Back to the Basics: Cnidarians Start to Fire

Abstract

The nervous systems of cnidarians, pre-bilaterian animals that diverged close to the base of the metazoan radiation, are structurally simple and thus have great potential to inform us about basic structural and functional principles of neural circuits. Unfortunately, cnidarians have thus far been relatively intractable to electrophysiological and genetic techniques and consequently have been largely passed over by neurobiologists. However, recent advances in molecular and imaging methods are fueling a renaissance of interest in and research into cnidarians nervous systems. Here, we review current knowledge on the nervous systems of some cnidarian species and propose that researchers should seize this opportunity and undertake the study of this phylum as strategic experimental systems with great basic and translational relevance for neuroscience.

The Jellyfish Cassiopea Exhibits a Sleep-like State

“…To our knowledge, our finding is the first example of a sleep-like state in an organism with a diffuse nerve net [7,8], suggesting that this behavioral state arose prior to the evolution of a centralized nervous system. Though at least 600 million years of evolution separate cnidarians from bilaterians [9–16], many aspects of the nervous system are conserved, including neuropeptides and neurotransmitters [15–20]. One such conserved molecule, melatonin [32], promotes sleep in diurnal vertebrates, including zebrafish [33] and humans [34], and induces quiescence in invertebrates [35].…”

“…Here we show that sleep is also present in Cnidaria [6–8], an earlier branching metazoan lineage. Cnidaria, along with Ctenophora, are the first metazoan phyla to evolve tissue-level organization and differentiated cell types, such as neurons and muscle [9–15]. In Cnidaria, neurons are organized into a non-centralized radially symmetric nerve net [11,13,15–17] that nevertheless shares fundamental properties with the vertebrate nervous system: action potentials, synaptic transmission, neuropeptides, and neurotransmitters [15–20].…”

“…Cnidaria, along with Ctenophora, are the first metazoan phyla to evolve tissue-level organization and differentiated cell types, such as neurons and muscle [9–15]. In Cnidaria, neurons are organized into a non-centralized radially symmetric nerve net [11,13,15–17] that nevertheless shares fundamental properties with the vertebrate nervous system: action potentials, synaptic transmission, neuropeptides, and neurotransmitters [15–20]. It was reported that cnidarian soft corals [21] and box jellyfish [22,23] exhibit periods of quiescence, a pre-requisite for sleep-like states, prompting us to ask if sleep is present in Cnidaria.…”

Rethinking the Role of the Nervous System: Lessons From the Hydra Holobiont

“…[11,12] In spite of this morphological simplicity, Hydra demonstrates a surprisingly rich behavioural repertoire: spontaneous periodic contractions and extension of the body and tentacles, contractions in response to mechanical stimuli and light, complex feeding behavior and somersaulting locomotion (refs. [17]). [17]).…”

“…All these behaviors are under the control of the nervous system, as evidenced by experiments on neuron-ablated polyps. [17]). However, the mechanisms behind these non-conventional functions remain largely unclear.…”