
He is biting her tail :D
This is a male Zebra Shark 'biting' the tail of a female Zebra Shark as part of their courting rituals. This can go on for hours, each time when he bites/chews on her tail, she goes into a tonic immobility trance, and she might lay down upside down exposing her belly.

The zebra shark is a species of carpet shark and the sole member of the family Stegostomatidae. It is found throughout the tropical Indo-Pacific, frequenting coral reefs and sandy flats to a depth of 62 m.
Similar species: Carpet Sharks
By Albert Kang
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Uploaded Mar 31, 2017. Captured Mar 10, 2017 17:51 in Dhevvadhoo Rasgefaanu Magu, Dhevvadhoo, Maldives.