noun
dorsal fin
Refers to the fin on the back of fish and some marine mammals.
サメの背鰭が水面から出ている。
The shark's dorsal fin is sticking out of the water.
この魚は背びれが長くて美しい。
This fish has a long, beautiful dorsal fin.
Compound of 背 (se, 'back') and 鰭 (hire, 'fin'), with rendaku voicing of hire to bire.