22. Oct, 2017

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.

Froese, Rainer, and Daniel Pauly, eds. (2014). "Chimaeriformes " in FishBase. November 2014 version

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.

https://www.britannica.com/animal/chimaera

https://themarinedetective.com/category/fish/ratfish-chimera/

https://en.wikipedia.org/wiki/Chimaera