サメの祖先は、今から約4億年前に出現しました。現在のサメは、約１億年前にあらわれ、その後あまり変化しておらず、世界の海に500種以上のサメが生息しています。エイはサメの仲間から分かれたといわれていて、世界の海に600種以上のエイが生息しています。サメとエイは、全身の骨が柔らかい軟骨魚類の仲間で、えら穴の位置で簡単に見分けることができます。えら穴が体の側面にあるのがサメ、腹面にあるのがエイになります。
The predecessor of sharks appeared on earth around 400 million years ago. The sharks we know today came into being about 100 million years ago, and hasn’t evolved much since then. Currently, there are some 500 shark species and 600 ray species in the world. Rays are thought to share the same ancestor as sharks, and have evolved differently about 200 million years ago. Both sharks and rays are chondrichthyes, which means they have soft bones. You can easily tell them apart by looking at their gill openings. The ones with openings on their sides are sharks, and those with openings on their abdomen are rays.
