The Chimaera: bird, fish and rat in one
In Greek antiquity the Chimaera was a mythical monster, depicted as an incongruent animal with the head of a roaring lion spitting fire, the body of goat sticking out in the middle and a vicious snake-like tail. The word also stands for a delusion; like in the french chimère. In our oceans the chimaera fish (alias: ghost shark, rabbit fish, spook fish) earned its name because of its strange body shape: a composition of a birds beak, a fishlike body and long slender tail (see picture at left). The rat-like tail is the reason why certain species are called ratfishes. Chimaera seems to have diverged from its shark relatives around 400 million years ago. The ways of evolution are often mysterious and hard to unravel. Which holds also for its products, the creatures that have lived in the seas for millions of years, and probably even more so for the chimaera.
Considering the diversity of 'shark like species' it is good to start with a taxonomic classification. Chimaera, skates, rays and sharks are all cartilaginous fish, belonging to the class of Chondrichthyes with a cartilaginous skeleton, and claspers in the males. There are two subclasses of cartilaginous fishes: the Elasmobranchii (with sharks, rays and skates and the sawfish) and the Holocephali (chimaera), indicating that members of Elasmobranchii are more closely related to another than to the chimaera. Further down the taxonomy we have families and species. Chimaera, or rather the order of Chimaeriformes consists of many different species with different outlooks and habitats. Some species have several synonyms or aliases which make the naming process a bit messy. But taken together they end up in 50 accepted species that are assembled in three families:
Families and species
-Callorhinchidae (Plownose chimaera: alias elephant fish and ghostshark, only one accepted species) are the oldest clade in the evolution thee. They have an elongate and flexible snout bearing a hooklike structure and are mostly found on the Southern hemisphere.
-Chimaeridae (Shortnose chimaera or ratfishes; 40 species) have a short and rounded snout. They are found in Atlantic, Pacific, and Indian oceans in temperate to tropical waters, mostly below 200 m. Some popular species are Chimaera monstrosa alias rabbit fish (the only species in the Mediterranean), the spotted ratfish (Hydrolagus colliei) in the north-eastern Pacific Ocean, and the small-eyed rabbitfish (Hydrolagus affinis).
-Rhinochimaeridae (Longnosed chimaera; 8 species) have a long and pointed snout, lacking a hooklike process and living worldwide in temperate and tropical seas. Some weird looking species are: Narrownose chimaera, alias Harriotta raleighana alias spookfish, the long nosed Rhinochimaera pacifica and the paddlenose chimaera (Rhinochimaera africana) with its flattened paddle-like nose.
Anatomy and habitat Chimaeriformes have large rabbit-like eyes, and a large head along with a tapered body. The large translucent-green eyes are adaptions of the darkness of water at greater depths. Although chimaera has some characteristics in common with sharks and rays, there also big differences. One example is the head with the small mouth and lips, and upper jaws that are fused with their skulls. Chimaera also miss the row of sharp teeth of sharks, but posses three bony tooth plates with fused teeth, forming a ideal beak to break hard shells. Their diet consists mainly of bottom-dwelling invertebrates like sea urchins, crabs, shell fish, crustaceans and starfish.
Chimaera has two large dorsal fins, the first erectile high. with a short base and preceded by an erectile poisonous spine, the second nonerectile low, and with a long base. Their bird-like style of swimming with the spread out big pectoral fins resembles the propulsion of rays. Like sharks they are equipped with electroreceptor cells on their snout for orientation. But unlike sharks and rays, chimaera has a single external gill opening, covered by a flaps or opercula as in the bony fishes, on each side of the body. Breathing water chiefly occurs through the nostrils.
Chimaera species vary in size between 60 cm and 1.5 m and live in temperate oceans. They tend to dwell on muddy or sandy seafloors, often down to 2,600 m deep, with few occurring at depths shallower than 200 m. This is also the reason why pictures of these species are rare. Exceptions include the members of Chimaeridae, like the rabbit fish and the spotted ratfish, which locally or periodically can be found at relatively shallow depths. Many species that live on or just above seafloors at greater depth have become the victim of deep sea trawlers.
Reproduction Chimaera are oviparous. Male chimaeras have claspers formed from the posterior portion of their pelvic, one of which is used to inseminate the femal. In addition, they possess a supplemental clasping organ, the tenaculum on the forehead, which is thought to aid in holding the female during mating. It is only visible during copulation and then used to clamp onto the female’s pectoral fin. This remarkable stalked club structure with little hooks must have been a clever adaptation of their evolutionary ancestors that lacked the rows of sharp teeth that male sharks use to hold their mating partner in a steady position. The females lay eggs in spindle-shaped, leathery egg case. Finally, there is a second grasping structure, the pre-pelvic tenaculum, just before the pelvic fins that also allow the male to anchor into position.
Sources and links
Nelson, J.S., 1994. Fishes of the world. Third edition. John Wiley & Sons, Inc., New York. 600 p.
Didier, D. A., Kemper, J. M., & Ebert, D. A. (2012). Phylogeny, biology, and classification of extant holocephalans. Biology of sharks and their relatives, 2nd edn. CRC Press, New York, 97-124.