|
The gulper shark (''Centrophorus granulosus'') is a long and slender dogfish usually about three feet in length generally found in deep, murky waters all around the world. It is a light grayish brown, paler ventrally, with a long snout and large greenish eyes.〔 This deep water shark has two dorsal fins with long, grooved spines and the second dorsal fin smaller than the first. Its upper teeth are blade-like and lower have finely serrated edges.〔 This tertiary consumer feeds on mainly fishes such as bony fishes, but also cephalopods such as squid and other invertebrates like crustaceans.〔 The gulper shark is currently a vulnerable species mainly because of exploitation by humans and their abnormally long gestation period and low fecundity, preventing their population from recovering.〔 ==Development and Reproduction== Gulper sharks reach maturity at around age 12 to 16 years for females, and age 7 to 8 years for males.〔Clarke, Connolly, Bracken, “Journal of Fish Biology”, 4 Apr 2015, “()”, 3/20/2013〕 The maturity of a gulper shark can be determined by the seven-stage maturity scale for aplacental and placental viviparous sharks.〔MedSudMed, “FAO Sicily”, Dec 2008, “()”, 3/21/2013〕 This scale is good for practical field use, but may not be as accurate as other maturity scales that have more than seven stages. Maturity for gulper sharks is considered when they are at stage 3 or above, which for males is when gonads are enlarged and filled with sperm, and sperm ducts are tightly coiled. For females, stage three is when ovaries are large and well rounded.〔 On average, male gulper sharks are smaller than females.〔Severino, Afonso-Dias, Delgado, “Life and Marine Sciences 26: 57-61”, 2009, “()”, 3/20/2013〕 The size of an average adult male is 80 to 95 cm. The size of an average adult female is from 90 to 100 cm long.〔 Differences in size between the sexes may be due to the need for space to support offspring.〔 It has been hypothesized that gulper sharks display a “depth distribution pattern associated with size” based on random human observation.〔 Male gulper sharks tend to outnumber females 2:1, which is common for many fish species. The life expectancy, longevity, of female gulper sharks ranges between 54 and 70 years.〔 Having a long life expectancy but a low net reproduction rate suggests that the population of gulper sharks would be at a very high risk if too many of them were killed from excessive fishing. Female gulper sharks typically have between 2 to 10 pups in their lifetime, with generally one pup per pregnancy, this is considered to be a low fecundity.〔 Once fertilized, females can hold up to 6 mature egg cells, or oocytes, in their body at a time. The length of time these egg cells are kept inside the female’s body is called the gestation period. Gulper sharks have a long gestation period, around two years.〔Guallart, Serena, Mancusi, Casper, Burgess, Ebert, Clarke, Stenberg, “IUCN Red List of Threatened Species”, 2006, “()”, 1 March 2013〕 Gulper sharks can have long resting periods between pregnancies.〔ARKive, “Gulper Shark Fact File”, “()”, 3/21/2013〕 They are ovoviviparous, meaning the only parental care they give their young is during the incubation period.〔 Since not all oocytes form into pups, when a pup or two is formed inside the female, they eat the remaining fertilized eggs, known as oophagy.〔 After they are born, they are on their own. Having a low fecundity, a long gestation period, breaks between pregnancies, and a late age of maturity all contribute to the gulper shark having a very low net reproduction rate.〔 It is believed that the gulper shark has the lowest reproduction rate of any elasmobranch species.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Gulper shark」の詳細全文を読む スポンサード リンク
|