A chimaera is a type of fish considered to be one of the oldest still in existence. Mostly deep water fish, chimaeras make up over 40 different species scattered throughout the world. Also known as a ghost shark, ratfish, or rabbitfish, the chimaera has a number of interesting features developed over millions of years.
Originally, the chimaera species sprung from the same ancestor as sharks, but differentiated extensively at least 400 million years ago. They are considered somewhat related to other cartilaginous species, such as rays and skates. Like sharks, they remain a cartilaginous group, featuring a skeleton made of stiffened cartilage but not bone. Unlike their shark cousins, however, these fish have a fused upper jaw and fused tooth plates that look like rodent or rabbit incisors, hence the nicknames of ratfish or rabbitfish.
Although many species have variations, most chimaera are recognizable by their long, slender tails and wide, flat heads. Some feature bright green eyes, or an long tapered snout. Coloration varies by species, with most ranging from brown to silver-gray. Some, such as the spotted ratfish, features a brown overall color broken up by white spots.
Like sharks and rays, they have smooth skin and no scales. Sizes vary among different species, with some, like the deep-dwelling long-nosed chimaera, reaching up to five feet (1.52 meters) in length. Females are generally considerably bigger than males. Males feature a distinct clasping organ near the fins, used to hold down the larger females during mating. The egg cases of the chimaera are distinct, featuring a leathery egg case shaped like a spindle.
Somewhat dangerous to humans, the chimaera feature a venom-injecting spine or fin that can be used for defense. If attacked or grabbed, the fish can raise this spine to ward off predators. Not all species feature this unusual spine, but it serves as an excellent defense mechanism for many chimaera species.
Despite their deadly venom, injury to humans is rare thanks to the extreme depths at which many of these creatures dwell. While a few species prefer near-shore waters of moderate depth, most live in the far deep of the world's oceans, often descending up to 8500 ft (2.59 kilometers) near continental shelves. As deep sea research has expanded thanks to improved submarine technology, many new species of chimaera have been found in deep places, bringing the total number of different types of chimaera to over 40.