Abstract
Placozoa are small disc-shaped animals, representing the simplest known, possibly ancestral, organization of free-living animals. With only six morphological distinct cell types, without any recognized neurons or muscle, placozoans exhibit fast effector reactions and complex behaviors. However, little is known about electrogenic mechanisms in these animals. Here, we showed the presence of rapid action potentials in four species of placozoans (Trichoplax adhaerens [H1 haplotype], Trichoplax sp.[H2], Hoilungia hongkongensis [H13], and Hoilungia sp. [H4]). These action potentials are sodium-dependent and can be inducible. The molecular analysis suggests the presence of 5-7 different types of voltage-gated sodium channels, which showed substantial evolutionary radiation compared to many other metazoans. Such unexpected diversity of sodium channels in early-branched animal lineages reflect both duplication events and parallel evolution of unique behavioral integration in these nerveless animals.
Highlights
Placozoans are the simplest known animals without recognized neurons and muscles
With only six morphological cell types, placozoans showed complex & rapid behaviors
Sodium-dependent action potentials have been discovered in intact animals
Voltage-gated sodium channels (Nav) in Placozoa support a rapid behavioral integration
Placozoans have more Nav channels that any studied invertebrate animal so far
Diversification of Nav-channels highlight the unique evolution of these nerveless animals
Competing Interest Statement
The authors have declared no competing interest.