The smalleye pygmy shark (''Squaliolus aliae'') is a little-known

species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...

of squaliform shark in the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Dalatiidae The Dalatiidae are the family of kitefin sharks of the order Squaliformes (the term "kitefin shark" also refers specifically to the species '' Dalatias licha''). Members of this family are small, under long, and are found worldwide. They have ci ...

, found in water deep near Japan, the

Philippines The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no), * bik, Republika kan Filipinas * ceb, Republika sa Pilipinas * cbk, República de Filipinas * hil, Republ ...

, and Australia. It migrates vertically daily, spending the day in deep water and the night in shallower water. One of the smallest shark species, the smalleye pygmy shark is known to reach only long. It has a blackish, spindle-shaped body with relatively small eyes, and a spine preceding the first

dorsal fin A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through c ...

, but not the second.

Bioluminescent Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...

photophore A photophore is a glandular organ that appears as luminous spots on various marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye; equipped with lenses, shutters, color filters and reflectors, h ...

s occur on its underside, which may serve to disguise its silhouette from

predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...

s. This species feeds on small

squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting ...

krill Krill are small crustaceans of the order Euphausiacea, and are found in all the world's oceans. The name "krill" comes from the Norwegian word ', meaning "small fry of fish", which is also often attributed to species of fish. Krill are consid ...

shrimp Shrimp are crustaceans (a form of shellfish) with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata of the decapod order, although some crustaceans outside of this order are refer ...

, and

bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartila ...

es. It is

aplacental viviparous Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a term used as a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals possess embryos that develop insi ...

. The

International Union for Conservation of Nature The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natur ...

has assessed it as

Least Concern A least-concern species is a species that has been categorized by the International Union for Conservation of Nature (IUCN) as evaluated as not being a focus of species conservation because the specific species is still plentiful in the wild. Th ...

, citing its wide distribution and lack of threat from fisheries.

Taxonomy

The first known specimen of the smalleye pygmy shark was a female long, caught off Donggang,

Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia, at the junction of the East and South China Seas in the northwestern Pacific Ocean, with the People's Republic of China (PRC) to the northwest, Japan to the northe ...

, on September 23, 1958. It was described by Taiwanese

ichthyologist Ichthyology is the branch of zoology devoted to the study of fish, including bony fish (Osteichthyes), cartilaginous fish (Chondrichthyes), and jawless fish (Agnatha). According to FishBase, 33,400 species of fish had been described as of October ...

Teng Huo-Tu, who gave it the

specific epithet In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

''alii'' after a woman. The name subsequently came to be rendered as ''aliae'' in scientific literature. Some authors questioned the validity of the species, and in 1977 Jeffrey Alan Seigel and colleagues synonymized ''S. aliae'' with '' S. laticaudus''. In 1987, Kunio Sasaki and Teruya Uyeno performed morphological comparisons and reaffirmed the distinctiveness of ''S. aliae''.

Distribution and habitat

The smalleye pygmy shark appears to be widely distributed, but in patches, in the western

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...

, having been reported from off southern Japan, the

, and northern and eastern Australia. It inhabits the upper and middle layers of the

water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...

near land, at depths of . It conducts a

diel vertical migration Diel vertical migration (DVM), also known as diurnal vertical migration, is a pattern of movement used by some organisms, such as copepods, living in the ocean and in lakes. The word ''diel'' comes from the Latin ''dies'' day, and means a 24-ho ...

, spending the day in deeper water and rising to shallower waters at night.

Description

Among the smallest of extant sharks, the smalleye pygmy shark attains a maximum recorded length of . It is shaped like a cigar and has a bulbous, pointed snout. The eyes are relatively small, with their diameters measuring 43–66% as long as the snout (compared to 61–82% in ''S. laticaudus''). The eyes of this species also differ from ''S. laticaudus'' in that the upper rim of the

eye socket In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents. In the adult human, the volume of the orbit is , of ...

is chevron-shaped rather than nearly straight. The nostrils lack substantially expanded skin flaps in front. The mouth is nearly transverse and bears thin lips; a pair of papillae (nipple-like structures) is on the upper lip that is absent in ''S. laticaudus''. There are 20–27 upper tooth rows and 18–23 lower tooth rows. The upper teeth are slender and upright. The larger, broader lower teeth have angled and knife-like cusps, and interlock to form a continuous cutting surface. The five pairs of

gill slit Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In co ...

s are tiny and uniform. The two ''Squaliolus'' species are the only sharks that have a spine on the first

, but not the second. The spine is usually exposed in males and covered by skin in females. The tiny first dorsal fin originates about over the rear tip of the

pectoral fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as ...

. The second dorsal fin is long and low, and originates over the front half of the

pelvic fin Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods. Structure and function Structure In actinopterygians, the pelvic fin consists of two e ...

bases. The

s are short with rounded margins, and the pelvic fins are long and low. The

anal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as s ...

is absent. The

caudal peduncle Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

is thin and bears slight lateral

keel The keel is the bottom-most longitudinal structural element on a vessel. On some sailboats, it may have a hydrodynamic and counterbalancing purpose, as well. As the laying down of the keel is the initial step in the construction of a ship, in B ...

s. Males have shorter abdomens and longer caudal peduncles than females. The

caudal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as s ...

is broad and triangular, with nearly symmetrical upper and lower lobes and a prominent notch in the trailing margin of the upper lobe. The

dermal denticle A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as w ...

s are flattened and not toothed or elevated on stalks. This species is dark brown to black in color, becoming light towards the fin margins. Its underside is covered by light-producing

Biology and ecology

Little is known of the natural history of the smalleye pygmy shark. It feeds primarily on midwater

s, and small

es such as

lanternfish Lanternfishes (or myctophids, from the Greek μυκτήρ ''myktḗr'', "nose" and ''ophis'', "serpent") are small mesopelagic fish of the large family Myctophidae. One of two families in the order Myctophiformes, the Myctophidae are represented ...

. The photophores on its underside may serve to camouflage its silhouette from

s by matching the downwelling light, a strategy known as "counter-illumination". Reproduction is

as in other members of its family, with the young being born under long. Males attain

sexual maturity Sexual maturity is the capability of an organism to reproduce. In humans it might be considered synonymous with adulthood, but here puberty is the name for the process of biological sexual maturation, while adulthood is based on cultural definit ...

around long.

Human interactions

The smalleye pygmy shark is infrequently caught by fisheries because of its small size, and has no economic value. This, coupled with its wide range, has led the IUCN to list it as

References

{{Taxonbar, from=Q3277595 Squaliolus Bioluminescent sharks Fish described in 1959