The
spined pygmy shark,
Squaliolus laticaudus, is a
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
of
dogfish sharkSqualiformes is an order of sharks that includes about 80 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
familyIn 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...
DalatiidaeDalatiidae 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 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 finA dorsal fin is a fin located on the backs of some fish, whales, dolphins, and porpoises, as well as the ichthyosaurs. Depending on the species, an animal can have up to three of them. Its main purpose is to stabilize the animal against rolling and assist in sudden turns...
, and an almost symmetrical caudal fin. It and its sister species
S. aliaeThe smalleye pygmy shark, Squaliolus aliae, is a sleeper shark of the family Dalatiidae found in the western Pacific Ocean from Japan to northern Australia, at depths down to 2,000 m. Its length is up to 22 cm....
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
bioluminescentBioluminescence 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...
organs called
photophoreA 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 their ventral surface. The shark is believed to use these photophores to match ambient light conditions and thus disguise its silhouette from predators.
Usually inhabiting nutrient-rich waters over upper continental and insular slopes, the spined pygmy shark feeds on small bony fishes and
squidSquid are marine 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 and two longer tentacles arranged in pairs...
. Like its prey it is a
diel vertical migratorDiel vertical migration refers to a pattern of movement that some organisms living in the ocean's photic zone undertake each day. The word diel comes from the Latin dies , and means a 24-hour period....
, spending the day at close to deep and moving towards a depth of at night. Reproduction is presumably ovoviviparous, with female giving birth to litters of 4 pups. This diminutive shark has no economic value. The International Union for Conservation of Nature (IUCN) has assessed this species as of
Least ConcernLeast Concern is an IUCN category assigned to extant species or lower taxa which have been evaluated but do not qualify for any other category. As such they do not qualify as threatened, nor Near Threatened, nor Conservation Dependent...
, 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 CommissionThe United States Commission of Fish and Fisheries was established on February 9, 1871 , as an independent commission with a mandate to investigate the causes for the decrease of commercial fish and aquatic animals in U.S...
SteamerA steamboat or steamship, sometimes called a steamer, is a ship in which the primary method of propulsion is steam power, typically driving propellers or paddlewheels....
AlbatrossThe second USS Albatross was an iron-hulled, twin-screw steamer in the United States Navy and reputedly the first vessel ever built especially for marine research....
. It was described by American ichthyologists
Hugh McCormick SmithHugh McCormick Smith was an American ichthyologist and administrator in the Bureau of Fisheries.-Biography:...
and
Lewis RadcliffeLewis Radcliffe was a naturalist, malacologist, and ichthyologist. He was Deputy Commissioner of the United States Bureau of Fisheries until 1932 and was the assistant naturalist under Hugh McCormick Smith for the 1907-1910 Philippines Expedition. During his life, he described numerous new species...
in a 1912 paper for the
scientific journalIn academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research. There are thousands of scientific journals in publication, and many more have been published at various points in the past...
Proceedings of the United States National Museum, based on two specimens collected in
Batangas BayBatangas Bay is a semi-enclosed body of water located in the Philippines. The municipalities of Mabini, Bauan, San Pascual, Tingloy on Maricaban Island in the Bay, and Batangas City are located on the coast of the bay. The water surface area of the bay is about 220 km² , and the coastline...
, south of
LuzonLuzon is the largest and most economically and politically important island in the Philippines and one of the three island groups in the country, with Visayas and Mindanao being the other two...
in the
PhilippinesThe Philippines officially known as the Republic of the Philippines, is a country in Southeast Asia with Manila as its capital city. It comprises 7,107 islands in the western Pacific Ocean....
. One of these, a long adult male, was designated the type specimen.
Smith and Radcliffe coined the new
genusIn biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...
Squaliolus for this shark, and gave it the specific epithet
laticaudus, from the
LatinLatin is an Italic language originally spoken in Latium and Ancient Rome. Through the Roman conquest, Latin spread throughout the Mediterranean and a large part of Europe...
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 organAn intromittent organ is a general term for an external organ of a male organism that is specialized to deliver sperm during copulation. Intromittent organs are found most often in terrestrial species, as most aquatic species fertilize their eggs externally, although there are exceptions.-Species...
s), the closest relative of the spined pygmy shark and the related
S. aliae is thought to be the
pygmy sharkThe pygmy shark, Euprotomicrus bispinatus, the second-smallest of all the shark species after the dwarf lanternshark, is a sleeper shark of the Dalatiidae family, the only member of the genus Euprotomicrus...
(
Euprotomicrus bispinatus).
