The frilled shark ( Chlamydoselachus anguineus ) is a rare and fascinating deep-sea shark, often referred to as a "living fossil" due to its primitive features and evolutionary stability over millions of years. This eel-like shark inhabits the depths of the Atlantic and Pacific Oceans, exhibiting unique adaptations for survival in its challenging environment.
Physical Characteristics and Habitat
Frilled sharks possess a distinctive appearance. They have a long, slender, eel-like body, typically chocolate brown in color, and can reach lengths of up to 2 meters (6.6 feet). Their most notable feature is the presence of six pairs of gill slits, which give the shark its name "frilled shark" because they are surrounded by frilly skin, unlike most modern sharks, which have five. The first gill slit is joined under their jaws forming a sort of collar.
The snout is short and the lower jaw is long. The moderately large eyes are horizontally oval (like a cat’s). Their mouth is located at the leading edge of their snout (terminal) rather than underneath like most sharks.
These sharks are wide ranging, with a patchy distribution across the Atlantic and Pacific Oceans (74°N - 58°S, 169°W - 180°E), and are occasionally reported in open waters. They are typically found near the sea floor in waters over outer continental and island (insular) shelves and upper slopes, usually at depths between 120 and 1,280 m but up to 1,570 m and occasionally even at the surface. In Suruga Bay, Japan they are most common at depths between 50 m and 200 m.
In the western Indian Ocean they are found off South Africa as C. africana. In the western Pacific, frilled sharks are known to live off Japan and south to New Zealand, New South Wales and Tasmania in Australia. In the eastern/central Pacific they have been observed off Hawaii, southern California to northern Chile. Frilled sharks have also been observed in the eastern Atlantic from waters off northern Norway to northern Namibia, and possibly off the eastern Cape of Good Hope in South Africa.
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In the central Atlantic, they have been caught at several locations along the Mid-Atlantic Ridge, from north of the Azores to the Rio Grande Rise off southern Brazil, as well as over the Vavilov Ridge off West Africa. In the western Atlantic, it has been reported from off New England, Georgia, and Suriname.
Frilled sharks are highly specialized for life in the deep sea with reduced, poorly-calcified skeletons and enormous livers filled with low-density lipids, which allows them to maintain their position in water with little effort. They are also one of the few sharks with an “open” lateral line, in which the mechanoreceptive hair cells are positioned in grooves that are directly exposed to the surrounding seawater.
Diet and Hunting Behavior
The frilled shark's diet primarily consists of cephalopods (mainly squid), other sharks, and small deep-water fishes (Taylor et al., 2002). Squid comprise some 60% of the diet of these sharks in Suruga Bay and this includes not only slow-moving, deep-dwelling squid such as Chiroteuthis and Histioteuthis, but also relatively large, powerful swimmers of the open ocean such as Onychoteuthis, Sthenoteuthis, and Todarodes.
Feeding behavior has not yet been observed by this weak-swimming species, though they are thought to capture active, fast-moving squid by taking advantage of injured squid or those that are exhausted and dying after spawning. Alternatively, they may surprise their prey by curving their body like a spring, bracing themselves with rear positioned fins, and launching quick strikes forward like a snake. They may also be able to close their gill slits creating negative internal pressure to suck prey quickly into their mouth.
The frilled shark possesses several adaptations that aid in its hunting success:
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- Specialized Teeth: They have many small, sharp, rear-pointing (recurved) teeth that function much like squid jigs which could easily snag the body or tentacles of a squid, particularly as they are rotated outwards when their jaws are protruded. Their rows of teeth are rather widely spaced, numbering 19-28 teeth in their upper jaws and 21-29 teeth in their lower jaws. Each tooth is small, with three slender, needle-like cusps alternating with two cusplets.
- Flexible Jaws: Using their long, extremely flexible jaws they should be able to swallow large prey (up to half its size!) whole, while their many rows of needle-like teeth would make escape essentially futile.
- Sensory Perception: They have a small lobe-like dorsal fin set far back over their pelvic fins with an anal fin that is larger than their dorsal fin. Their pectoral fins are small and paddle-shaped and their very long caudal fin (tail fin) has a small ventral lobe and without a subterminal notch. They are also one of the few sharks with an “open” lateral line, in which the mechanoreceptive hair cells are positioned in grooves that are directly exposed to the surrounding seawater. This configuration is thought to be the most primitive in sharks and may enhance their sensitivity to minute movements of prey in their proximity.
Observations of captive frilled sharks swimming with their mouths open might also suggest that the small teeth, light against their dark mouths, may even fool squid into attacking and entangling themselves.
Most captured individuals have been found with no or barely identifiable stomach contents, suggesting that they have a fast digestion rate and/or long intervals between feedings. One 1.6 m long individual, caught off Japan, was found to have swallowed an entire 590 g Japanese catshark, Apristurus japonicus.
Reproduction and Life Cycle
Frilled sharks are aplacental viviparous (aka ovoviviparity) where the embryos emerge from their egg capsules inside their mother’s uterus and are nourished by their yolk until birth. Frilled sharks’ gestation period may be as long as three and a half years, the longest of any vertebrate. Between 2 and 15 young are born at a time (average is 6) measuring 40-60 cm long, and there appears to be no distinct breeding season (which is expected as these sharks inhabits depths at which there is little to no seasonal influence). Male frill sharks attain sexual maturity at 1.0-1.2 m long and females at 1.3-1.5 m. A possible mating aggregation of 15 male and 19 female frilled sharks was recorded over a seamount on the Mid-Atlantic Ridge.
There is little, if any, information on parental investment in frilled sharks. Frilled sharks have never been kept in captivity.
Conservation Status and Threats
Frilled shark are classified as a near threatened species on the IUCN Red List. As bycatch, this species is variously either used for meat, fishmeal, or discarded. Occasionally kept in aquaria (Japan). There is some concern that expansion of deepwater fisheries effort (geographically and in depth range) will increase the levels of bycatch. Although little is known of its life history, this deepwater species is likely to have very little resilience to depletion as a result of even non-targeted exploitation.
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There are few known predators of frilled sharks, and humans may take these sharks incidentally as fishing bycatch. They may be ground up for fishmeal and fish food.
Evolutionary Significance and "Living Fossil" Status
The frilled shark is considered a "living fossil" due to its ancient lineage and the retention of primitive characteristics. Its physical features closely resemble those of sharks that lived millions of years ago, providing valuable insights into the evolutionary history of sharks.