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Mammals and birds experience REM sleep -- and apparently dream. I gather that most cold-blooded animals do not, and I presume that fish also do not. Does anyone have any further information on this?

Dinyar

From what I can find, fish do not undergo REM sleep, only behavioral rest/sleep. The best explanation I could find in journal articles on my university's library website was that ectotherms (fish, reptiles, amphibians, etc.) have different nerve structures in their brains and do not show evidence of REM activity like endotherms (mammals, etc). I'll let you know if I can find a more specific explanation.

Maybe fish only daydream instead....

Some fish, particularly pelagic schooling species like tuna, don't appear to sleep (in the sense that we know).

Apparently, marine fish that sleep do so more soundly than freshwater fish, and it is hypothesized that the lability of the freshwater environment decreases the advantage of sleep (in the sense of resetting a biological clock).

Silurus wrote:[...] it is hypothesized that the lability of the freshwater environment decreases the advantage of sleep (in the sense of resetting a biological clock).

First, why should lability (changeability) decrease the advantage of sleep for FW fish? In endotherms, learning (an adaptation to environmental lability) is facilitated by sleep.

Second, what do you mean by "sleep (in the sense of resetting a biological clock)"?

Maybe we should get Achim in on this discussion. He's a circadian rhythm researcher, is he not?

First, why should lability (changeability) decrease the advantage of sleep?

In the sense of obtaining food, vulnerability to predators, etc. according to the review in Reebs (2003):

Reebs, SG, 2003. Plasticity of diel and circadian activity rhythms in fishes. Reviews in Fish Biology and Fisheries, 12 [2002]: 349-371.

This is not a field I am competent (or particularly interested) in, so I reserve further comment.

there are plenty freshwaterfishes that "sleep" as soundly as any Reef-inhabitant....
check out Discus or Angelfish for example. at night you can handpick them right out of the water!
the same goes for the Sandcichlids of Tanganyika.

most cichlidae (and AFAIK many Characins, Cyprinids and Cyprinodonts as well) are completely torpid at night.

so I don't think it's a freshwater versus marine issue, but something else.....

Yes they do, mostly of Denise Richards and Sandra Bullock

Denise I'm not familiar with, but heck, even I dream about Sandra Bullock!

does it mean I'm a fish?
or something else?