Chimaeras: Strange Fish with a Cartilaginous Skeleton
What are Chimaeras?
Chimaeras are strange fish that have a network of lines over their surface. The lines often look like seams and make the fish look as though it's been stitched together from pieces of other creatures. The appearance is reminiscent of the chimera, a creature in Ancient Greek mythology that consisted of parts of different animals joined together.
A chimaera has a big head with large eyes and a snout in front of its eyes. The snout looks like a duck's bill. Some chimaeras are known as ratfish because their body tapers into a long, rat-like tail and their teeth look somewhat like rat incisors. Some are called rabbitfish because their face reminded early naturalists of a rabbit's face. The name "elephant fish" arose because in some species the tip of the snout has a curled projection that looks like a miniature elephant's trunk. Rhinochimaeras have a long projection from their snout.
Chimaeras move by flapping their large pectoral fins, which makes them like they are flying through the water. Many species have a sharp, poisonous spine in front of the first dorsal fin on the top of their body. The chemicals released by the spine seem to be only mildly poisonous to humans. The spine can inflict a painful wound, however.
A Chimaera of the Deep Sea
Biological Classification of Chimaeras
Chimaeras are distantly related to sharks. They have some unique characteristics and look quite different from sharks, however. The skeleton of both a chimaera and a shark is made of cartilage instead of bone.
According to the traditional classification scheme, all cartilaginous fish belong to the class Chondrichthyes. This class is divided into two subclasses: the subclass Elasmobranchii, which contains sharks, skates and rays, and the subclass Holocephali, which contains chimaeras.
The subclass Holocephali contains just one order - the Chimaeriformes. The order contains three families, which are discussed in this article. The families are listed below.
- Chimaeridae: rat fish (ratfish) and rabbit fish (rabbitfish)
- Callorhinchidae: elephant fish (elephantfish) or plough-nosed chimaeras
- Rhinochimaeridae: long-nosed chimaeras
Some fish in both of the first two families listed above are known as ghost sharks. In addition, there are bony fish (class Osteichthyes) with the same common name as some chimaeras. There are bony rabbitfish, for example. This is one reason why scientific names are useful in identifying an animal.
Spotted Ratfish Appearance (Family Chimaeridae)
The spotted ratfish (Hydrolagus colliei) lives in the northeastern area of the Pacific ocean off the coast of North America. The skin has a pattern of white spots on a brown or grey background. It's smooth and scaleless and often has an attractive sheen.
Like the bony fish - the large class that contains most fish - the spotted ratfish has a cover over its gills called an operculum. However, the ratfish gill cover is soft and fleshy while the bony fish gill cover is made of bone. Sharks lack an operculum.
The ratfish has two dorsal fins on its back. There is a spine in front of the first one. The second dorsal fin consists of two small lobes and can be mistaken for two separate fins. There is a caudal fin above and below the elongated and narrow tail. The fish has a pair of triangular pectoral fins (one on each side) towards the front of its body and a pair of pelvic fins towards the back of its body.
The chimera was a fire-breathing monster in Ancient Greek mythology. It had the head of a lion, the body of a goat and the tail of a snake. The chimaera probably reminded earlier naturalists of the chimera due to the body parts that resemble different animals and the seam-like lateral line system of the fish.
Eyes, Teeth and Lateral Line
Ratfish eyes are large, which allows them to absorb as much light as possible in deep, dark water. The eyes contain a membrane called the tapetum lucidum. This membrane reflects any light that passes through the retina (the light-sensitive layer in the eye) back on to the retina. The process produces a glowing effect in the eyes under certain conditions, such as when a light is shone on a fish while it's in a dark environment.
The fish has three pairs of tooth plates (two in the upper jaw and one in the lower) that often protrude from the mouth like a rodent's incisors. This is one reason why the fish is called a "rat" fish. The teeth enable a ratfish to grind the shells of its prey very effectively.
There is a line traveling along each side of the fish's body. There are also lines above and below the eyes and around the snout. The lines make up the lateral line system, which detects vibrations and movement in the water. In the front and lower sections of the head, the line becomes a series of dots. The dots are the location of sensory structures that detect electric fields.
A Spotted Ratfish Feeding
Diet and Feeding
Most chimaeras live in deep water, which makes them hard to study. The spotted ratfish is often seen in shallow water and is easier to observe. It's also a common fish in some places. Researchers therefore know more about the life of the spotted ratfish than about the lives of many other chimaeras.
The ratfish is often seen in groups but may hunt on its own as well. It travels close to the ocean bottom, in shallow to deep water. It's a carnivore and often feeds at night, eating crabs, clams, sea stars and shrimp as well as softer prey like worms and small fish. Its mouth faces downwards, which helps it to pick up food from the sea bed.
The fish finds its prey primarily by smell. Like other cartilaginous fish and some bony ones, a ratfish can detect the weak electric fields produced by living organisms. This ability is believed to aid its hunt for food, especially in dim light.
