Thresher shark - Thresher shark

A general term for a family or genera of the order Lamniformes in the class Chondrichthyes. The Alopiidae (thresher sharks) are made up of one genus, Alopias . Sharks in this family can be easily distinguished from other sharks by their extremely long tail fin, which is about half the length of their body. The genus name Alopias means "fox" and is derived from their long tail. There are three known species of thresher sharks in the world, and these three species are found in the waters around Japan.

The Japanese white-spotted flycatcher A. superciliosus is found in the tropical waters of the Pacific, Indian and Atlantic Oceans, and is characterized by its very large, vertically long eyes, which are located from the side to the back of the head, a groove from the back of the head to the top of the pectoral fin, which looks like an eight-shaped split when viewed from above, and a thick tail fin. It grows to about 5 meters in length. The Japanese blue-spotted flycatcher A. pelagicus and the Japanese long-tailed magpie A. vulpinus are similar in that they both have round eyes located on the side of the body and a long, slender tail fin, but the Japanese blue-spotted flycatcher can be distinguished by the small terminal lobe of its tail fin, which is almost the same size as its anal fin, and the white area on the ventral surface of its body does not extend above the pectoral fin (the terminal lobe of the tail fin of the Japanese long-tailed magpie is much larger than the anal fin, and the white area on the ventral surface of its body extends above the pectoral fin).

Thresher sharks are ovoviviparous, meaning that the embryos grow in the uterus on their own egg yolk until they reach a certain size, after which they feed on small nutrient eggs supplied by the ovaries. They give birth to 2-6 young, each 1-1.5 meters long. Thresher sharks have a very interesting habit: they approach schools of small fish or squid, swing their long tail fin up at their prey from below, and strike the prey to weaken it before eating it. All three species are caught using longlines and other methods, but there is a fishery that makes good use of their habit of striking prey with their tail fin, and many are caught with the hook still attached to their tail fin. Their meat is used as an ingredient in steaks and high-quality fish paste products. On the International Union for Conservation of Nature (IUCN) Red List, the blue-and-white flycatcher is listed as "endangered," while the brown-and-white flycatcher and the blue-and-white flycatcher are listed as "vulnerable" (as of September 2021).

[Kazuhiro Nakatani October 20, 2021]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

軟骨魚綱ネズミザメ目の科や属の総称。オナガザメ科Alopiidae（英名thresher sharks）はオナガザメ属Alopiasの1属からなる。同科のサメは、体の半分ほどもあるきわめて長い尾びれにより容易にほかのサメ類と区別できる。属名Alopiasは「キツネ」の意味で、尾が長いことに由来している。全世界に3種のオナガザメ類が知られ、日本近海にもこの3種が分布している。

　ハチワレA. superciliosusは、太平洋、インド洋、大西洋の温熱帯海域に分布し、目が縦長で非常に大きく、頭の側面から背面にあること、後頭部から胸びれ上方にかけて溝があり、上から見るとその溝が「八」の字状に割れて見えること、尾びれが太いことなどにより特徴づけられる。全長5メートルほどになる。ニタリA. pelagicusとマオナガA. vulpinusは、目が丸く、体側に位置していること、尾びれが細長いことなどで似ているが、ニタリは尾びれの末端葉が小さく、臀(しり)びれとほぼ同大であること、体腹面の白色部分が胸びれの上まで広がらないことなどで区別できる（マオナガの尾びれ末端葉は臀びれよりかなり大きく、体腹面の白色は胸びれの上まで広がる）。

　オナガザメ類は食卵型胎生で、胎仔(たいし)は子宮内である程度の大きさになるまでは自分のもつ卵黄で成長し、その後は卵巣から供給される小形の栄養卵を食べて成長する。全長1～1.5メートルの子を2～6尾産む。オナガザメ類にはたいへん興味ある習性があり、小魚やイカの群れに近づき、下側から獲物に向かって、その長い尾びれを振り上げ、獲物をたたいて弱らせてから食べる。3種とも延縄(はえなわ)などで漁獲されるが、獲物を尾びれでたたくという習性をうまく利用した漁業があり、多くが尾びれに針がかかった状態で釣りあげられる。肉はステーキや上等の練り製品の原料となる。国際自然保護連合（IUCN）のレッド・リストでは、ニタリは絶滅危惧(きぐ)種中の「危機」（EN）に、ハチワレとマオナガは「危急」（VU）に指定されている（2021年9月時点）。

［仲谷一宏　2021年10月20日］

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例