A chimaera, or rabbitfish, on the sea bed
Text: Dorien Schröder
Chimaeras are also known as ghost sharks, even though they are technically not sharks. Both belong to the class Chondrichthyes, the cartilaginous fish, but sharks and rays belong to the subclass Elasmobranchi, whereas chimaeras belong to the subclass Holocephali. The chimaera branched off from the sharks and rays about 400 million years ago and are the only order left of the Holocephali, all others have disappeared.
Chimaereas mainly live in the deep waters of temperate oceans, up to a depth of 2,600 meters. They have long, soft bodies, with a bulky head and only one gill-opening. Chimaeras grow up to a length of 1.5 meters and most species have a venomous spine in the dorsal fin. Some of the species have amazing shapes.
There are a few similarities with sharks, for instance, male chimaeras have claspers for internal fertilisation of the females and they also lay eggcases, which are spindle-shaped. The elongated snouts of chimaeras have electroreception to locate their prey.
Chimaera, picture by Doug Perrine
However, unlike sharks, male chimaeras also have retractable sexual appendages on the forehead and in front of the pelvic fins. They also differ from sharks in that their upper jaws are fused with their skulls and they have separate anal and urogenital openings. They lack sharks’ many sharp and replaceable teeth, having instead just three pairs of large permanent grinding tooth plates. They also have gill covers or opercula like bony fishes.
The chimaeras are an enigmatic and understudied group of fishes particularly vulnerable to impacts of deep-sea fisheries. This vulnerability is compounded by taxonomic uncertainties, because it shares similarities with both bony and cartilaginous fishes, and a lack of life history information.
