Scientists Discover New Placozoan Habitat in an Unexpected Place

Placozoan species Hoilungia hongkongensis. Credit: Hans-Jürgen Osigus, Stiftung Tierärztliche Hochschule Hannover

DNA traces in the stomachs of predatory snails have provided new ecological insights into placozoans, according to research led by LMU geobiologist Gert Wörheide.

Placozoans, some of the simplest animals in the world, are found in coastal waters globally. These tiny organisms, only a few millimeters in size, were once thought to inhabit either solid surfaces—like rocks, corals, and mangrove roots—or to drift in open coastal waters in their “swarmer” stages.

Through analysis of DNA traces in the stomachs of predatory sea slugs, a team led by LMU geobiologist Professor Gert Wörheide has demonstrated that the animals also live in the seabed sediment, a habitat they were previously thought not to colonize. In addition, they are more genetically diverse than had been known, as the researchers report in the journal Ecology and Evolution.

With their flat, disk-shaped bodies, all placozoans worldwide look strikingly similar. Nevertheless, Wörheide and his team have already demonstrated in previous studies that there are huge genetic differences between them. “These differences are comparable with those between humans and mice,” emphasizes the geobiologist.

Due to their diminutive size and inconspicuousness, placozoans are challenging to study in their natural environments. To gain a better insight into the ecology of the animals, the researchers exploited the fact that small shell-less sea slugs from the Rhodopidae family feed on placozoans.

Prof. Gert Wörheide investigated publicly accessible genetic data for snails for traces of placozoan DNA. Credit: LMU

Among the undigested meals of sea snails

“We hoped we could find undigested remains of placozoans in the stomach contents of the snails, which we could then perform molecular analyses on,” recounts Dr. Michael Eitel, lead author of the study. “To this end, we bioinformatically investigated publicly accessible genetic data for the snails for traces of placozoan DNA.”

To the researchers’ surprise, they also identified the DNA of placozoans in the stomachs of snails that live exclusively in seabed sediments – a habitat that all experts had previously ruled out for the very fragile placozoans. “Clearly, however, their presence in sediments is a normal occurrence and could even play a key role in their biology, particularly in their sexual reproduction, about which we have only rudimentary knowledge,” says Eitel.

Furthermore, the scientists discovered an unexpectedly large genetic diversity. In the stomach contents of just two snails, they found five genetically different lineages, of which three had never been described before. The researchers believe that the diversity of placozoans is much greater than previously assumed. “Our results will have a big impact on our picture of the developmental history of one of the oldest phyla on Earth,” says Wörheide. “At the same time, the major new habitat discovery literally adds another dimension to the ecology of placozoans.”

Reference: “Beauty in the beast – Placozoan biodiversity explored through molluscan predator genomics” by Michael Eitel, Hans-Jürgen Osigus, Bastian Brenzinger and Gert Wörheide, 11 April 2024, Ecology and Evolution.
DOI: 10.1002/ece3.11220