| Velvet belly lanternshark | |
|---|---|
 | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Division: | Selachii |
| Order: | Squaliformes |
| Family: | Etmopteridae |
| Genus: | Etmopterus |
| Species: | E. spinax |
| Binomial name | |
| Etmopterus spinax | |
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| Range of the velvet belly lantern shark | |
| Synonyms | |
* ambiguous synonym | |
Thevelvet belly lanternshark (or simplyvelvet belly) (Etmopterus spinax) is a species ofdogfish shark in thefamilyEtmopteridae. One of the most common deepwater sharks in the northeasternAtlantic Ocean, the velvet belly is found fromIceland andNorway toGabon andSouth Africa at a depth of 20–2,490 m (66–8,169 ft).[2][3] A small shark generally no more than 45 cm (18 in) long, the velvet belly is so named because its black underside is abruptly distinct from the brown coloration on the rest of its body. The body of this species is fairly stout, with a moderately long snout and tail, and very smallgill slits. Like otherlanternsharks, the velvet belly isbioluminescent, with light-emittingphotophores forming a species-specific pattern over its flanks and abdomen. The ventral photophores are thought to function incounter-illumination, whichcamouflages the shark againstpredators and prey.[4][5] The bioluminescent flank markings may play a role in intraspecific communication.[6]
Young velvet bellies feed mainly onkrill and smallbony fish, transitioning tosquid andshrimp as they grow larger. There is evidence that individuals also move into deeper water as they age. This species exhibits a number ofadaptations to living in the deep sea, such as specializedT-cells andliverproteins for dealing with the higher concentrations ofheavy metals found there. Velvet bellies often carry a heavyparasite load. It isovoviviparous, giving birth to litters of six to 20 young every two to three years. This species has virtually no commercial value, but large numbers are caught asbycatch in deepwatercommercial fisheries. It has been assessed as Near Threatened by theInternational Union for Conservation of Nature, the heavy fishing pressure throughout its range and its slow reproductive rate are raising conservation concerns.
The velvet belly was originally described asSqualus spinax by Swedish natural historianCarl Linnaeus, known as the "father oftaxonomy", in the 1758tenth edition ofSystema Naturae. He did not designate atype specimen; thespecific epithetspinax is in reference to the spinydorsal fins. This species was later moved to the genusEtmopterus via thesynonymy ofConstantine Samuel Rafinesque'sEtmopterus aculeatus withSqualus spinax.[7]
The velvet belly is grouped with theCaribbean lanternshark (E. hillianus),fringefin lanternshark (E. schultzi),brown lanternshark (E. unicolor),broadbanded lanternshark (E. gracilispinis),combtooth lanternshark (E. decacuspidatus), anddwarf lanternshark (E. perryi) in having irregularly arranged, needle-shapeddermal denticles.[8] Itscommon name comes from this shark's black ventral surface, which is sharply delineated from the rest of its body like a patch ofvelvet.[9]
The range of the velvet belly is in the eastern Atlantic, extending from Iceland and Norway to Gabon, including theMediterranean Sea, theAzores, theCanary Islands, andCape Verde. It has also been reported offCape Province, South Africa. This shark mainly inhabits the outercontinental and insular shelves and upper slopes over mud or clay, from close to the bottom to the middle of the water column.[7][10] It is most common at a depth of 200–500 m (660–1,640 ft), though in theRockall Trough, it is only found at a depth of 500–750 m (1,640–2,460 ft).[11][12] This species has been reported from as shallow as 20 m (66 ft),[2] and as deep as 2,490 m (8,170 ft).[3]
The velvet belly is a robustly built shark with a moderately long, broad, flattened snout. The mouth has thin, smooth lips. The upper teeth are small, with a narrow centralcusp and usually fewer than three pairs of lateral cusplets. The lower teeth are much larger, with a strongly slanted, blade-like cusp at the top and interlocking bases. The five pairs ofgill slits are tiny, comparable in size to thespiracles. Both dorsal fins bear stout, grooved spines at the front, with the second much longer than the first and curved. The first dorsal fin originates behind the short and roundedpectoral fins; the second dorsal fin is twice the size of the first and originates behind thepelvic fins. Theanal fin is absent. The tail is slender, leading to a longcaudal fin with a small lower lobe and a low upper lobe with a prominent ventral notch near the tip.[7]
Thedermal denticles are thin with hooked tips, arranged without a regular pattern well-separated from one another. The coloration is brown above, abruptly transitioning to black below. There are thin black marks above and behind the pelvic fins, and along the caudal fin.[7] The velvet belly possesses numerous photophores that emit a blue-green light visible from 3–4 m (9.8–13.1 ft) away.[9] Varying densities of photophores are arranged in nine patches on the shark's sides and belly, creating a pattern unique to this species: photophores are present along thelateral line, scattered beneath the head but excluding the mouth, evenly on the belly, and concentrated around the pectoral fins and beneath thecaudal peduncle.[11][13] The maximum reported length is 60 cm (24 in), although few are longer than 45 cm (18 in).[11] Females are larger than males.[14]