Distribution and habitat
The spined pygmy shark has a wide distribution around the world. In the
Atlantic OceanThe Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about 106.4 million square kilometres , it covers approximately one-fifth of the Earth's surface and about one-quarter of its water surface area. The first part of its name refers to the Atlas of Greek...
, it occurs off
BermudaBermuda is a British overseas territory in the North Atlantic Ocean. Located off the east coast of the United States, it is situated around 1,770 kilometres northeast of Miami, Florida, and 1,350 kilometres south of Halifax, Nova Scotia, Canada...
, the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
,
SurinameSuriname , officially the Republic of Suriname, is a country in northern South America....
, southern
BrazilBrazil , officially the Federative Republic of Brazil , is a country in South America. It is the fifth largest country by geographical area, occupying nearly half of South America, the fifth most populous country, and the fourth most populous democracy in the world. Bounded by the Atlantic Ocean...
, and northern
ArgentinaArgentina, officially the Argentine Republic , is the second largest country in South America, constituted as a federation of 23 provinces and an autonomous city, Buenos Aires. It is the eighth largest country in the world by land area and the largest among Spanish-speaking nations, though Mexico,...
in the west, and off northern
FranceFrance , officially the French Republic , is a country located in Western Europe, with several overseas islands and territories located on other continents. Metropolitan France extends from the Mediterranean Sea to the English Channel and the North Sea, and from the Rhine to the Atlantic Ocean...
,
MadeiraMadeira is a Portuguese archipelago in the mid Atlantic Ocean that lies between and . It is one of the Autonomous regions of Portugal, with Madeira Island and Porto Santo Island being the only inhabited islands...
,
Cape VerdeThe Republic of Cape Verde is an island country, spanning an archipelago located in the Macaronesia ecoregion of the North Atlantic Ocean, off the western coast of Africa, opposite Mauritania and Senegal....
, and the
AzoresThe Azores is a Portuguese archipelago in the Atlantic Ocean, about from Lisbon and about from the east coast of North America. The two westernmost Azorean islands actually lie on the North American plate...
in the east. In the
Indian OceanThe Indian Ocean is the third largest of the world's oceanic divisions, covering about 20% of the water on the Earth's surface. It is bounded on the north by South Asia ; on the west by Africa; on the east by Indochina, the Sunda Islands, and Australia; and on the south by the Southern Ocean...
, this species has only been recorded off
SomaliaSomalia , officially the Republic of Somalia and formerly known as the Somali Democratic Republic, is a country located in the Horn of Africa...
. In the
Pacific OceanThe Pacific Ocean is the largest of the Earth's oceanic divisions. Its name is derived from the Latin name Tepre Pacificum, "peaceful sea", bestowed upon it by the Portuguese explorer Ferdinand Magellan. It extends from the Arctic in the north to Antarctica in the south, bounded by Asia and...
, it is found off southern
Japanis an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, People's Republic of 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...
,
TaiwanTaiwan , also known as Formosa , is the largest island of the Republic of China in East Asia. Taiwan is located east of the Taiwan Strait, off the southeastern coast of mainland China...
, and the
PhilippinesThe Philippines officially known as the Republic of the Philippines, is a country in Southeast Asia with Manila as its capital city. It comprises 7,107 islands in the western Pacific Ocean....
. The spined pygmy shark is found at depths of and seldom approaches the surface, unlike the related pygmy shark and
cookiecutter sharkThe cookiecutter shark, Isistius brasiliensis, also known as the cigar shark or luminous shark, is a small, rarely-seen dogfish shark.-Anatomy and morphology:...
(
Isistius brasiliensis). This shark prefers areas of high biological productivity over upper continental and insular slopes. It may also be found over outer
shelvesThe continental shelf is the extended perimeter of each continent and associated coastal plain, and was part of the continent during the glacial periods, but is undersea during interglacial periods such as the current epoch by relatively shallow seas and gulfs. The continental rise is below the...
, 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 slitGill 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 minute and uniform in size.
The two species of
Squaliolus are the only sharks with a spine on the first
dorsal finA dorsal fin is a fin located on the backs of some fish, whales, dolphins, and porpoises, as well as the ichthyosaurs. Depending on the species, an animal can have up to three of them. Its main purpose is to stabilize the animal against rolling and assist in sudden turns...
but not the second. The spine is sexually dimorphic, being typically exposed in males and enclosed by skin in females. The first dorsal fin is tiny and originates over the trailing margin of the pectoral fins. The second dorsal fin is low, with a base twice as long as that of the first, and originates over the anterior half of the pelvic fin bases. The pectoral fins are short and triangular, with the rear margin slightly curved. The pelvic fins are long and low, and there is no anal fin. The caudal peduncle is slender and laterally expanded into weak keels. The caudal fin is broad and paddle-like, with the upper and lower lobes of similar size and shape, and a deep notch in the trailing margin of the upper lobe.
