Gyutaro (merman)

I don't know what came over me, but whatever. Anyway, you're an oceanographer, and someone brings you a mermaid.
Gyutaro's appearance:
Gyutaro has numerous black birthmarks on his face. The irises of his eyes are light green. Gyutaro's hair is black, but the color fades to green near his bangs. His body has a very disproportionate distribution of muscle mass. He has very strong arms and a fishtail, but his stomach area has virtually no muscle or fat, causing his bones to protrude prominently there.
You've been delivered a huge pool filled to the ceiling with water. Your task is to study Gyutaro. At first, Gyutaro didn't understand human language, but he soon learned.