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1. Study on expelled but viable zooxanthellae from giant clams, with an emphasis on their potential as subsequent symbiont sources.

Author

Shin-Ya Morishima, Hiroshi Yamashita, Shizuka O-Hara, Yuji Nakamura, Vanessa ZhiQin Quek, Momo Yamauchi, and Kazuhiko Koike

Subjects
Medicine, Science
Abstract

Unlike most bivalve shellfishes, giant clams (tridacnines) harbor symbiotic microalgae (zooxanthellae) in their fleshy bodies. Zooxanthellae are not maternally inherited by tridacnine offspring, hence, the larvae must acquire zooxanthellae from external sources, although such algal populations or sources in the environment are currently unknown. It is well known that giant clams expel fecal pellets that contain viable zooxanthellae cells, but whether these cells are infectious or just an expelled overpopulation from the giant clams has not been investigated. In this study, we observed the ultrastructural and photosynthetic competencies of zooxanthellae in the fecal pellets of Tridacna crocea and further tested the ability of these cells to infect T. squamosa juveniles. The ultrastructure of the zooxanthellae cells showed that the cells were intact and had not undergone digestion. Additionally, these zooxanthellae cells showed a maximum quantum yield of photosystem II (Fv/Fm) as high as those retained in the mantle of the giant clam. Under the assumption that feces might provide symbionts to the larvae of other giant clams, fecal pellets from Tridacna squamosa and T. crocea were given to artificially hatched 1-day-old T. squamosa larvae. On the 9th day, 15-34% of the larvae provided with the fecal pellets took up zooxanthellae in their stomach, and on the 14th day, zooxanthellae cells reached the larval margin, indicating the establishment of symbiosis. The rate reaching this stage was highest, ca. 5.3%, in the larvae given whole (nonhomogenized) pellets from T. crocea. The composition of zooxanthellae genera contained in the larvae were similar to those in the fecal pellets, although the abundance ratios were significantly different. This study is the first to demonstrate the potential of giant clam fecal pellets as symbiont vectors to giant clam larvae. These results also demonstrate the possibility that fecal pellets are a source of zooxanthellae in coral reefs.

Published
2019
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2. Study on expelled but viable zooxanthellae from giant clams, with an emphasis on their potential as subsequent symbiont sources

Author

Vanessa ZhiQin Quek, Shin-ya Morishima, Momo Yamauchi, Kazuhiko Koike, Hiroshi Yamashita, Shizuka Ohara, and Yuji Nakamura

Subjects
0106 biological sciences, Life Cycles, Light, Marine and Aquatic Sciences, 01 natural sciences, Larvae, Medicine and Health Sciences, Photosynthesis, Larva, Multidisciplinary, geography.geographical_feature_category, biology, Coral Reefs, Physics, Electromagnetic Radiation, Stomach, Eukaryota, Coral reef, Artificial Light, Zooxanthellae, Physical Sciences, Corals, Dinoflagellida, Medicine, Anatomy, Research Article, Bivalves, animal structures, Science, Photoperiod, Pellets, Zoology, Marine Biology, 010603 evolutionary biology, Symbiosis, Sea Water, Animals, Feces, geography, 010604 marine biology & hydrobiology, fungi, Ecology and Environmental Sciences, Giant clam, Organisms, Biology and Life Sciences, Aquatic Environments, Molluscs, biology.organism_classification, Marine Environments, Invertebrates, Tridacna, Gastrointestinal Tract, Species Interactions, Earth Sciences, Reefs, Digestive System, Developmental Biology
Abstract

Unlike most bivalve shellfishes, giant clams (tridacnines) harbor symbiotic microalgae (zooxanthellae) in their fleshy bodies. Zooxanthellae are not maternally inherited by tridacnine offspring, hence, the larvae must acquire zooxanthellae from external sources, although such algal populations or sources in the environment are currently unknown. It is well known that giant clams expel fecal pellets that contain viable zooxanthellae cells, but whether these cells are infectious or just an expelled overpopulation from the giant clams has not been investigated. In this study, we observed the ultrastructural and photosynthetic competencies of zooxanthellae in the fecal pellets of Tridacna crocea and further tested the ability of these cells to infect T. squamosa juveniles. The ultrastructure of the zooxanthellae cells showed that the cells were intact and had not undergone digestion. Additionally, these zooxanthellae cells showed a maximum quantum yield of photosystem II (Fv/Fm) as high as those retained in the mantle of the giant clam. Under the assumption that feces might provide symbionts to the larvae of other giant clams, fecal pellets from Tridacna squamosa and T. crocea were given to artificially hatched 1-day-old T. squamosa larvae. On the 9th day, 15–34% of the larvae provided with the fecal pellets took up zooxanthellae in their stomach, and on the 14th day, zooxanthellae cells reached the larval margin, indicating the establishment of symbiosis. The rate reaching this stage was highest, ca. 5.3%, in the larvae given whole (nonhomogenized) pellets from T. crocea. The composition of zooxanthellae genera contained in the larvae were similar to those in the fecal pellets, although the abundance ratios were significantly different. This study is the first to demonstrate the potential of giant clam fecal pellets as symbiont vectors to giant clam larvae. These results also demonstrate the possibility that fecal pellets are a source of zooxanthellae in coral reefs., This work was supported by MEXT/JSPS KAKENHI Grant Number 18K19232 to KK.

Published
2019

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