The description is the same as for Poseidonia gen. nov. with the following additions. Poseidoniaceae fam. nov. was previously known as and “Group II-∝” and “Marine Group IIa”. Eleven genus-level clades I, J1, J2, J3, K1, K2, L1, L2, L3, L4, and Poseidonia (M) representing the rank of genera were assigned to this family. All members of the family were inferred to be marine, motile heterotrophs, encoding phototrophic and non-phototrophic lifestyles with a distribution from surface waters to the deep sea (1m-500m). Type genus: Poseidonia gen. nov.