Friends in unlikely places: key coral microbes in giant clam poop

By Carly E. Karrick

We all poop, but did you know that giant clams do, too? Lucky for us, a team of researchers in Japan did. They even went so far as to ask: what’s in giant clam poop, and could it help reef-building corals? 

Corals rely on tiny algae for survival, but a key question is where they get these algae

Reef-building corals form the basis of coral reefs, amazing ecosystems with important functions for humans and animals alike. To create reef structures, these corals rely on a partnership with tiny algae called Symbiodiniaceae [1]. These algae create food for corals through photosynthesis, like plants. In fact, if corals lose their algal partners, they starve and die. Because this partnership is so important, scientists have long been interested in where corals get their algae. This is an important question because, for many coral species, coral babies don’t have algae at first. In 2020, a team of researchers in Japan explored an unexpected source of these algae: giant clam poop.

Giant clam poop has algae in it, but can corals partner up with these algae?

Giant clams (Tridacna crocea) are other reef organisms that, like coral, contain Symbiodiniaceae. In 2019, Masami Umeki and their team wanted to see whether the algae in giant clam poop could transfer to coral babies. So, they collected three giant clams and three corals (Acropora tenuis) in Japan. After the coral produced larvae, they kept the larvae in filtered water that didn’t contain microalgae. Then, they put the giant clams in jars. After just five hours, they were able to collect 20-30 grams of poop from the bottom of the jars! For three days, they applied the giant clam feces to the coral babies. After that, they measured the amount of algae in each coral baby.

Researchers put giant clams into jars with filtered seawater (left) and they collected fecal pellets (black arrows) after five hours (right). 

Coral babies partnered with algae from giant clam poop!

After just eight days, the researchers checked whether the coral babies had algae in them. They used a fancy microscope that shows algae as bright red, fluorescent circles. This means that giant clam poop could be a source of algae for corals!

After eight days, researchers found algae (bright red circles) in corals that were exposed to giant clam poop.

What about other animal poop?

This exciting research is related to a lot of research going on right now about environmental sources of algae! For example, another team of researchers recently found lots of algae in coral-eating fish poop [2]. However, not all poop is created equal. Researchers have also found evidence that other microorganisms in fish poop like bacteria could be harmful to coral [3]. Going forward, it’s important to understand how coral get their algal partners. In a changing climate, the coral-algae partnership is threatened by increasing temperatures. As the water heats up, corals lose their algae. If they don’t partner with new algae, the corals will die, and we will lose coral reefs. Environmental sources of algae could be beacons of hope for corals. Whether it’s from giant clam poop, fish poop, or some other animal’s poop, maybe rethinking reef processes like eating and pooping will help unlock a safe future for coral [4].

Read the full article here: Umeki, M., Yamashita, H., Suzuki, G., Sato, T., Ohara, S., and Koike, K. (2020). Fecal pellets of giant clams as a route for transporting Symbiodiniaceae to corals. PLoS One, 15(12), e0243087. https://doi.org/10.1371/journal.pone.0243087 

References

[1] LaJeunesse, T.C., Parkinson, J.E., Gabrielson, P.W., Jeong, H.J., Reimer, J.D., Voolstra, C.R., and Santos, S.R. (2018). Systematic revision of Symbiodiniaceae highlights the antiquity and diversity of coral endosymbionts. Current Biology, 28(16), 2570-2580. https://doi.org/10.1016/j.cub.2018.07.008 

[2] Grupstra, C.G.B., Rabbitt, K.M., Howe-Kerr, L.I., and Correa, A.M.S. (2021). Fish predation on corals promotes the dispersal of coral symbionts. Animal Microbiome, 3, 25. https://doi.org/10.1186/s42523-021-00086-4 

[3] Grupstra, C.G.B., Howe-Kerr, L.I., van der Meulen, J.A., Veglia, A.J., Coy, S.R., and Correa, A.M.S. (2023). Consumer feces impact coral health in guild-specific ways. Frontiers in Marine Science, 10. https://doi.org/10.3389/fmars.2023.1110346 

[4] Grupstra, C.G.B., Lemoine, N.P., Cook, C., and Correa, A.M.S. (2022). Thank you for biting: dispersal of beneficial microbiota through ‘antagonistic’ interactions. Trends in Microbiology, 30(10): 930-939. https://doi.org/10.1016/j.tim.2022.03.006