What do spiders dream about?

Amazing stuff! What little we still know about sleep and dreaming! This new research raises interesting questions about meaning of dreaming. What do death and dreaming have in common?

Even newly born spider babies already dream.

"... The team ... of researchers analyzed videos of the sleeping arachnids and found they exhibited a rapid-eye movement (REM) dream-like state, which they could directly observe because juvenile spiders have translucent exoskeletons. The researchers also documented limb movements characteristic of dreaming, including leg twitching and curling. The study is believed to be the first time REM sleep-like behavior has been documented in a terrestrial invertebrate. ...
It makes me wonder if creatures that aren’t visual also dream. Jumping spiders are really visual, but there are lots of spiders that aren’t visual but have very similar brains. It makes me wonder if dreaming happens in a lot of different ways. ...
What is characteristic for REM sleep is not only the eye movements but also the muscle atonia during that time, which is basically motor function being greatly diminished so someone doesn’t run off while they dream. ... In spiders, the whole system is based on hydraulics. They do not use muscles to extend their legs, but they use muscles just in the head to basically push fluid into the legs to extend them. When muscle atonia happens, what we think is happening is that the muscles in the head relax and that leads the legs to decrease in pressure, and so they curl up, basically like a dead spider. (All dead spiders have their legs curled up.) For us, this muscle atonia proves they are not awake and suggests the body is trying to suppress motion. ..."

From the abstract:

"Sleep and sleep-like states are present across the animal kingdom, with recent studies convincingly demonstrating sleep-like states in arthropods, nematodes, and even cnidarians. However, the existence of different sleep phases across taxa is as yet unclear. In particular, the study of rapid eye movement (REM) sleep is still largely centered on terrestrial vertebrates, particularly mammals and birds. The most salient indicator of REM sleep is the movement of eyes during this phase. Movable eyes, however, have evolved only in a limited number of lineages—an adaptation notably absent in insects and most terrestrial arthropods—restricting cross-species comparisons. Jumping spiders, however, possess movable retinal tubes to redirect gaze, and in newly emerged spiderlings, these movements can be directly observed through their temporarily translucent exoskeleton. Here, we report evidence for an REM sleep–like state in a terrestrial invertebrate: periodic bouts of retinal movements coupled with limb twitching and stereotyped leg curling behaviors during nocturnal resting in a jumping spider. Observed retinal movement bouts were consistent, including regular durations and intervals, with both increasing over the course of the night. That these characteristic REM sleep–like behaviors exist in a highly visual, long-diverged lineage further challenges our understanding of this sleep state. Comparisons across such long-diverged lineages likely hold important questions and answers about the visual brain as well as the origin, evolution, and function of REM sleep."

Harvard researchers find REM sleep in jumping spiders – Harvard Gazette New study documents REM-like state in spiders, raises questions about function of nighttime activity

Regularly occurring bouts of retinal movements suggest an REM sleep–like state in jumping spiders (open access)