Chimaeriformes

(Chimaeras)

Class Chondrichthyes

Order Chimaerformes

Number of families 3

Evolution and systematics

The chimaeras are an ancient lineage of cartilaginous fishes related to sharks, skates, and rays. All chimaeras, sharks, skates, and rays are united in the class Chondrichthyes, which is further subdivided into two subclasses: Holocephali, consisting of the chimaeras characterized by a unique jaw and tooth morphology, and the Elasmobranchii, which includes the sharks, skates, and rays. Fossil evidence indicates that the chimaeras probably evolved nearly 300 million years ago. What is especially remarkable is that many of the modern forms look very much like their fossil ancestors.

Within the Order Chimaeriformes there are three families, each of which is distinguished by a unique snout morphology. The plow-nosed chimaeras of the family Callorhinchidae have a delicate flap of skin in the shape of a hoe projecting from the snout; the long-nosed chimaeras of the family Rhinochimaeridae are characterized by elongate, pointed snouts; and the ratfishes, family Chimaeridae, have blunt fleshy snouts. As of 2002 there are 33 described species of chimaeras with at least 10 additional species that are known, but not yet formally described.

Physical characteristics

Chimaeras are characterized by large heads and elongate bodies that taper to a whip-like tail. They range in size from small, slender-bodied fishes of 1–2 ft in total length (about 30.5–61 cm), to massive fishes, nearly 4 ft in length (122 cm), with gigantic heads and large girth. The skin is smooth and rubbery, completely lacking in scales or denticles. The four gill openings on each side of the head are covered by a fleshy operculum. The mouth is small with teeth that are formed into three pairs of non-replaceable tooth plates, two pairs in the upper jaw and one pair in the lower jaw. These tooth plates tend to protrude from the mouth like a rodent's incisors, suggesting the common names ratfish or rabbitfish for some of the species. The pectoral fins of chimaeras are broad and wing-like and serve to propel the fish through the water by a flapping motion much like underwater flying. All chimaeras have two dorsal fins; the first is preceded by a stout and often toxic spine, and the second is spineless. The lateral line canals are visible externally, and in many species are formed as open grooves. Chimaeras are sexually dimorphic. Adult males possess three unique secondary sexual characteristics: a bulbous, denticulate frontal tenaculum that rests in a pouch atop the head; blade-like prepelvic tenaculae that are hidden in pouches anterior to the pelvic fins; and pelvic claspers that extend from the posterior edge of the pelvic fins.

Distribution

These fishes are entirely marine and are distributed in all of the world's oceans with the exception of Arctic and Antarctic waters. Most species live in deep waters of the shelf and slope, generally at depths greater than 1,500 ft (457 m), and the deepest recorded capture was near 9,000 ft (2,743 m). Although most chimaeras are deepwater dwellers, several species occur in shallower waters, and some migrate inshore. Many species are known from a very widespread geographic range, sometimes throughout an ocean basin spanning the northern and southern hemisphere, while other species appear to be more restricted in their range both vertically and horizontally.

Habitat

Chimaeras usually live on or near muddy bottoms. They tend to remain close to the bottom and are not known to move into the pelagic zone. Most species occur near continental landmasses or off oceanic islands and on the slopes of seamounts and underwater ridges.

Behavior

Some species are locally migratory and congregate near the shore for breeding and spawning. It also has been observed that some chimaeras tend to aggregate into single-sex groups and to separate into groups based on age.

Feeding ecology and diet

The diet consists primarily of benthic invertebrates. The tooth plates are used to crush hard-bodied prey such as crabs, clams, and echinoderms. Chimaeras also are known to prey upon other fishes. Very little information exists with regard to predation of chimaeriformes; their main predators are sharks and humans.

Reproductive biology

Most species reach sexual maturity at about 18 in (45.7 cm) body length measured from the distal edge of the gill opening to the origin of the dorsal lobe of the caudal fin. Females are generally larger than males. Like their shark relatives, chimaeras have internal fertilization in which males, equipped with pelvic claspers, transfer sperm directly into the female reproductive tract. Males also possess two additional organs used in copulation. Unique to chimaeroids is the club-like frontal tenaculum that emerges from the top of the head in sexually mature males and has been observed to be used to grasp the posterior edge of the pectoral fin of the female during courtship. Additionally, a pair of blade-like prepelvic tenaculae aid in anchoring the male during copulation. Sperm storage in females has been observed in one species and is likely to occur in all species.

All species of chimaeras are oviparous and reproduce by laying eggs. Fertilized eggs are encased in egg capsules and deposited onto the ocean floor. Egg capsules are laid in pairs with each egg capsule containing only a single egg. The egg capsules are generally spindle shaped, sometimes with broad lateral web-like flanges that vary in size and shape depending on the species. Embryological development may take six to twelve months, and fully developed hatchlings measure about 5 in (12.7 cm) in length and look like miniature adults. Very little is known about details of reproduction and development for most species of chimaeras.

Conservation status

There is insufficient data to determine if any species of chimaeras are threatened. However, chimaeras may be inadvertently subject to overexploitation from fisheries due to lack of understanding of their age, growth, and population structure, and seemingly low fecundity.

Significance to humans

A few species of chimaeras are fished commercially for human consumption, particularly in the southern hemisphere; however, most species of chimaeras are little used and are unlikely to become an important fishery resource. Chimaeras are sometimes taken as minor bycatch in trawls and can be processed for oil and fishmeal.

