The spined pygmy shark (Squaliolus laticaudus) is a species of squaliform shark in the family Dalatiidae found widely in all oceans. Growing no larger than roughly , it is one of the smallest sharks alive, with this record beaten by the dwarf lanternshark. This shark has a slender, cigar-shaped body with a sizable conical snout, a long but low second dorsal fin, and an almost symmetrical caudal fin. Its sister species S. aliae and it are the only sharks with a spine on the first dorsal fin and not the second. Spined pygmy sharks are dark brown to black, with numerous bioluminescent organs called photophores on their ventral surface. The shark is believed to use these photophores to match ambient light conditions, which break up its silhouette and help the shark to avoid being seen by predators below.
Usually inhabiting nutrient-rich waters over upper continental and insular slopes, the spined pygmy shark feeds on small bony fishes and squid. Like its prey, it is a diel vertical migrator, spending the day at close to deep and moving towards a depth of at night. Reproduction is presumably aplacental viviparous, with females giving birth to litters of up to four pups. This diminutive shark has no economic value. The International Union for Conservation of Nature has assessed this species as of Least Concern, as it faces little threat from commercial fisheries and has a wide distribution.
Taxonomy and phylogeny
The spined pygmy shark was one of many new species discovered during the course of the 1907–1910 Philippine Expedition of the U.S. Fish Commission Steamer Albatross. It was described by American ichthyologists Hugh McCormick Smith and Lewis Radcliffe in a 1912 paper for the scientific journal Proceedings of the United States National Museum, based on two specimens collected in Batangas Bay, south of Luzon in the Philippines. One of these, a 15-cm-long adult male, was designated the type specimen.
Smith and Radcliffe coined the new genus Squaliolus for this shark, and gave it the specific epithet laticaudus, from the Latin latus meaning "broad" or "wide", and cauda meaning "tail". The spined pygmy shark may also be referred to as the dwarf shark or the bigeye dwarf shark. Based on similarities in their claspers (male intromittent organs), the closest relative of the spined pygmy shark and the related S. aliae is thought to be the pygmy shark (Euprotomicrus bispinatus).
Distribution and habitat
The spined pygmy shark has a wide distribution around the world. In the Atlantic Ocean, it occurs off Bermuda, the United States, Suriname, southern Brazil, and northern Argentina in the west, and off northern France, Madeira, Cape Verde, and the Azores in the east. In the Indian Ocean, this species has only been recorded off Somalia. In the Pacific Ocean, it is found off southern Japan, Taiwan, and the Philippines. The spined pygmy shark is found at depths of and seldom approaches the surface, unlike the related pygmy shark and cookiecutter shark (Isistius brasiliensis). This shark prefers areas of high biological productivity over upper continental and insular slopes. It may also be found over outer shelves, but avoids central ocean basins. The range of this species does not overlap that of the pygmy shark, which has a similar ecology, and is also largely separate from that of the cookiecutter shark.
Description
One of the world's smallest sharks, the spined pygmy shark attains a maximum recorded length of for males and for females. This species has an elongated, spindle-shaped body with a long, bulbous, moderately pointed snout. The eyes are large, with the upper rim of the orbit almost straight. Each nostril is preceded by a short flap of skin. The mouth has thin, smooth lips and contains 22–31 tooth rows in the upper jaw and 16–21 tooth rows in the lower jaw. The upper teeth are narrow and smooth-edged with single upright cusps. The bases of the lower teeth are broad and interlocked to form a continuous cutting surface, with each tooth bearing a single upright, smooth-edged, knife-like cusp. The openings of the five pairs of gill slits are minute and uniform in size.