Wake-like skin patterning and neural activity during octopus sleep.

While sleeping, many vertebrate groups alternate between at least two sleep stages: rapid eye movement and slow wave sleep1-4, in part characterized by wake-like and synchronous brain activity, respectively. Here we delineate neural and behavioural correlates of two stages of sleep in octopuses, marine invertebrates that evolutionarily diverged from vertebrates roughly 550 million years ago (ref. 5) and have independently evolved large brains and behavioural sophistication. 'Quiet' sleep in octopuses is rhythmically interrupted by approximately 60-s bouts of pronounced body movements and rapid changes in skin patterning and texture6. We show that these bouts are homeostatically regulated, rapidly reversible and come with increased arousal threshold, representing a distinct 'active' sleep stage. Computational analysis of active sleep skin patterning reveals diverse dynamics through a set of patterns conserved across octopuses and strongly resembling those seen while awake. High-density electrophysiological recordings from the central brain reveal that the local field potential (LFP) activity during active sleep resembles that of waking. LFP activity differs across brain regions, with the strongest activity during active sleep seen in the superior frontal and vertical lobes, anatomically connected regions associated with learning and memory function7-10. During quiet sleep, these regions are relatively silent but generate LFP oscillations resembling mammalian sleep spindles11,12 in frequency and duration. The range of similarities with vertebrates indicates that aspects of two-stage sleep in octopuses may represent convergent features of complex cognition.

Sensory cells associated with the tentacular tunic of the ascidian Polyandrocarpa misakiensis (Tunicata: Ascidiacea).

New probable mechanosensory cell groups were found in Polyandrocarpa misakiensis. In this species, the tunic with epithelium penetrates into the oral and atrial tentacles (oral and atrial tentacular tunic), which is continuous with a tunic layer intervening between the descending and ascending epithelium of the siphons. In the oral tentacles, the tunic only extends into the basal part, but in the atrial siphon the tunic extends the full length of the tentacle. Intraepithelial sensory cells were found in the basal part of the oral and atrial tentacular tunic. These sensory cells are usually solitary or paired in the atrial tentacles, and a few cells are grouped together in the oral tentacles. In the oral tentacles, the apicolateral parts of the sensory cells are joined together by two types of junctions, i.e., adherent junctions and modified tight junctions. The supporting cells and sensory cells are connected by adherent junctions. Certain sensory cells are coupled to what seem to be neurosecretory cells. The sensory cells have one apical cilium surrounded by microvilli and a basal axonal process. The apical cytoplasm of the sensory cells has a few organelles. The basal cytoplasm of the sensory cells is dense with rough endoplasmic reticulum, mitochondria, lipid droplets, and dense bodies. The morphology of these sensory cells is comparable with that of the ciliated intraepithelial sensory neurons found in many tunicates, such as those of cupular organ and capusular organ.