Biology · Homeostasis

Marine teleosts are hypoosmotic to seawater, so they face osmotic loss of water and diffusional gain of NaCl across the gill. 

Myxine, or hagfish, are isotonic to seawater, meaning their internal osmotic pressure matches the external environment, so they do not need to actively regulate their osmotic balance.

Skates maintain an isotonic condition with seawater through the retention of urea, which balances osmotic pressures without needing significant regulatory mechanisms.

Sharks are isotonic to seawater due to the presence of urea and trimethylamine N-oxide in their blood, aligning their internal osmotic conditions with the surrounding environment.

Teleosts are not isotonic to seawater; they are hypotonic. They lose water osmotically and face a constant influx of salts, requiring active osmoregulation to maintain homeostasis.