How do marine sharks osmoregulate?
Internal salt concentration less than seawater, so salt tends to diffuse in. However, not hypoosmotic to water. Contains urea in its tissues. Also contains trimethylamine oxide, which protects proteins from damage from urea. This combination of salts, urea, and TMAO results in similar osmolarity to seawater which is why they are considered osmoconformers. However, they are actually slightly hyperosmotic to seawater, which results in water flowing in (not too much). Also sharks get water from food. Sharks remove this water by urine, which also removes some salts.