Sea Turtle
noun
A sea turtle is a sea turtle. Cowabunga!
The reading is the kun'yomi readings of these kanji, put together. I suppose it's because it's like two separate words put together into one. That makes this word read as うみがめ, where the かめ gets rendaku'd.
その種の海亀は、絶滅の危機にあります。
That kind of sea turtle is in danger of extinction.