Species accounts

List of Species

Ghost shark

Callorhinchus milii

family

Callorhinchidae

taxonomy

Callorhinchus milii Bory de St. Vincent, 1823, Australia.

other common names

English: Elephant shark, whitefish; Maori: Reperepe.

physical characteristics

Distinguished by a plow-shaped snout. Unlike other chimaeras, all of which have whip-like tails, callorhinchids have externally heterocercal tails with a large dorsal lobe and smaller ventral lobe. Body color is silvery and is black along the dorsal midline and top of the head with black saddle-like bands along the dorsal surface of the trunk.

distribution

Southern coasts of New Zealand and Australia.

habitat

Prefers coastal waters, living on or near sandy, muddy, or rocky bottoms.

behavior

Known for seasonal migration inshore to spawn in shallow coastal waters.

feeding ecology and diet

Feeds on benthic invertebrates, particularly small bivalves. It may also eat other fishes.

reproductive biology

This is an oviparous species, with eggs fertilized within the female reproductive tract. Females lay two eggs at a time, each

contained within its own egg capsule, over a period of months. Development appears to take 6–12 months.

conservation status

Large fluctuations in population size have been recorded in New Zealand with a general trend toward decreasing numbers over the years. This species may be impacted by overfishing.

significance to humans

Commercially fished and used for human consumption in New Zealand and Australia.

Spotted ratfish

Hydrolagus colliei

family

Chimaeridae

taxonomy

Hydrolagus colliei Lay and Bennett, 1839, Monterey, California.

other common names

None known.

physical characteristics

Head contains a bluntly pointed snout. Body color is a reddish to dark brown with silvery-blue and gold highlights, as well as numerous small white spots on the head and along sides and back of the trunk. Ventrally the color is an even pale cream or gray.

distribution

Southeastern Alaska to Baja, California, and the northern Gulf of California. It has been recorded at depths ranging from the surface to 2,995 ft (912.9 m).

habitat

Usually occurs near muddy, sandy, or rocky bottoms.

behavior

Known to migrate from deeper to shallower waters. It tends to aggregate into groups based on age and sex.

feeding ecology and diet

Feeds on benthic invertebrates and other fishes.

reproductive biology

Oviparous, with eggs fertilized within the female reproductive tract. Two egg capsules, each containing a single embryo, are laid every 7–10 days for a period of months. Development appears to take 6–12 months.

conservation status

Not threatened.

significance to humans

At one time this species was fished locally for the oil extracted from the liver. There is no known commercial value, and it is considered a nuisance fish by local fishermen.

Pacific spookfish

Rhinochimaera pacifica

family

Rhinochimaeridae

taxonomy

Rhinochimaera pacifica Mitsukuri, 1898, Japan.

other common names

English: Knifenose chimaera; Spanish: Tucán; Japanese: Tengu-ginzame.

physical characteristics

A long, narrow conical snout extends forward from the head. The body is elongate and tapering to a whip-like tail. Body color is usually a uniform brown or grayish brown, with fins a darker shade. The skin along the ventral side of the snout and around the mouth is generally white in color.

distribution

Widely distributed throughout the western Pacific Ocean from Japan to subantarctic waters. Also reported from the southeastern Pacific of Peru.

habitat

Inhabits deep water slopes and seamounts usually associated with muddy bottoms.

behavior

Nothing is known.

feeding ecology and diet

Diet consists of a wide variety of benthic invertebrates and possibly other fishes.

reproductive biology

Oviparous, with eggs fertilized within the female reproductive tract. Females lay two egg capsules at a time, each containing a single embryo. Very few egg capsules and embryos have ever been observed, and almost nothing is known of spawning and embryological development in rhinochimaerids.

conservation status

Insufficient information is available.

significance to humans

Not known to be commercially fished, although it may be caught as bycatch and processed for oil or fishmeal.

Resources

Books

Eschmeyer, W. N., E. S. Herald, and H. Hammann. A Field Guide to Pacific Coast Fishes of North America. Boston: Houghton Mifflin Company, 1983.

Hart, J. L. Pacific Fishes of Canada. Fisheries Research Board of Canada, Bulletin No. 180, 1980.

Last, P. R., and J. D. Stevens. Sharks and Rays of Australia. CSIRO Australia, 1994.

Paulin, C., A. Stewart, C. Roberts, and P. McMillan. New Zealand Fish: A Complete Guide. National Museum of New Zealand Miscellaneous Series No. 19. Wellington: New Zealand, 1989.

Periodicals

Didier, D. A. "Phylogenetic Systematics of Extant Chimaeroid Fishes (Holocephali, Chimaeroidei)." American Museum Novitates 3119 (1995): 1–86.

——, E. E. LeClair, and D. R. Vanbuskirk. "Embryonic Staging and External Features of Development of the Chimaeroid Fish, Callorhinchus milii (Holocephali, Callorhinchidae)." Journal of Morphology 236 (1998): 25–47.

Lund, R., and E. D. Grogan. "Relationships of the Chimaeriformes and the Basal Radiation of the Chondrichthyes." Reviews in Fish Biology and Fisheries 7(1997): 65–123.

Mathews, C. P. "Note on the Ecology of the Ratfish, Hydrolagus colliei, in the Gulf of California." California Fish and Game 61 (1975): 47–53.

Quinn, T. P., B. S. Miller, and R. C. Wingert. "Depth Distribution and Seasonal and Diel Movements of Ratfish, Hydrolagus colliei, in Puget Sound, Washington." Fishery Bulletin 78 (1980): 816–821.

Dominique A. Didier, PhD