Expanding of Life Strategies in Placozoa: Insights From Long-Term Culturing of Trichoplax and Hoilungia.

Placozoans are essential reference species for understanding the origins and evolution of animal organization. However, little is known about their life strategies in natural habitats. Here, by maintaining long-term culturing for four species of Trichoplax and Hoilungia, we extend our knowledge about feeding and reproductive adaptations relevant to the diversity of life forms and immune mechanisms. Three modes of population dynamics depended upon feeding sources, including induction of social behaviors, morphogenesis, and reproductive strategies. In addition to fission, representatives of all species produced "swarmers" (a separate vegetative reproduction stage), which could also be formed from the lower epithelium with greater cell-type diversity. We monitored the formation of specialized spheroid structures from the upper cell layer in aging culture. These "spheres" could be transformed into juvenile animals under favorable conditions. We hypothesize that spheroid structures represent a component of the innate immune defense response with the involvement of fiber cells. Finally, we showed that regeneration could be a part of the adaptive reproductive strategies in placozoans and a unique experimental model for regenerative biology.

Description of a metacercaria of a zoogonid trematode Steganoderma cf. eamiqtrema Blend and Racz, 2020 (Microphalloidea: Zoogonidae), with notes on the phylogenetic position of the genus Steganoderma Stafford, 1904, and resurrection of the subfamily Lecithostaphylinae Odhner, 1911.

Metacercariae of the zoogonid trematode Steganoderma cf. eamiqtrema ex crab Chionoecetes bairdi caught in the Sea of Okhotsk were described using morphological and molecular-genetic (ITS2 region, 28S rRNA and nd1 genes) data. These are the first molecular-genetic data for the genus Steganoderma. The studied trematodes differed from S. eamiqtrema in having a much larger body size. The phylogenetic analysis based on the 28S rRNA gene supported neither the current taxonomic hypothesis that Steganoderma belongs to the subfamily Lepidophyllinae nor the earlier views that the Steganodermatinae and the Lecithostaphylinae are synonymous. The topology of the phylogenetic tree shows that the Steganodermatinae and the Lecithostaphylinae are independent subfamilies. However, morphological differences between them are obscure. Until morphological evidence for the Steganodermatinae is found, we propose to distinguish the subfamily Lepidophyllinae sensu stricto with the genera Lepidophyllus and Urinatrema, and the subfamily Lecithostaphylinae sensu lato uniting all the other former lepidophyllines. Thus, for now, we propose to consider the Steganodermatinae as a conditional synonym for Lecithostaphylinae sensu lato.