The most primitive metazoan animals, the placozoans, show high sensitivity to increasing ocean temperatures and acidities

Abstract

The increase in atmospheric carbon dioxide (CO₂) leads to rising temperatures and acidification in the oceans, which directly or indirectly affects all marine organisms, from bacteria to animals. We here ask whether the simplest-and possibly also the oldest-metazoan animals, the placozoans, are particularly sensitive to ocean warming and acidification. Placozoans are found in all warm and temperate oceans and are soft-bodied, microscopic invertebrates lacking any calcified structures, organs, or symmetry. We here show that placozoans respond highly sensitive to temperature and acidity stress. The data reveal differential responses in different placozoan lineages and encourage efforts to develop placozoans as a potential biomarker system.

Keywords: biomarkers; evolutionary constraints; global warming; ocean acidification; placozoa.

Figure 1

The experimental setup for the temperature experiment. 1—Aquarium filled with artificial seawater, 2—heater, 3—glass bowls turned over, 4—covered Petri dishes with the experimental animals placed on the glass bowls, 5—surface line of artificial seawater

Figure 2

The experimental setup for the pH experiment. 1—Aquarium filled with artificial seawater, 2—heater, 3—glass bowls turned over, 4—covered Petri dishes with the experimental animals placed on the glass bowls, 5—surface line of artificial seawater, 6—CO ₂‐reactor, and 7—pH meter

Figure 3

Population growth rate (PGR) at different temperatures for the three placozoan lineages (a) H1_gre, (b) H2_ros, and (c) H2_pan

Figure 4

Population growth rate (PGR) at the two different pH levels for the lineages (a) H1_gre, (b) H2_ros, and (c) H2_pan

Figure 5

Changes in morphology of Trichoplax adhaerens under acidity stress. (a) Unusually enlarged specimens, (b) extremely long specimen, (c) normal to very long shaped specimens, (d) very tiny, round‐shaped specimens