What is a Chimaera?

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.

A chimaera, also known as a ghost shark, is a mysterious and elusive cartilaginous fish related to sharks and rays. They are deep-sea dwellers, often found at depths of up to 2,600 meters. Chimaeras have elongated bodies, wing-like pectoral fins, and distinctive, retractable sexual appendages on their foreheads for males.

Chimaeras differ from sharks in several ways. They have smooth skin without the typical denticles found on sharks, and their gill openings are covered by a flap of skin. Unlike sharks, which have multiple gill slits, chimaeras possess a single external gill slit. They also have a more elongated, tapering body compared to the robust form of sharks.

Chimaeras primarily feed on the ocean floor, consuming invertebrates like mollusks, crustaceans, and echinoderms. Their tooth plates are adapted for grinding rather than cutting, which is ideal for their bottom-dwelling prey. Their diet reflects their benthic lifestyle, foraging in the deep sea's sediment.

Some chimaera species are considered vulnerable or endangered due to deep-sea fishing practices and habitat destruction. The International Union for Conservation of Nature (IUCN) has listed certain species like the Giant Black Chimaera as Near Threatened, indicating that they may become endangered if the circumstances threatening them continue.

Chimaeras have a unique reproductive system. They are oviparous, meaning they lay eggs. These eggs are encased in leathery capsules, often referred to as "mermaid's purses," which are deposited on the ocean floor. Male chimaeras have claspers, like sharks, for internal fertilization, and a retractable sexual appendage on their forehead known as a tenaculum to hold onto the female during mating.

Chimaeras are rarely kept in aquariums due to their specific deep-sea habitat requirements. They need cold, deep water, which is difficult to replicate in captivity. Additionally, their elusive nature and sensitivity to changes in pressure and temperature make them unsuitable for most public aquariums. There are very few instances of chimaeras being displayed successfully in aquarium settings.