High cell diversity and complex peptidergic signalling underlie placozoan behaviour
Placozoans, together with sponges, are the only animals devoid of a nervous system and muscles, yet both respond to sensory stimulation in a coordinated manner. How behavioural control in these free-living animals is achieved in the absence of neurons and, more fundamentally, how the first neurons evolved from more primitive communication cells during the rise of animals is not yet understood [1-5]. The placozoan Trichoplax adhaerens is a millimeter-wide, flat, free-living marine animal composed of six morphologically identified cell types distributed across a simple bodyplan [6-9]: a flat upper epithelium and a cylindrical lower epithelium interspersed with a loose layer of fiber cells. Its genome encodes several proneuropeptide genes and genes involved in neurosecretion in animals with a nervous system [10-12]. Here we investigate neuropeptide signalling in Trichoplax adhaerens. We found specific expression of several neuropeptides in non-overlapping cell populations distributed over the three cell layers, revealing an unsuspected cell-type diversity of Trichoplax adhaerens. Using live imaging, we uncovered that treatments with 11 different neuropeptides elicited striking and consistent effects on the animals shape, patterns of movement and velocity that we categorized under three main types: (i) crinkling, (ii) turning, and (iii) flattening and churning. Together, the data demonstrate a crucial role for peptidergic signalling in nerveless placozoans and suggest that peptidergic volume signalling may have predated synaptic signalling in the evolution of nervous systems.