Chimaeras, also known as ghost sharks, rat fish, spookfish, or rabbit fish, are cartilaginous fish belonging to the order Chimaeriformes. As relatives of sharks and rays, they are part of the broader group Chondrichthyes. Chimaeras are characterized by their elongated, cone-shaped bodies, bulky heads, long tapering tails, and smooth, scaleless skin dotted with electroreceptor organs.
They inhabit all oceans except the Arctic and Antarctic. There are currently over 50 extant species of chimaeras, categorized into three families: plough-nosed chimaeras (Callorhinchidae), short-nosed chimaeras (Chimaeridae), and long-nosed chimaeras (Rhinochimaeridae).
They range between 24 and 80 inches (60 to 200 cm) in length.
These soft-bodied vertebrates are shark-like, characterized by their bulky heads and long, tapering tails. As members of the class Chondrichthyes, their skeletons are entirely cartilaginous. Unlike sharks and rays, adult chimaeras have smooth skin without tooth-like placoid scales, although hatchlings retain a few rows of denticles on their backs, and adult males possess denticulated sexual organs.
Their gill arches are condensed into a pouch-like bundle, covered by a sheet of skin called the operculum, with a single gill opening in front of the pectoral fins. Males have accessory sexual organs to aid in mating, including paired claspers on the pelvic fins and retractable appendages called tentacula on the forehead (cephalic tentaculum) and in front of the pelvic fins (pre-pelvic tentacula).
Chimaeras have large ventral pectoral fins that generate lift during underwater propulsion, giving them a resemblance to flying fish. Behind these pectoral fins are small pelvic fins, and some species also have an anal fin located in front of the tail (caudal fin).
Chimaerids and rhinochimaerids have leptocercal tails, characterized by a whip-like shape with two lobes of similar size. In contrast, callorhinchids have heterocercal tails similar to those of sharks, with a larger upper lobe inclined upwards.
On the dorsal side of a chimaera’s body are two fins. The first dorsal fin is larger and triangular, while the second is small and rectangular. Some species, such as Chimaera monstrosa and Hydrolagus affinis, have a venomous spine on the front edge of the first dorsal fin.
These fish possess a holostylic skull, marked by the fusion of the upper jaw cartilage or palatoquadrate to the neurocranium or cranial cartilage. This arrangement is in contrast to modern sharks with hyostyly, where the palatoquadrate is flexible.
Unlike sharks with multiple teeth that are constantly replaced, chimaeras have ever-growing, permanent tooth plates. These plates are arranged in three pairs, with one pair at the lower jaw and the other two along the upper jaws. All the teeth plates collectively form a protruding, beak-like structure that helps crush and grind their food.
While most vertebrates with mineralized teeth have a hard covering called enamel composed of hydroxyapatite, each tooth plate in chimaeras has a core of osteodentin, covered by an enamel-like hypermineralized tissue called pleromin arranged into sheets or beaded rods.
Some classification schemes consider Holocephali (the group within which chimaeras are placed) a subclass, while others consider it a class. Currently, there are over 50 extant species of chimaeras divided into three families: Callorhinchidae, Rhinochimaeridae, and Chimaeridae.
Chimaeras are exclusively marine and inhabit all oceans of the world except the Arctic and the Antarctic. They are found in shallow coastal waters (though very few, like the spotted ratfish, live in regions shallower than 660 ft or 200 m) as well as in depths as low as 8,500 ft (2,600 m).
In addition, some chimaeras, like the Cape elephantfish (Callorhinchus capensis) and Australian ghostshark (Callorhinchus milii), are also kept in public aquaria.
They are opportunistic feeders that primarily feed on bottom-dwelling invertebrates, like crabs, mollusks, octopuses, marine worms, and sea urchins. Since most of their diet includes animals with hard-shelled exoskeletons, they are said to be durophagous.
During mating, the male grasps their female counterpart with claspers, clutches her pectoral fins using his frontal tentaculum, and anchors themselves with serrated hooked plates in the pre-pelvic tentacula. He then releases sperm into her body, fertilizing her eggs internally.
The females are oviparous and lay eggs directly on sandy or muddy sea beds. These eggs are spindle-shaped and covered with leathery and horny egg cases. Usually, two eggs are released simultaneously, and the embryos hatch after 6 to 12 months.
Although chimaeras are rarely encountered due to their demersal or bottom-dwelling lifestyle, they are often subject to commercial exploitation and overfishing through bycatch. According to the IUCN Red List, four species, including the silver chimaera and striped rabbitfish, are classified as Vulnerable (VU), while four others, such as Mitsukuri’s chimaera and Ogilby’s chimaera, are listed as Near Threatened (NT).
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