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Alligator gar (''Atractosteus spatula'') are ray-finned euryhaline fishes related to bowfin in the infraclass Holostei (ho'-las-te-i). The fossil record traces the existence of alligator gars back to the Early Cretaceous over a hundred million years ago. They are the largest in the gar family, and among the largest freshwater fishes in North America. Gars are often referred to as "primitive fishes", or "living fossils" because they have retained some morphological characters of their earliest ancestors, such as a spiral valve intestine which is also common to the digestive system of sharks, and they can breathe both air and water. Their common name was derived from their resemblance to American alligators, particularly their broad snout and long sharp teeth. Anecdotal scientific reports suggest that alligator gars can grow up to in length and weigh as much as ; however in 2011 the largest alligator gar ever caught and officially recorded was long, weighed , and was around the girth. Their bodies are torpedo shaped, usually brown or olive fading to a lighter gray or yellow ventral surface. They do not have scales like other fish, rather they are armored for protection against predation with hard, enamel-like, jagged diamond-shaped ganoid scales that are nearly impenetrable. Unlike other gar species, mature alligator gars have a dual row of large sharp teeth in the upper jaw which they use for impaling and holding prey. They are stalking, ambush predators that are primarily piscivores, but will also ambush and eat water fowl and small mammals that may be floating on the surface. Alligator gars have been extirpated from much of their historic range through habitat destruction, indiscriminate culling, and unrestricted harvests. Populations are now located primarily in the southern portions of the United States extending into Mexico. They are considered euryhaline because they can adapt to varying salinities ranging from freshwater lakes and swamps to brackish marshes, estuaries, and bays along the Gulf of Mexico. For nearly a half-century, alligator gars were considered "trash fish", or a "nuisance species" that were detrimental to sport fisheries, and therefore targeted for elimination by state and federal authorities in the United States, but the last ten years has seen a greater emphasis placed on the importance of alligator gars to the ecosystems they inhabit. As a result, they were afforded protection by restricted licensing. They are also protected under the Lacey Act which makes it illegal to transport fish in interstate commerce when in violation of state law or regulation. Several state and federal resource agencies are monitoring populations in the wild, and have initiated outreach programs to educate the public. Alligator gars are being cultured in ponds, pools, raceways and tanks by federal hatcheries for mitigation stocking, by universities for research purposes, and in Mexico for consumption. ==Anatomy and physiology== Alligator gars are the largest species of gar, and among the largest freshwater fishes found in North America. Mature alligator gars commonly measure in length, and weigh over 100 lbs. (45 kg). However, anecdotal reports suggest they can grow up to 10 ft (3m) in length, and weigh as much as 350 lbs. (159 kg). The largest alligator gar officially recorded was inadvertently caught in the net of fisherman Kenny Williams of Vicksburg, Mississippi while he was fishing the oxbow lakes of the Mississippi River on February 14, 2011. Williams was pulling up his net on Lake Chotard expecting to find buffalo fish, but instead discovered a large alligator gar tangled in his net. The gar was long, weighed , and its girth was . According to wildlife officials, the fish was estimated to be somewhere between 50 and 70 years old. Williams donated it to the Mississippi Museum of Natural Science in Jackson where it will remain on display. All gars have torpedo-shaped bodies, but some distinguishing characteristics of adult alligator gars include their large size, heavy bodies, broad heads, short broad snouts, large sharp teeth and double row of teeth on their upper jaw. They are usually brown or olive fading to a lighter gray or yellow ventral surface. The dorsal and anal fins are positioned toward the back of their bodies, and their caudal fin is abbreviate-heterocercal, or non-symmetrical.〔 Alligator gars have gills, but unlike other fishes they also have a highly vascularized swim bladder lung that supplements gill respiration. The bladder not only provides buoyancy but also enables them to breathe in air which is why they are able to inhabit bodies of water in which most other fishes would die of suffocation. The swim bladder is connected to their foregut by a small pneumatic duct which allows them to breathe or gulp air when they break the surface, an action that is seen quite frequently on lakes in the southern United States during the hot summer months. The scales of alligator gars are not like the scales of other fishes; their bodies are protected by overlapping, enamel-like ganoid scales that are diamond-shaped with jagged edges and composed of a hard inner layer of bone and an outer layer of ganoin that is nearly impenetrable. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Alligator gar」の詳細全文を読む スポンサード リンク
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