Predatory saltwater fish. Tropical in origin, they are one of the world's poisonous fish. Lionfish rarely bite--their venom is in their stiff and highly sharp dorsal spines they use strictly for defense. (Lions do not hunt with their venom.) They rarely sting except when feeling threatened, but the occasional specimen realizes it is poisonous and stings without provocation.
Lionfish are striped, usually brown and white, although some are black or red-rust on white. The species most commonly available in pet stores is the volitans lion, which will reach about a foot in length, although some of the dwarf lions like the fuzzy lionfish and the fu-manchu lionfish, which both top out at about 6 inches, are becoming more common as well.
Lions are easily identifiable by their flowing graceful pectoral fins that flare out from their sides like fans. The rays will often wear and break with age, but when the fish is young, the rays may be longer than its body. In an old individual, rays half the length of the fish's body are considered excellent. Patterns, often resembling peacock spots, frequently deepen on the webbing between fin rays with age.
Captive lionfish will eat live food such as ghost shrimp, feeder comet goldfish, or rosey minnows but can easily be weaned onto frozen food such as silversides, or freezedried foods such as krill over time. They need a high protein diet to stay healthy. Lionfish are highly agressive and can only be put with other predator fish such as puffers, triggerfish, and groupers. If it fits in their mouth, they will eat it.
Lionfish are poisonous, but their sting is rarely lethal to humans, unless the person stung is allergic. (This is a fairly rare allergy, but if one is allergic to bees, ants, wasps, or any other animal protein venom, they are more prone to also being allergic to lionfish.) If stung, immediately run the sting under water as hot as you can stand--the heat starts breaking down the venom before it can do damage to your nerves. Expect the area to be stiff, sore, and slightly swolen for up to two weeks.
It's come to my attention that lionfish, nowadays, are most often identified as "hey, look, its that fish from Deuce Bigalow, Male Gigolo" for there is apparently a scene involving a prize lionfish whose tank is clumsily destroyed in highly stupid ways. But there's more to the fish than the stupid movie. (Grrr! you come to the fish store where I am, and give me the above line, I will hurt you. Seriously. The only movie/fish comment I hate more than this is "OMG, NEMO NEMO NEMO!")