MO2, behaviour data and supplementary material for the first physiological evidence of sleep in sharks

Sharks represent the earliest group of jawed vertebrates and as such, they may provide original insight for understanding the evolution of sleep in more derived animals. Unfortunately, beyond a single behavioural investigation, very little is known about sleep in these ancient predators. As such, recordings of physiological indicators of sleep in sharks have never been reported. Reduced energy expenditure arising from sustained restfulness and lowered metabolic rate during sleep have given rise to the hypothesis that sleep plays an important role for energy conservation. To determine whether this idea applies also to sharks, we compared metabolic rates of draughtsboard sharks (Cephaloscyllium isabellum) during periods ostensibly thought to be sleep, along with restful and actively swimming sharks across a 24-h period. We also investigated behaviours that often characterise sleep in other animals, including eye closure and postural recumbency, to establish relationships between physiology and behaviour. Overall, lower metabolic rate and a flat body posture reflect sleep in draughtsboard sharks, whereas eye closure is a poorer indication of sleep. Our results support the idea for the conservation of energy as a function of sleep in these basal vertebrates.

鲨鱼是现存最古老的有颌脊椎动物类群，因此可为解析演化更高级动物的睡眠起源提供独特视角。遗憾的是，除了一项单一的行为学研究外，学界对这类古老捕食者的睡眠认知仍极为匮乏。因此，迄今尚无关于鲨鱼睡眠相关生理指标记录的相关报道。睡眠期间持续静息所带来的能量消耗减少与代谢率降低，催生了“睡眠在能量守恒中发挥重要作用”这一假说。为验证该假说是否同样适用于鲨鱼，本研究在24小时周期内，对比了斑纹绒毛鲨（Cephaloscyllium isabellum）在疑似睡眠时段、静息状态以及主动游泳状态下的代谢率。此外，我们还观测了其他动物睡眠中常见的特征行为，包括眼部闭合与躯体俯卧姿态，以厘清生理指标与行为之间的关联。总体而言，较低的代谢率与平直的躯体姿态可作为斑纹绒毛鲨睡眠的可靠标志，而眼部闭合则并非睡眠的良好指示指标。本研究结果证实，睡眠的能量守恒功能同样适用于这类基底脊椎动物。