The
dermal denticleDenticles are body surface structures found on some fish and insects. Literally, the term means "small tooth" and refer to the structures' shape....
s are flat and blocky, not elevated on stalks or bearing marginal teeth. The coloration is dark brown to black, with light fin margins. The underside is densely carpeted by light-emitting
photophoreA 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, which extend to the tip of the snout and around the eyes and nostrils, and thin to almost non-existent on the back. This species has on average only 60
vertebraA vertebra is an individual bone in the flexible column that defines vertebrate animals, e.g. humans. The vertebral column encases and protects the spinal cord, which runs from the base of the cranium down the dorsal side of the animal until reaching the pelvis. From there, vertebra continue into...
e, the fewest of any shark.
Biology and ecology
The diet of the spined pygmy shark consists mainly of bony fishes (including the
dragonfishStomiidae is a family of deep-sea ray-finned fish, including the barbeled dragonfishes, stareaters and loosejaws.Stomiids are generally elongated fish with black or near-black bodies, but they are highly variable in form, and are sometimes grouped into multiple different families as a result. The...
Idiacanthus, the
lanternfishLanternfishes are small, deep sea 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...
Diathus, and the bristlemouth
Gonostoma) and
squidSquid are marine 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 and two longer tentacles arranged in pairs...
(including members of the genera
ChiroteuthisChiroteuthis is a genus of chiroteuthid squid, comprising two subgenera. The hectocotylus is absent from all members of the genus; instead, a penis extending from the mantle opening is utilised. The genus is characterised by enlarged, lidded photophores present at the end of the tentacular club...
and
HistioteuthisHistioteuthis is a genus of squid and the only member of the Histioteuthidae family. It goes by the English name cock-eyed squid, because in all species the right eye is normal-sized, round, blue and sunken; whereas the left eye is at least twice the diameter of the right eye, tubular,...
). Catch records suggest that the spined pygmy shark follows its prey on their
diel vertical migrationDiel vertical migration refers to a pattern of movement that some organisms living in the ocean's photic zone undertake each day. The word diel comes from the Latin dies , and means a 24-hour period....
s, spending the day close to a depth of and ascending towards a depth of at night. The ventral photophores of the spined pygmy shark have been theorized to function in
counter-illuminationCountershading, or Thayer's Law, is a form of camouflage. Countershading, in which an animal’s pigmentation is darker dorsally, is often thought to have an adaptive effect of reducing conspicuous shadows cast on the ventral region of an animal’s body...
, a form of camouflage in which the shark disguises its silhouette from would-be predators by matching the ambient light welling down from above. There is no evidence that this shark swallows its shed teeth like the pygmy and cookiecutter sharks.
Reproduction in the spined pygmy shark is ovoviviparous like the rest of its family, with the developing
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s being sustained by a
yolk sacThe yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
until birth. Adult females have two functional ovaries that may each contain up to 12 mature eggs. However, the actual litter size is much smaller; a pregnant female caught off southern Brazil in 1999 contained four near-term pups. The young are born at long. Males mature sexually at a length of , and females at a length of . The spined pygmy shark was widely considered to be the smallest living shark species until the discovery of the
dwarf lanternsharkThe dwarf lanternshark is a little-known species of dogfish shark in the family Etmopteridae and possibly the smallest shark in the world, reaching a maximum known length of . It is known to be present only on the upper continental slopes off Colombia and Venezuela, at a depth of...
(
Etmopterus perryi), though the
pygmy ribbontail catsharkThe pygmy ribbontail catshark, Eridacnis radcliffei, is a species of finback catshark, family Proscylliidae, distributed patchily in the western Indo-Pacific from Tanzania to the Philippines. It occurs around the edges of continental and insular shelves at a depth of , typically on or near mud...
(
Eridacnis radcliffei) is also known to mature at a size comparable to these two species. Whether one of these sharks is definitively smaller than the others cannot yet be stated with certainty, because of the difficulties involved in assessing reproductive maturity in sharks.
Human interactions
Spined pygmy sharks have no commercial value; they sometimes appear in the bycatch of trawl fisheries, but are generally too small to be captured. In light of its wide distribution and the absence of substantial threats from human activity, the International Union for Conservation of Nature (IUCN) has assessed this species as of
Least ConcernLeast Concern is an IUCN category assigned to extant species or lower taxa which have been evaluated but do not qualify for any other category. As such they do not qualify as threatened, nor Near Threatened, nor Conservation Dependent...
.