Cyclical alternation between quiet and active sleep states in octopuses observed under controlled laboratory conditions and in the natural environment

Abstract

The alternation of long episodes of quiet sleep interrupted rhythmically by short episodes of active sleep, a pattern once thought to be exclusive to amniotes, has recently been reported in some invertebrates, such as Drosophila (Insecta), jumping spiders (Arachnida), and cuttlefish (Cephalopoda). However, studies indicating the existence of a cyclic pattern with distinct sleep phases in invertebrates have been conducted only in captive animals, under laboratory conditions. This leaves open questions about the natural sleep cycle of invertebrates and how it is affected by environmental factors. To investigate in detail the behavioral structure of sleep in octopuses, we recorded uninterrupted videos in the laboratory for 12 hours over 4 consecutive days of four adult specimens of Octopus insularis, a tropical reef octopus from Brazil. The animals were captured in Pirangi (6°0’14.29”S, 35° 6’21.01”W) and kept in tanks measuring 1.0 x 0.7 x 0.6 meters in the city of Natal, RN, Brazil. After acclimation, we quantified several variables across the different behavioral states and during the transitions between them. Changes in skin color and texture, as well as eye and mantle movements, were assessed using mathematical tools for image processing, developed for this purpose. To measure the arousal threshold in each state, the latency of the behavioral response to sensory stimuli was measured. Two distinct behavioral states with lack of reactivity to stimulation were identified. The first was a quiet sleep state characterized by uniformly pale skin, closed pupils, and long episode durations (median 415.2 s). The second was an active sleep state characterized by dynamic patterns of skin color and texture, rapid eye movements, and short episode durations (median 40.8 s). The active sleep episodes were markedly periodic (60% of recurrences between 26-39 min) and occurred mainly after quiet sleep episodes (82% of transitions). These results show that O. insularis exhibits an ultradian sleep cycle analogous to that of amniotes. To compare these results with the sleep patterns of animals in their natural environment, we established a collaboration to analyze existing data on the behavior of Octopus insularis, recorded in a minimally invasive way using remote cameras placed at den entrances in the Turks and Caicos Islands (21.5112°N, 71.5190°W). Ten adult specimens were recorded during the day and night (total of 240 h). These videos were analyzed using the same behavioral categories found for laboratory animals. All octopuses exhibited a sleep cycle pattern similar to that observed in the laboratory, with a periodic alternation of quiet sleep episodes (median 643.2 s) and active sleep (median 37.8 s), with quiet sleep episodes preceding active sleep episodes 98% of the time. We also observed that the animals slept for 44% of the sampled time, demonstrating that sleep occupies a substantial fraction of the life span in O. insularis. The documentation and quantification of the cyclic sleep in specimens of O. insularis, investigated both in the laboratory and in the natural environment, helps to understand the selective pressures that drove the evolution of the sleep cycle in this species and in cephalopods.