Smalleye pygmy shark
Encyclopedia
The smalleye pygmy shark (Squaliolus aliae) is a little-known species
of dogfish shark
in the family
Dalatiidae
, found in water 150–2000 m (492.1–6,561.7 ft) deep near Japan
, the Philippines
, 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 22 cm (8.7 in) long. It has a blackish, spindle-shaped body with relative small eyes, and a spine preceding the first dorsal fin
but not the second. There are bioluminescent
photophore
s on its underside, which may serve to disguise its silhouette from predators. This species feeds on small squid
, krill
, shrimp
s, and bony fishes. It is aplacental viviparous. The International Union for Conservation of Nature (IUCN) has assessed it as Least Concern
, citing its wide distribution and lack of threat from fisheries.
on September 23, 1958. It was described by Taiwanese ichthyologist Teng Huo-Tu
, who gave it the specific epithet 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.
, having been reported from off southern Japan
, the Philippines
, and northern and eastern Australia
. It inhabits the upper and middle layers of the water column near land, at depths of 150–2000 m (492.1–6,561.7 ft). It conducts a diel vertical migration
, spending the day in deeper water and rising to shallower waters at night.
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 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; there are a pair of papillae (nipple-like structures) on the upper lip that are 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
s are tiny and uniform.
The two Squaliolus species are the only sharks that have a spine on the first dorsal fin
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 of the pectoral fin. The second dorsal fin is long and low, and originates over the front half of the pelvic fin bases. The pectoral fins are short with rounded margins, and the pelvic fins are long and low. The anal fin is absent. The caudal peduncle is thin and bears slight lateral keels. Males have shorter abdomens and longer caudal peduncles than females. The caudal fin 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 denticles 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
photophore
s.
, krill
, shrimp
s, and small bony fishes such as lanternfish
. The photophores on its underside may serve to camouflage its silhouette from predators by matching the downwelling light, a strategy known as "counter-illumination". Reproduction is aplacental viviparous as in other members of its family, with the young being born at under 10 cm (3.9 in) long. Males attain sexual maturity
at around 15 cm (5.9 in) long.
.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of dogfish shark
Squaliformes
Squaliformes is an order of sharks that includes about 97 species in seven families.Members of the order have two dorsal fins, which usually possess spines, no anal fin or nictitating membrane, and five gill slits. In most other respects, however, they are quite variable in form and size...
in the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Dalatiidae
Dalatiidae
Dalatiidae is a family of sharks in the order Squaliformes, commonly known as kitefin sharks . Members of this family are small, under long, and are found worldwide. They have cigar-shaped bodies with narrow heads and rounded snouts...
, found in water 150–2000 m (492.1–6,561.7 ft) deep near Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, the Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
, and Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. It migrates vertically daily
Diel vertical migration
Diel vertical migration, also known as diurnal vertical migration, is a pattern of movement that some organisms living in the ocean and in lakes undertake each day. Usually organisms move up to the epipelagic zone at night and return to the mesopelagic zone of the oceans or to the hypolimnion zone...
, 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 22 cm (8.7 in) long. It has a blackish, spindle-shaped body with relative small eyes, and a spine preceding the first dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
but not the second. There are bioluminescent
Bioluminescence
Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...
photophore
Photophore
A photophore is a light-emitting organ which 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...
s on its underside, which may serve to disguise its silhouette from predators. This species feeds on small squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
, krill
Krill
Krill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world...
, shrimp
Shrimp
Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...
s, and bony fishes. It is aplacental viviparous. The International Union for Conservation of Nature (IUCN) has assessed it as Least Concern
Least Concern
Least Concern is an IUCN category assigned to extant taxon or lower taxa which have been evaluated but do not qualify for any other category. As such they do not qualify as threatened, Near Threatened, or Conservation Dependent...
, citing its wide distribution and lack of threat from fisheries.
