Dalatias licha

The kitefin shark or seal shark (Dalatias licha) is a species of squaliform shark in the family Dalatiidae, and the only species in its genus. It is found sporadically around the world, usually close to the sea floor at depths of 200–600 m (660–1,970 ft). With a sizable oil-filled liver to maintain neutral buoyancy, this shark is able to cruise slowly through the water while expending little energy. The kitefin shark has a slender body with a very short, blunt snout, large eyes, and thick lips. Its teeth are highly differentiated between the upper and lower jaws, with the upper teeth small and narrow and the lower teeth large, triangular, and serrated. Its typical length is 1.0–1.4 m (3.3–4.6 ft). Armed with large teeth and a strong bite, the kitefin shark is a powerful, solitary predator that takes many different types of prey, ranging from bony fishes, sharks and rays, to cephalopods, crustaceans, polychaete worms, siphonophores, and possibly carrion. It also takes bites out of animals larger than itself, similar to its smaller relative, the cookiecutter shark (Isistius brasiliensis). This shark is aplacental viviparous and gives birth to 10–14 young. The kitefin shark is fished commercially for its meat, skin, and liver oil, primarily by Portugal and Japan. A fishery targeting this species existed off the Azores from the 1970s to the 1990s, but collapsed due to overfishing and falling liver oil prices; the rapid depletion of the Azores stock is often cited as an example of the susceptibility of deep-sea sharks to human exploitation. The low reproductive rate of this species renders it susceptible to overfishing and, coupled with known population declines, has led it to be assessed as Vulnerable by the International Union for Conservation of Nature (IUCN). Scientists stumbled upon a teeny male bioluminescent (glow-in-the-dark) kitefin shark in 2010 while studying sperm whales in the Gulf of Mexico. It wasn't observed again until 2013, when National Oceanic and Atmospheric Administration (NOAA) researcher Mark Grace found it in a pool of less luminous specimens. This 5.5-inch (14 cm) American pocket shark (Mollisquama mississippiensis) is the first of its kind to be discovered. The kitefin shark was originally described as Squalus licha by French naturalist Pierre Joseph Bonnaterre, in his 1788 Tableau encyclopédique et méthodique des trois regnes de la nature; the type specimen from 'Le cap Breton' has since been lost. This species was later placed in its own genus, Dalatias, which came from the synonymy of Constantine Rafinesque's 1810 Dalatias sparophagus with S. licha. However, some authorities dispute this on the grounds that D. sparophagus is a nomen dubium, and prefer to use the next available genus name Scymnorhinus. The genus name Dalatias is derived from the Greek dalos or dalou, meaning 'torch'. The specific epithet licha comes from la liche, the French name for this shark. Additional common names used for the kitefin shark include black shark and darkie Charlie. Cladistic studies have consistently found that the closest relatives of the kitefin shark are the cookiecutter sharks (Isistius), with which they share several dentitional, skeletal, and muscular similarities. Dalatias and Isistius are believed to have evolutionarily diverged shortly after the transition between the Cretaceous and Tertiary periods (65.5 Ma), as part of a larger adaptive radiation of dogfish sharks from the deep sea into relatively shallower habitats. The oldest fossil teeth that definitively belong to the kitefin shark date to the Middle Eocene epoch, such as those recovered from Bortonian-stage deposits (43.0–37.0 Ma) in New Zealand. Dalatias fossil teeth dating to various ages have also been discovered in Europe, the former USSR, Japan, and western India. The fossil material now recognized as belonging to this species were historically described under a multitude of different names. The kitefin shark has a moderately elongated body with a very short, rounded snout. The eyes and spiracles are large. The lips are thick with pleats or fringes, though are not modified to be suctorial. There are 16–21 tooth rows in the upper jaw and 17–20 tooth rows in the lower jaw. The upper teeth are small and spike-shaped, curving slightly towards the corners of the mouth. The lower teeth are very large, knife-shaped, and serrated, with their bases interlocking to form a continuous cutting surface. The first dorsal fin is slightly smaller and shorter-based than the second, and neither has spines. The first dorsal fin originates behind the free rear tip of the pectoral fins, while the second originates above the middle of the pelvic fin bases. The pectoral fins are short and rounded. The caudal fin has a prominent upper lobe with a well-developed notch near the tip, and a barely present lower lobe. The form and arrangement of the fins is similar to the Portuguese dogfish (Centroscymnus coelolepis), from which this species can be distinguished by the lack of fin spines. The dermal denticles are small and flat, with a single horizontal ridge ending in a point.