Along with theblackmouth catshark (Galeus melastomus) and thePortuguese dogfish (Centroscymnus coelolepis), the velvet belly is one of the most abundant deep-sea sharks in the northeastern Atlantic.[15] It is found individually or in smallshoals.[16] Samplings in the Mediterranean have found females outnumbering males across all ages; this imbalance increases in the older age classes.[17] In theRockall Trough and theCatalan Sea, large adults are found in deeper waters than juveniles, which may serve to reduce competition between the two groups.[15] However, this pattern has not been observed at other sites in the eastern Mediterranean.[3]
The velvet belly's liver accounts for 17% of its body mass, three-quarters of which isoil, making it nearlyneutrally buoyant.[18] To deal with the higher concentrations of heavy metals in the deep sea, the velvet belly hasT-cells in itsbloodstream that can identify and mark toxic compounds for elimination. These T-cells are produced by alymphomyeloidgland in itsesophagus called a "Leydig's organ", which is also found in some other sharks and rays. In its liver, specialized proteins are also capable of detoxifyingcadmium,copper,mercury,zinc, and other toxic contaminants.[11] The velvet belly's bioluminescence is thought to function in counter-illumination, which eliminates the shark's silhouette and camouflages it from upward-looking predators.[13] Its bioluminescence may also serve a social function, such as finding mates or co-ordinating groups, as the pattern is species-specific. The velvet belly is an important food of larger fishes such as other sharks; a major predator of this species is thelongnosed skate (Dipturus oxyrinchus).[11][16]
Numerousparasites are known for this species, and both juveniles and adults often carry heavy parasite loads. Known internal parasites include themonogeneanSqualonchocotyle spinacis, thetapewormsAporhynchus norvegicus,Lacistorhynchus tenuis, andPhyllobothrium squali, and thenematodesAnisakis simplex andHysterothylacium aduncum. Some of these parasites use the velvet belly's prey as intermediate hosts and are acquired via ingestion, while others use the shark itself as an intermediate host.[16] ThebarnacleAnelasma squalicola, an external parasite, attaches to the shark's dorsal spine socket and penetrates deeply into the muscle, in the process often providing an attachment site for a second (and rarely a third) barnacle. Infestation by this barnacle reduces its host'sfecundity by impairing the development of thereproductive organs.[14]
As generalist predators, velvet bellies feed on crustaceans (e.g.pasiphaeid shrimp andkrill), cephalopods (e.g.ommastrephid squid andsepiolids), and bony fishes (e.g.shads,barracudinas,lanternfishes, andpouts).[11] Sharks offItaly also eat small amounts of nematodes,polychaete worms, and other cartilaginous fishes.[19] Studies of velvet bellies off Norway andPortugal, and in the Rockall Trough, have found small sharks under 27 cm (11 in) long feed mainly on the krillMeganyctiphanes norvegica and the small fishMaurolicus muelleri. As the sharks grow larger, their diets become more varied, consisting mainly of squid and the shrimpPasiphaea tarda, as well fishes other thanM. muelleri.[12][16][20] It has been speculated that smaller velvet bellies may be too slow to catch fast-moving cephalopods.[16] The cephalopod diet of adults overlaps with that of the Portuguese dogfish; the latter species may avoid competition with the velvet belly by living in deeper water.[15] The bite force exerted by the velvet belly is only around 1N.[21]
The velvet belly isovoviviparous, with theembryos hatching inside theuterus and being sustained by ayolk sac. The reproductive cycle may be two to three years long, withovulation occurring in early autumn,fertilization in the summer (or possibly in the winter if females are capable of storing sperm), and parturition in late winter or early spring. Thegestation period is under one year.[14][22] The litter size is six to 20, with the number of young increasing with female size. At birth, the young measure 12–14 cm (4.7–5.5 in) long.[7][22] The shark's bioluminescence develops before birth; the yolk sac isfluorescent before any photophores have formed, suggesting the mother transfers luminescent materials to her offspring. The first luminous tissue appears when the embryo is 55 mm (2.2 in) long, and the complete pattern is laid down by the time it is 95 mm (3.7 in) long. At birth, the young shark is already capable of counter-illumination with 80% of its ventral surface luminescent.[13]
The growth rate of the velvet belly is slow, though faster than some other deep-sea sharks, such as theleafscale gulper shark (Centrophorus squamosus) or theshortspine spurdog (Squalus mitsukurii). Malesmature sexually at 28–33 cm (11–13 in) long and females at 34–36 cm (13–14 in) long.[11][17] The average age at maturity is 4.0 years for males and 4.7 years for females, though four-year-old mature individuals of both sexes have been caught in the wild, along with immature females over eight years old.[22] Males and females eight and 11 years old, respectively, have been caught in the wild; the potential lifespan of this species has been estimated at 18 years for males and 22 years for females.[17][22]
Throughout their range, substantial quantities of velvet bellies are caught asbycatch inbottom trawls meant forshrimp andlobsters, anddeepwater longlines meant for other fish. Lacking commercial value, these sharks are almost always discarded with extremely high mortality, though occasionally they aredried and salted or made intofishmeal.[7][17] TheIUCN has listed the velvet belly underLeast Concern overall, as its population remains stable across much of its range, and it is afforded some protection in the Mediterranean from a 2005 ban on bottom trawling below 1,000 m (3,300 ft). It has been assessed asNear Threatened, as its numbers have declined by almost 20% from 1970 to 1998–2004.[23] The slow reproductive rate of this species limits its capacity to recover from population depletion.[22]
