[dv_dt]

This begs the question how do fish get water in seawater environments.

[jacobolus]

Another 1 minute web search turns up https://en.wikipedia.org/wiki/Salt_gland https://en.wikipedia.org/wiki/Osmoregulation https://en.wikipedia.org/wiki/Euryhaline

[dv_dt]

By inference something along those lines had to exist somewhere along the ocean food chain...