by Piter Kehoma Boll
A long time ago I talked about a polemic group of bilaterian animals, the acoelomorphs, whose exact phylogenetic position is uncertain but the evidence is starting to build up toward the idea that they are part of the first branch of bilaterians to diverge from the others, so that they are neither protostome nor deuterostomes. No acoelomorph has been presented as a Friday Fellow before, so today it is time for their debut. The species I selected is a very interesting one. Its name is Waminoa litus and, of course, it lacks a common name, so I decided to call it the Pacific waminoa because it is found in the Pacific, especially around Australia.
A marine species, the Pacific waminoa has the typical anatomy of the acoels, the subgroup of acoelomorphs to which it belongs. It lacks, for example, a gut, and its mouth opens directly into the internal tissues. The mouth is located at the venter, like in many flatworms, such as planarians. In fact, acoelomorphs were for a long time thought to be flatworms until molecular analyses revealed that they are not.
With a flat and circular, slightly heart-shaped body, the Pacific waminoa lives on the surface of corals. They are, in fact, kind of an ectoparasite to corals, as they feed on the coral’s mucus. But more than that, the Pacific waminoa also harbors symbiotic dinoflagellates (zooxanthellae) living directly inside its parenchyma, the tissue that fills their body. Zooxanthellae are famous as common symbiotic organisms living inside corals, and they share part of the nutrients they make via photosynthesis with their host. The corals in which the Pacific waminoa lives also harbor zooxanthellae. However, while all zooxanthellae living in the corals belong to the genus Symbiodinium, the ones living in the Pacific waminoa belong to two genera, Symbiodinium and Amphidinium. This is very unusual, as zooxanthellae belonging to two different genera in the same animal is a very rare occurrence. The beautiful golden color of the Pacific waminoa is a direct result of the presence of the zooxanthellae.
At first it was thought that, due to this close relationship between these acoels and the corals, the lineages of Symbiodinium living inside both organisms would be closely related, but this is not the case. Molecular studies revealed that Symbiodinium living in the Pacific waminoa are not the same that live inside the corals, which indicates that both organisms acquired them independently, i.e., the Pacific waminoa does not captures them from the coral. In fact, they are passed directly from the mother to the offspring inside the eggs. The Pacific waminoa is born with the dinoflagellates already inside it.
The Pacific waminoa is, therefore, an incredible creature not only because it belongs to a very divergent lineage of bilaterian animals but because it is the only case known in bilaterians about symbionts being transferred directly from the mother to the offspring. Perhaps one day the zooxanthellae will evolve into a new organelle of a clade that will evolve from this amazing species.
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Barneah, O., Brickner, I., Hooge, M., Weis, V. M., LaJeunesse, T. C., & Benayahu, Y. (2007). Three party symbiosis: acoelomorph worms, corals and unicellular algal symbionts in Eilat (Red Sea). Marine Biology, 151(4), 1215-1223.
Hikosaka-Katayama, T., Koike, K., Yamashita, H., Hikosaka, A., & Koike, K. (2012). Mechanisms of maternal inheritance of dinoflagellate symbionts in the acoelomorph worm Waminoa litus. Zoological science, 29(9), 559-567.
Kunihiro, S., & Reimer, J. D. (2018). Phylogenetic analyses of Symbiodinium isolated from Waminoa and their anthozoan hosts in the Ryukyu Archipelago, southern Japan. Symbiosis, 76(3), 253-264.
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