|Range of the swellshark (in blue)|
The swellshark, Cephaloscyllium ventriosum, is a catshark of the family Scyliorhinidae. It is found in the subtropical eastern Pacific Ocean between latitudes 40° N and 37° S, from the surface to 460 m.
It grows to about 100 centimeters (39 in) in length, and can expand its body to about double its regular size to prevent its predators such as seals and larger sharks from pulling it out from rocky reefs, under ledges, and in crevices. The swellshark's appearance resembles that of the leopard shark in that it has spots. Younger swellsharks are lighter in color than the older ones. Swellsharks are nocturnal, sleeping in reef crevices and caves during the day. Sometimes, these sharks will grab onto their tail with their mouth in a ring shape to prevent other fish from being able to attack them. During the night they hunt molluscs, crustaceans and bony fishes. The swellshark is oviparous, and the females lay two flattened egg sacks which contain the embryo which is attached by two tendrils to a reef. Swellsharks are commonly found in aquariums and are completely harmless to humans, usually staying completely motionless when encountered by divers. The World Conservation Union has assessed this species as a least concern species worldwide, as it is usually not fished by divers and fishermen due to its poor flesh quality. However the occasional swellshark is killed by fishermen and this along with its low egg production may put the species at risk.
When discovered in 1880, the swellshark was first described as Scyllium ventriosum, but was later changed to Cephaloscyllium ventriosum. The Greek word kephale which means "head" and skylla which is the name of a mythological creature was used for the term Cephaloscyllium, while ventriosum, Latin for "large belly" was used to describe the shark's ability to suck in water.
Distribution and habitat
The swellshark can be found in the Eastern Pacific, the central California coast, the Gulf of California, and the southern tip of Mexico. Swellsharks have been reported as far south as Chile, but that is believed to be a different species. It can be found over continental shelves and subtropical waters and can be seen as deep as 457 m (1,499 ft) or as close as five meters below the surface. Swellsharks are often found over algae-covered rock bottoms. In the California area, it shares the same habitat as the horn shark and unlike that shark, the swellshark is more tolerant of water temperatures below 20°C (70°F).
Anatomy and appearance
Swellsharks have a rounded dorsal fin located in the pelvic area. Another fin is located in the anal area. The fins are light brown with dark patches and are covered along with the rest of its body with black dots. Usually the younger sharks are lighter in color than the adults. Its spotted coloration closely resembles that of the leopard shark. The gills of a swellshark are usually very small and tight. Swellshark eyes are large and gold and the lower eyelid is nictitating. The largest swellsharks can be up to 110 centimeters (43 in) but are usually around 90 centimeters (35 in) in length. Every swellshark has around 55–60 teeth, which contains mostly small teeth.
Biology and ecology
Diet and behavior
When hunting at night, the swellshark hunts for bony fish, molluscs, and crustaceans by lying on the bottom of the ocean, and waiting until its prey is a few centimeters away before it attacks. Another hunting tactic used by the swellshark is to lie on the bottom of the ocean with its mouth open, waiting for a fish to swim into its mouth. Swellsharks also enter lobster traps to get an easy meal. Swellsharks are nocturnal and sleep in reef crevices during the day. When the shark feels threatened, it will bend its body into a U–shape, grab its tail fin with its mouth, and suck in water. Doing so makes it more difficult for predators to remove it from rocky crevices, allows the shark to double in size, and makes it harder for larger sharks and seals to eat it. During the time when the air is in its stomach, the swellshark floats upward to the water surface. When letting the air out, the swellshark makes a dog–like bark. This fish is very sociable with other sharks, and is commonly seen sleeping next to or on top of one another.
The swellshark was reported to be biofluorescent in 2014. The researchers presented species-specific emission patterns, indicating that biofluorescence potentially functions in intraspecific communication and evidence that fluorescence can be used for camouflage.
The swellshark is oviparous and they lay two green or amber flattened eggs which are attached to the reef with tendrils. After a study, it was determined that the length of the tendrils depends on the amount of surf action the region is under. The egg case which contains the embryo is approximately 2.5 centimeters (0.98 in)–5.1 centimeters (2.0 in) by 7.6 centimeters (3.0 in)–13 centimeters (5.1 in) . The embryos will feed solely on yolk before they hatch. The hatching time for swellshark eggs solely depends on water temperature but usually will hatch between 7.5–12 months. When born after this period of time in the egg, the swellshark pup will immediately begin feeding on molluscs and crustaceans.
Relationship to humans
When confronted by a human in the water, the swellshark will become motionless. However, if it is threatened, it will swell up to double its regular size. The swellshark is common in public aquariums because it can live for several years in captivity.
It is usually not caught purposely, but if it is, usually it will be released because its flesh is of poor quality. Since the swellshark often enters lobster traps to find food, it will sometimes get caught incidentally and killed when brought to the surface. This accidental catching threatens the species mainly because of its age at maturity and the low number of eggs it produces. Despite this, the swellshark is listed as Least Concern by the World Conservation Union.
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