of catfishes are known with extant members found on all continents except Antarctica; found in almost all freshwater environments ranging from mountain streams to slow-moving rivers; some species found in coastal marine habitats.
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