Carcharhinus melanopterus (Quoy & Gaimard, 1824)
不気味に見えるサメの目
The Eerie Eyes of a Shark
体長1.5mほど、最大で1.8mに達する中型のサメです。インド洋から西・中央太平洋の熱帯域に広く分布し、サンゴ礁や礁湖などの浅い沿岸域を主な生活場所としています。浅瀬に現れるサメとして非常に身近な存在で、背ビレを水面から突き出して泳ぐ姿もよく見られます。
体は灰色から黄褐色で腹側は白く、第一背ビレや尾ビレなどの先端が黒く染まっています。特に第一背ビレの黒い模様がよく目立ち、「ツマグロ」という和名の由来にもなっています。主に小魚やイカ、タコなどの頭足類、エビやカニなどの甲殻類を食べます。魚の群れを複数の個体で追い込むような行動も観察されています。
サメの目には、私たちとは異なる防御機構があります。魚には人間のようなまぶたはありませんが、ツマグロを含むメジロザメ科の仲間には「瞬膜」と呼ばれる膜状の構造があります。獲物に襲いかかる際などに瞬膜を目の前へ動かして眼球を保護するため、攻撃の瞬間には目が白く覆われたように見えます。人間から見ると、この独特な目の動きがサメの不気味な印象につながっているのかもしれません。
一方、ホホジロザメなど瞬膜を持たない一部のサメでは、獲物に噛みつく際に眼球そのものを眼窩の奥へ引き込むようにして保護します。サメの目が独特に見える背景には、こうした捕食時の負傷を防ぐための特殊な仕組みがあります。
The Blacktip Reef Shark is a medium-sized shark that grows to around 1.5 m in length, with a maximum recorded length of about 1.8 m. It is widely distributed throughout tropical waters from the Indian Ocean to the western and central Pacific, inhabiting shallow coastal waters such as coral reefs and lagoons. It is a familiar shark in shallow waters, and its dorsal fin can often be seen breaking the surface as it swims.
The body is gray to yellowish-brown, with a white underside, while the tips of several fins are distinctly black. The black markings on the first dorsal fin and lower lobe of the caudal fin are particularly conspicuous and give the species its Japanese name, “Tsumaguro,” meaning “black-tipped.” It feeds mainly on small fish, cephalopods such as squid and octopus, and crustaceans including shrimp and crabs. Groups of individuals have also been observed working together to drive schools of fish into shallower water.
Sharks have a distinctive mechanism for protecting their eyes. Fish do not have eyelids like humans, but members of the requiem shark family, including the Blacktip Reef Shark, possess a membrane called the nictitating membrane. When attacking prey, they can move this membrane over the eye to protect the eyeball, making the eye appear to be covered in white at the moment of attack. To humans, this unusual movement may contribute to the eerie impression often associated with shark eyes.
Some sharks, such as the Great White Shark, lack a nictitating membrane and instead protect their eyes by retracting and rotating the eyeballs into the eye sockets when biting prey. The distinctive appearance of shark eyes is therefore closely related to specialized adaptations that protect them from injury while hunting.
参考文献
“Everything You Need to Know About Blacktip Reef Sharks” bluereef. 2025年3月29日閲覧
松浦 圭一「したたかな魚たち」角川新書, 2017, p.128 – 129.