Snakehead fish is a term given to 25 different species of freshwater fish of the genus Channa that can be found throughout southeastern Asia (one species is found as far west as the African tropics). Snakeheads are apparently tasty (I don't really like the taste of fish in general so I can't back this up myself) but what are probably the most noteable things about them are their voracious appetites and ability to waddle their way from pond to pond, over dry land, if their food supply runs out.
Adult snakeheads will eat just about anything smaller than them, particularly other fish (even other snakeheads), frogs, aquatic birds, baby turtles, and smaller types of mammals that come close enough to the water. The juvenile snakeheads feed on worms, insects, tadpoles, dragonfly larvae, and similar small creatures they have access to. Snakeheads have even been known to eat creatures roughly equivalent in size to themselves. Snakeheads in captivity can be trained to eat strips of meat held between their keepers' fingers. Snakeheads are able to leap out of the water to catch small animals (thus, captive snakeheads can escape their tanks if the tops aren't covered). If a snakehead has a large enough food supply, it will gorge itself until it can barely swim.
As one might figure out from the name, snakeheads are named for their head's resemblence to those of snakes. Specifically, the shape and position of the eyes is much like that of a snake. As juveniles, snakeheads vary in colour but as they age, they become nearly uniform in their colour, which can range from greenish to brown. The fish have several darkly coloured daigonal bands that form rectangular spots across their body, which are sometimes complimented by rows of spots of a colour that could be described as mother of pearl. The fish's dorsal and anal fins are usually red. Channa obscura, the African species, is generally regarded as the most beautiful.
Channa micropeltes (AKA giant snakehead, red snakehead, or toman), the largest of the snakeheads, can grow to a meter in length. Keeping a toman in captivity would require a 100 gallon tank, minimum. The fish grow at about an inch per year, so a bigger tank would be nice for them. Snakeheads shouldn't be kept with smaller or even some equally sized fish (this includes other snakeheads), as snakeheads will most likely eat them. Snakeheads are constantly hungry and fiercely predatory. Live fish are their preferred meal, though earthworms and chopped beef heart are adequate substitutes. Breeding in captivity is difficult to accomplish, as a very large tank is required. Channa asiatica and Channa gachua are regarded as the best species to raise in an aquarium, as they are the smallest, reaching their maximum length in only one or two years.
Snakeheads have an interesting breeding ritual. First of all, if one of the pair is smaller than the other, the likely outcome is not the two mating but the larger fish eating the smaller one. Assuming no cannibalism occurs, the two fish will approach the surface of the water in the morning and begin circling each other in gradually narrowing movements until they come together. At this point, the female lays yellow-coloured eggs, each contained within an air bubble, and the male fertilizes them. The fish (in some species, only the male) then guard the eggs for two to three days, after which the young will hatch.
The young use the large yolk sac from their eggs to cling to aquatic plants for around eight days before they begin swimming normally. By this time, most of the sacs will have been consumed. While juveniles, snakeheads swim together in schools and retreat to quiet, less noticeable areas to digest their food after eating. Once snakeheads are large enough to feed on larger types of prey, they'll separate and typically spend most of their time either alone or in pairs after that, finding an area to claim as their own territory.
Snakeheads are incredibly tolerant of environmental changes. The fish can live in waters with a pH value ranging between four and nine. Snakeheads are able to survive outside a body of water in open air, provided they can stay moist, for up to four days (this is how they're able to survive migrating from one waterhole to another in search of food and makes them valued by cooks who value the freshness of the fish they prepare). In the event that a drought or similar event causes their habitat to dry up, snakeheads can settle themselves into the mud of drying waterholes, burrowing deeper in the drying it gets, and live in this manner for several months. In this state, the fish live off atmospheric oxygen and fat stored in their bodies until they're able to swim and eat again.
In mid-2002, Channa argus argus, the northern snakehead (native to China), was found in a pond in the United States (Maryland, specifically). As the snakeheads have no natural predators in Maryland and a tendency to eat everything they can, the Maryland Deparment of Natural Resources Fisheries Service has asked that anyone who comes across a northern snakehead in wild kill the fish, specifically by cutting it as they can survive out of water for up to four days, and report the encounter. Authorities believe the fish reached Maryland by being transported for the purposes of being sold at an Asian-style market in the area and somehow were released into the wild. Though snakeheads are legal to own in Maryland (they are not legal to own in thirteen other states), pet store workers doubt someone who would buy one would release it into the wild.
The fish present a problem to the ecosystem of Maryland because they could wipe out tons of naturally-occuring fish in the area simply by feeding and then moving on to another pond, lake, marsh, etc (the walking catfish, Clarias batrachus, caused a similar problem in Florida when some arrived there from southeastern Asia in the 1960s). In addition, environmental authorities are unsure of how to handle the problem. Poisoning bodies of water believed to be home to the fish would kill off all other wildlife living in or around those bodies of water. Draining the ponds or setting off small explosives in the water presents the same problem. One possible solution is to run a low electric current through the water in order to stun all the fish living inside. The unconscious fish will float to the surface, whereupon authorities can remove any snakeheads from the crowd of stunned fish, then allow the remaining species to eventually wake back up and return to their normal life. This doesn't sound particularly healthy for any fish involved but it is at least better than guaranteeing them all death.
In the meantime, the media and local fishers have seized the opportunity to catch (or, at least, catch footage) of something exotic. At the time of this writing, the remaining snakehead(s) (one was caught and killed but based on the account of a fisherman who caught and released one in May, not knowing what it was, authorities believe there is at least one more in the pond) continue to elude capture, as the hot weather has driven most fish deeper into the pond (so they can more easily breathe), with the snakehead(s) in pursuit (so they can more easily eat the other fish).
UPDATE (5 August, 2002): Northern snakeheads have been found outside the pond in Maryland already mentioned, meaning they've been able to spread beyond it.
UPDATE (30 September, 2002): czeano says re snakehead fish: i'm pretty sure they killed 'em all though, or I'd certainly be hearing more about it in the local news (THIS JUST IN! EVACUATE! RUN FOR YOUR LIVES!)
As much as I prefer balance be kept in the environment, I would really like it if not only did the snakeheads in Maryland survive, but also mutate into unstoppable killing machines. Bear with me on this: A group of city officials are standing around a poisoned pond, patting themselves on the back about their excellent handling of the situation, when all of the sudden, a northern snakehead fish leaps out of the eeriely still and opaque water, several feet into the air, and flies straight into a city counsel member's neck. Using its sharp teeth, the snakehead manages to quickly chew its way through the neck, decapitating the counsel member!
Chaos erupts as the snakehead falls to the ground and begins waddling toward more victims. There is much running and screaming from the humans as more snakeheads leap out of the water and attack. The snakeheads proceed into town, killing more and overturning cars (it's not important how), which of course burst into flames. Amidst all this, a lone snakehead waddles his way into a church, where it looks up at a confused priest and in a tiny, scratchy, and evil voice, tells him "Darrrrwiiiin was riiiiight."* The snakehead then waddles back to the chaotic battle raging outside.
*This makes more sense if you've ever seen those Darwin fish-with-feet some people have on their cars.