A Group of Ratfish
Like sharks, ratfish have internal fertilization. Also like sharks, a male ratfish has claspers behind his pelvic fins which are used to insert sperm into the female's body. A ratfish has two pairs of claspers while a shark has only one pair, however. The second pair of claspers are retractable.
In addition to claspers, the male ratfish has an appendage called a tentaculum on his forehead. Like the second pair of claspers, the tentaculum is retractable. When it's not in use it's visible as a little white bulge on the male's forehead. During courtship, the tentaculum is extended and placed in a hollow on the back of the female's head in order to keep the fish attached while they mate.
The spindle or spoon-shaped egg capsule is about five inches long and has a leathery texture. Eighteen to thirty hours are needed for an egg to be extruded. For part of this time the capsule hangs by a filament from the female's body. Eventually it's dropped on to seaweed or on to the ocean floor. Tendrils on the capsule help it to stick to its surroundings. The female releases just two eggs per spawning. There are multiple spawnings in a year, however.
The young fish develops slowly and may not leave the capsule for six months to a year after the capsule is released from the female's body. The youngster is a little over five inches in length when it emerges from the capsule.
The IUCN - International Union for Conservation of Nature - classifies the spotted ratfish population in its "Least Concern" category. The fish may be abundant in some parts of their range.
A Rabbitfish in Norway
The Rabbitfish (Family Chimaeridae)
Chimaera monstrosa is found in the eastern Atlantic Ocean and the western Mediterranean sea. It's sometimes known as a rabbitfish. The fish has brown stripes and blotches on a bluish or greenish background. The background has a lovely silvery sheen. The tail is very long, even longer than that of the spotted ratfish.
The rabbitfish lives in very deep water. It's a poor swimmer and moves by flapping its large pectoral fins, which look almost like bird wings. Like the spotted ratfish, it feeds mainly on hard-shelled prey, which it crushes with its plate-like teeth.
Rabbitfish are thought to have a long life expectancy. Preliminary estimates for the lifespan are thirty years for a male and twenty six years for a female, although researchers think that the maximum lifespan is probably longer.
The IUCN classifies the rabbitfish in the "near threatened" category. Deep water fishing in the Atlantic Ocean is killing rabbitfish, which are caught accidentally by trawlers aiming to catch other fish. In addition, human demand for rabbitfish oil (or ratfish oil) is increasing. The oil is extracted from the livers of rabbitfish and used as a supplement. Buyers of the supplement believe that it has a number of health benefits.
The Elephant Fish (Family Callorhinchidae)
Elephant fish belong to the Callorhinchus genus. They have a flexible extension from their snouts that looks like an elephant's trunk. Their tail looks very different from the tail of the spotted ratfish or the rabbitfish and is more shark-like. Elephant fish are also known as plough-nose chimaeras.
The elephant fish uses its snout extension to explore the ocean bottom as it hunts for small invertebrates and fish. The extension acts as a probe and is sensitive to movement and weak electrical currents.
There are three or four species of Callorhinchus, depending on the classification system that is being used. In the waters around Australia and New Zealand, Callorhinchus milii is known as an elephant shark or a ghost shark, even though the fish is a chimaera and not a shark. It's a beautiful fish that has an iridescent, silvery grey skin with dark blotches. In the video below, the elephant fish is swimming with the typical flying motion of chimaeras, using its enlarged pectoral fins for propulsion.
A Swimming Elephant Fish
Elephant fish are also known as Australian ghost sharks. Their flesh is enjoyed as food. They are often caught to provide the fish for fish and chips meals.
Rhinochimaeras have a łong extension on their snout that is somewhat reminiscent of a rhino's horn. The extension is either straight or hooked. The body surface has the typical seamed appearance of a chimaera. The eyes contain a tapetum lucidum that reflects light, like a cat's eye, giving the fish an eerie, ghost-like appearance. Other chimaeras share this feature, which is one reason why some species are known as ghost sharks.
Not much is known about the biology of rhinochimaeras. Many of them live in deep water close to the ocean floor. Highly specialized equipment is needed to observe them. The photos and videos that have been obtained so far are fascinating. The fish are sometimes caught as bycatch and frequently make news headlines in this situation. Bycatch is an animal that is caught accidentally when fishing for another species.
Despite the way in which the name of the fish is spelled in the video below, "rhinochimaera" is the accepted spelling. The different spelling does remind us of the Ancient Greek myth and the strangeness of this intriguing group of fish, however.
Interesting and Unusual Fish
Even though some chimaeras are common fish, many people are unfamiliar with these creatures. They are interesting animals which have some characteristics of sharks, some characteristics of bony fish and some characteristics that are uniquely their own. Chimaeras have existed on Earth for a long time and hopefully will continue to do so for many years to come.
© 2012 Linda Crampton