Cell types diversity of H4 haplotype Placozoa sp.

Placozoa is one of the five basal metazoan lineages critical for our understanding of animal evolution in general, and the origin of neuromuscular organization in particular. All Placozoa have the simplest known animal body plan, without neurons and muscles, but relatively complex behaviors. Totally 19 haplotypes of placozoans have been identified including two genera, Trichoplax adhaerens (H1) and Hoilungia hongkongensis (H13), plus a number of less characterized ecological groups also known as H2–H19 Placozoa sp. However, microscopic anatomy had been characterized for H1 (Trichoplax adhaerens) only. Here, using scanning and confocal microscopy, we described morphological organization of H4, the haplotype similar to Hoilungia. All six basal morphological cell types have been found in H4. Nevertheless, we recognized a greater heterogeneity of cell populations (in their shape, density, mitochondrial distribution and activity) than described elsewhere for H1. This study expands our knowledge about the biology of these enigmatic groups of marine organisms.