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Ghost sharks, rat fish, spook fish and rabbit fish
DID YOU KNOW? In Greek mythology the ‘Chimaera’ was a monstrous fire-breathing hybrid creature composed of the parts of more than one animal.
With skeletons made of cartilage, chimaera are closely related to sharks, skates and rays. However this enigmatic group diverged from their shark relatives around 400 million years ago. A series of physical characteristics now differentiate them from sharks, including upper jaws which are fused to their skull, four gills with one external opening and just three pairs of large permanent grinding tooth plates, in contrast to sharks' many sharp and replaceable teeth.
To date, 50 different species of chimaera have been recorded worldwide, which scientists separate into three distinct groups: plow-nose chimaeras, long-nose chimaeras and short-nose chimaeras. Yet relatively little is known about these mysterious deep-sea animals; most chimaera species are found at depths of over 500m (a few inhabit shallower coastal waters), and are difficult to study in their natural habitat.
Adult chimaera range from 60cm to 200cm in length, and are characterised by long tapering bodies with very large heads. They vary in colour from black to pale blue to brownish grey, with smooth skin. Large translucent-green eyes help chimaera to see in the permanent darkness of the deep-sea. These large eyes, as well as their nostrils and teeth, give them a rabbit-like appearance, hence their collective nickname ‘rabbit fish’ (not to be confused with a particular species of chimaera known as the Rabbitfish (Chimaera monstrosa). Most species also have a venomous spine which is used as defence against predators.
Chimaera appear to be opportunist feeders, with diets consisting mainly of bottom-dwelling invertebrates, such as crabs, molluscs, octopuses, marine worms and sea-urchins, which they crush with their three rows of tooth plates. Like their shark relatives, chimaeras use electroreception to locate their prey in the total darkness of the deep-sea.
Like skates and some species of shark, chimaera are oviparous – that is, they reproduce by laying eggs. Eggcases are deposited directly on to flat sandy or muddy sea-beds, with the size and shape of the eggcase varying depending on species. Females deposit two eggs simultaneously, with several pairs deposited each season, although the exact number remains unknown. Depending on species, gestation may take from 6-12 months.
Chimaera inhabit all of the world’s oceans, with the exception of the Antarctic. They are found at depths ranging from 200m-2,600m, over continental shelves and slopes, as well as around oceanic islands, seamounts and underwater ridges. Chimaera appear to remain within a few metres of the seafloor, with different species observed occupying a variety of habitats including volcanic boulders and cobbles, high rocky relief, as well as soft sediments such as sand and mud.
Some chimaera species are widespread, for example the Rabbitfish which is found throughout the North Atlantic and Mediterranean. Others, however, have a more limited range: the Galapagos Ghostshark – as the name suggests – appears to be endemic to the Galapagos Islands.
CHIMAERA IN BRITISH WATERS
Scientific surveys have recorded eight species of chimaera in UK and Irish waters to-date: the Rabbitfish, Opal Chimaera, Small-eyed Rabbitfish, Large-eyed Rabbitfish, Pale Chimaera, Smallspine Spookfish, Longnose Chimaera and the Straightnose Rabbitfish.
CHIMAERA IN FISHERIES
Chimaera are directly targeted in a small number of fisheries around the world, particularly in coastal waters: for example, in New Zealand and Australian waters, the Elephant Fish is commercially targeted and sold as whitefish fillets. In other parts of the world chimaera may be kept as ‘valued bycatch’ or discarded back into the sea – although for the majority of chimaera species, discarding has a low survival rate due to the depths at which they are caught.
Like sharks, chimaera display biological characteristics which make them incredibly vulnerable to commercial fishing pressure: they are late to reach sexual maturity, produce few young, and are long-lived (Rabbitfish are estimated to reach a maximum age of 30 years, likely longer). As deep-sea species, chimaera are at further risk from deep-sea trawling, a largely indiscriminate method of fishing in which they are caught as bycatch.
Of the 46 chimaera species assessed on the IUCN Red List, one (Ogilby’s Ghostshark) is listed as Vulnerable, with another two species assessed as Near Threatened (the Rabbitfish and Large-eyed Rabbitfish). These listings reflect a combination of biological vulnerability, a restricted distribution and exposure to deep-sea trawling. A further 23 chimaera are listed as Least Concern, while the remaining 20 are considered Data Deficient, due to the lack of knowledge on their biology and ecology.
With the exception of the Elephant Fish in New Zealand and Australian waters, there are currently no management or conservation measures in place for any species of chimaera.
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