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

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.

A sleeping paradox may extend to the spider

“…Before I summarize the exhilarating findings of Rößler et al. ( 5 ) and consider the provocative implications, it is important to note what this study does not show. With an absence of tests for sleep behavior or physiology, the authors make it clear that they are not definitively demonstrating sleep in their subjects.…”

“…There is no reason to think that jumping spiders do not sleep, so if future studies confirm that the stereotyped behavior reported by Rößler et al. ( 5 ) is sleep and these REM-like behaviors occur when the spiders are less responsive, an exploration of REM sleep in a species far removed from our usual vertebrate subjects could offer great potential for considering the evolutionary origins and functional nature of this paradoxical state.…”

“…As you can easily see in the movies in the work by Rößler et al. ( 5 ), the retinal tubes shift, dart, and jerk about as the body dangles relatively motionless and upside down from a silk thread. At the same time, the spider’s legs are curled, and parts of the body occasionally twitch.…”

“…Rößler et al. ( 5 ) have boldly challenged us to consider a very distant relative of the other REM sleepers by cleverly analyzing the movements of retinal tubes visible through the translucent cuticle of juvenile jumping spiders.…”

Laboratory rearing alters activity and sleep patterns in the olive fruit fly

“…Sleep and sleep-like states are present in all animals and many studies have shown sleep-like states exist even in arthropods and nematodes [11]. The circadian system is responsible for controlling daily activity patterns, such as locomotion, mating, and also sleep [12,13].…”