nosoop4u246
New member
That's actually a surprisingly complex question. Freshwater fish are hypertonic to their environment, so they don't really urinate. They can flush out their solutes, including waste solutes like ammonia, simply by diffusion. The liquid waste exits the fish in the area where there is the greatest rate of diffusion-- the gills. So, as one user put it, fish basically pee out their gills.
Saltwater fish, on the other hand, are hypotonic to their environment, so they do everything possible to conserve freshwater (if they didn't, they'd dehydrate due to the salt). There is no free expulsion of solutes via diffusion due to all the salt in the water, so saltwater fish actually do urinate from the same oraface as their anus. To conserve water, the kidneys produce a highly saline urine composed of metabolic wastes (ammonia), salt, and very little water.
Saltwater fish, on the other hand, are hypotonic to their environment, so they do everything possible to conserve freshwater (if they didn't, they'd dehydrate due to the salt). There is no free expulsion of solutes via diffusion due to all the salt in the water, so saltwater fish actually do urinate from the same oraface as their anus. To conserve water, the kidneys produce a highly saline urine composed of metabolic wastes (ammonia), salt, and very little water.