Taxonomy
The first known specimen of the smalleye pygmy shark was a female 18 cm (7.1 in) long, caught off Donggang, TaiwanTaiwan
Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...
on September 23, 1958. It was described by Taiwanese ichthyologist Teng Huo-Tu
H. T. Teng
Teng Huo-Tu was an ichthyologist with the Taiwan Fisheries Research Institute . Much of his work involved classification of chondricthyes, especially sharks....
, who gave it the specific epithet 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
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...
S. aliae with S. laticaudus
Spined pygmy shark
The spined pygmy shark is a species of dogfish shark in the family Dalatiidae found widely in all oceans. With a maximum known length of , it is one of the smallest living sharks. This shark has a slender, cigar-shaped body with a sizable conical snout, a long but low second dorsal fin, and an...
. In 1987, Kunio Sasaki and Teruya Uyeno performed morphological
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
comparisons and reaffirmed the distinctiveness of S. aliae.
Distribution and habitat
The smalleye pygmy shark appears to be widely but patchily distributed in the western Pacific OceanPacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
, having been reported from off southern Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, the Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
, and northern and eastern Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. It inhabits the upper and middle layers of the water column near land, at depths of 150–2000 m (492.1–6,561.7 ft). It conducts a diel vertical migration
Diel vertical migration
Diel vertical migration, also known as diurnal vertical migration, is a pattern of movement that some organisms living in the ocean and in lakes undertake each day. Usually organisms move up to the epipelagic zone at night and return to the mesopelagic zone of the oceans or to the hypolimnion zone...
, 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 22 cm (8.7 in). It is shaped like a cigarCigar
A cigar is a tightly-rolled bundle of dried and fermented tobacco that is ignited so that its smoke may be drawn into the mouth. Cigar tobacco is grown in significant quantities in Brazil, Cameroon, Cuba, the Dominican Republic, Honduras, Indonesia, Mexico, Nicaragua, Philippines, and the Eastern...
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 is chevron
Chevron (anatomy)
A chevron is one of a series of bones on the ventral side of the tail in many reptiles, dinosaurs , and some mammals such as kangaroos and manatees....
-shaped rather than nearly straight. The nostrils lack substantially expanded skin flaps in front. The mouth is nearly transverse and bears thin lips; there are a pair of papillae (nipple-like structures) on the upper lip that are 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 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, rays, sawfish, and guitarfish. Most of these have five pairs, but a few species have 6 or 7 pairs...
s are tiny and uniform.
The two Squaliolus species are the only sharks that have a spine on the first dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
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 of the pectoral fin. The second dorsal fin is long and low, and originates over the front half of the pelvic fin bases. The pectoral fins are short with rounded margins, and the pelvic fins are long and low. The anal fin is absent. The caudal peduncle is thin and bears slight lateral keels. Males have shorter abdomens and longer caudal peduncles than females. The caudal fin 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 denticles 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
Bioluminescence
Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...
photophore
Photophore
A photophore is a light-emitting organ which 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...
s.
Biology and ecology
Little is known of the natural history of the smalleye pygmy shark. It feeds primarily on midwater squidSquid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
, krill
Krill
Krill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world...
, shrimp
Shrimp
Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...
s, and small bony fishes such as lanternfish
Lanternfish
Cooper Lanternfishes are small mesopelagic fish of the large family Myctophidae. One of two families in the order Myctophiformes, the Myctophidae are represented by 246 species in 33 genera, and are found in oceans worldwide. They are aptly named after their conspicuous use of bioluminescence...
. The photophores on its underside may serve to camouflage its silhouette from predators by matching the downwelling light, a strategy known as "counter-illumination". Reproduction is aplacental viviparous as in other members of its family, with the young being born at under 10 cm (3.9 in) long. Males attain sexual maturity
Sexual maturity
Sexual maturity is the age or stage when an organism can reproduce. It is sometimes considered synonymous with adulthood, though the two are distinct...
at around 15 cm (5.9 in) 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 International Union for Conservation of Nature (IUCN) to list it under Least ConcernLeast Concern
Least Concern is an IUCN category assigned to extant taxon or lower taxa which have been evaluated but do not qualify for any other category. As such they do not qualify as threatened, Near Threatened, or Conservation Dependent...
.