AsianScientist (Sep. 5, 2019) – Giant clams found in healthy coral reefs acquire symbiotic algae via the feces of their parents, according to research by marine biologists in Japan and Singapore. Their findings, which have implications for the health of coral reefs, are published in the journal PLOS One.
Tridacna gigas, a species of giant clam, can live for more than 100 years and grow to more than a meter wide. Their size and beautiful shells have led to their popularity as ornaments and as a delicacy, but this has resulted in their endangerment. Importantly, T. gigas plays host to symbiotic algae: Zooxanthellae, which in turn provide food for corals and clams through products of photosynthesis.
About 80 percent of coral reef nutrition and 65-70 percent of giant clam nutrition comes from the symbiotic relationship with Zooxanthellae. However, these algae are not able to survive without a host and do not appear to be present in large amounts in the water or sediment surrounding the coral reef.
In this study, researchers led by Professor Kazuhiko Koike at Hiroshima University, Japan, with collaborators in Japan and Singapore, tried to find the mode of transmission of algae from clam to clam. While researching the giant clams, the researchers noticed that the fecal pellets of clams were full of Zooxanthellae.
When the scientists studied the algae under a fluorescent microscope, they observed that the algae were alive and active, with intact chloroplasts, and therefore capable of photosynthesizing. The team then grew juvenile clams in the laboratory and fed them fecal pellets from adult giant clams originally harvested in Okinawa. 34 percent of the larvae took up the Zooxanthellae from the fecal pellets, and five percent of larvae established symbiosis with feces-borne algae.
Although this is a preliminary study, Koike believes that the findings could help solve a mystery of coral reef maintenance. Under conditions of elevated ocean temperatures due to global warming, more heat-resistant Zooxanthella grow in the clams, and more are expelled in their fecal pellets. Koike thinks that giant clams could thus supply heat-resistant Zooxanthellae to coral reefs to mitigate coral bleaching caused by climate change.
The article can be found at: Morishima et al. (2019) Study on Expelled but Viable Zooxanthellae From Giant Clams, With an Emphasis on Their Potential as Subsequent Symbiont Sources.
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Source: Hiroshima University; Photo: Kazuhiko Koike/Hiroshima University.
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