Do fish feel thirsty and piss?

[Semon]

1. do fish feel thirsty?
2. do fish piss?

 

[Giambattista]

They probably don't think about it all that much. Why do you ask?

 

[Hercules Rockefeller]

do fish feel thirsty?

All organisms require water. For fish, obtaining water is relatively easy as they live in it. They simply swallow a mouthful of water every now and again. Marine fish excrete the salt in the sea water through their kidneys and urine (see next question). Whether fish feel “thirsty”, as such, is a rather anthropomorphic question. I doubt that fish are ‘aware’ of their constant water uptake.

do fish piss?

Yes. Fish have kidneys that remove metabolic waste from the blood and excrete it in urine. Unlike humans who have separate excretion systems for urine and feces, many (most?, all?) fish have a cloaca. A cloaca is a common opening for the digestive and urinary systems. Urine from the kidney tubules and fecal matter from the intestine empty into the cloaca and are excreted together out through the anus. Same with birds.<P>

 

[Idle Mind]

I'm not sure all fish use the cloaca for urinary excretion. I believe at least some fish excrete urine through the gill slits. My reasoning for this is that there is a parasitic fish (candiru - Vandellia cirrhosa) that uses urea from fish urine to find a fish and its gills, and feeds off blood for a few minutes. At least, it is thought it uses urea and ammonia (which is what I was taught in my Parasitology course) but isn't confirmed. I'm not sure how host-specific the parasite is, however, and it may be a particular fish that urinates out its gills.

Although, I am aware that you Hercules have worked with D. rerio, and are probably comfortable with their anatomy and general anatomy of fish. I should also state that I did no research on the matter before posting and am very comfortable with being corrected.

 

[Hercules Rockefeller]

I believe at least some fish excrete urine through the gill slits.

Thanks. Yes, you are quite right. Fish can eliminate ammonia and urea through their gills (branchial epithelium). I think it is via passive diffusion, although I seem to recall the existence of urea transporters which implies it is also by active transport. However, this is removal directly from the blood rather than from ‘urine’. As such, I wouldn’t describe it as "urinating" through their gills.<P>