Long-term dynamics of placozoan culture: emerging models for population and space biology

Abstract

As the simplest free-living animal, Trichoplax adhaerens (Placozoa) is emerging as a powerful paradigm to decipher molecular and cellular bases of behavior, enabling integrative studies at all levels of biological organization in the context of metazoan evolution and parallel origins of neural organization. However, the progress in this direction also depends on the ability to maintain a long-term culture of placozoans. Here, we report the dynamic of Trichoplax cultures over 11 years of observations from a starting clonal line, including 7 years of culturing under antibiotic (ampicillin) treatment. This study revealed very complex population dynamics, with seasonal oscillation and at least partial correlations with the solar radio emission flux and the magnetic field disturbance parameters. Notable, the analysis of the distribution of Fe²⁺ in living animals revealed not only its high abundance across most cells but also asymmetric localizations of Fe²⁺ in unidentified cells, suggesting that these Fe²⁺ intracellular patterns might be coupled with the animal's bioenergetics. We hypothesize that placozoans might have magnetoreception, which can be experimentally tested in future studies. In sum, Trichoplax, in particular, and Placozoa, in general, can be viewed as prospective reference species in traditional evolutionary and system biology but have the yet unexplored potential for planetary ecology and space biomedicine.

Keywords: Placozoa; Trichoplax; aging of culture; behavior; long-term culturing; space biology.

FIGURE 1

11 years of long-term culturing of placozoans. (A) Photo of Trichoplax adhaerens, H1 haplotype (B) Mode 1: stable population growth rate under culture condition. (C) The number of Petri dishes (earch dot represents one dish, colors mark different years) as used in the study. (D) Oscillations of the population growth (meant +/− standard errors) under three exparimental conditions (control/CCL, ampicillin/ACL, and doxycycline/DCL). See details in Supplementary Table S1. (B) modified from Romanova et al. (2022).

FIGURE 2

Annual and monthly population dynamics during long-term cultivation of Trichoplax adhaerens. (A) Long-term culturing from 2014 to 2024 (up to December). Oscillations of the population growth (meant +/− standard errors); continuous error bands are a graphical representation of error or uncertainty shown as a shaded region around a main trace (upper purple line in the gradient graph (the same in in Figure 1D) in gradient graph). (B) Dynamic of culturing during January-February 2024. On the graph: all colors–sequential Petri dishes, horizontal line for every dish is mean values of animals’ count (mean +/− standard error).

FIGURE 3

Decade of oscillation in population growth rate (PGR) (control group). (A) Long-term culturing of control clonal individuals for 11 years (dot represent numbers of animal per a dish). (B) Dynamic of PGR for normal clonal live and antibiotic treatment groups with ap and F10.7 indices (blue and red lines respectively). (C) Pearson scatterplot for the observable solar F10.7 index versus numbers of placozoans. (D) Pearson scatterplot for the ap index versus numbers of placozoans. (E) Numbers of animals in the individual Petri dishes during 2024 overlaped with data of ap index for the current period.

FIGURE 4

Fe²⁺-selective staining in Trichoplax. (A) Top view Red: HMRhoNox-M, blue–DAPI. (B) Focus on upper epithelia with large inclusions, and damaged rim [high magnification, see (D)]. (C) Middle layer focus with microcavities. (D) Rim area: epithelial cells with fluorescent signal inside. (E) The part of damaged rim epithelium: some cells show assymetric distribution of fluorescent signal location. White arrows point to areas of inclusions presumably enriched in Fe²⁺. Scale bar: (A)-100 µm, (B)-20 µm, (C)-50 µm, (D, E) – 10 µm.