Establishment of the photosymbiosis in the early ontogeny of three giant clams |
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Authors: | E Hirose K Iwai T Maruyama |
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Institution: | (1) Department of Chemistry, Biology and Marine Science, Faculty of Science,, University of the Ryukyus, Nishihara, 903-0213 Okinawa, Japan;(2) Yaeyama Branch Laboratory, Okinawa Fisheries Experiment Station, 907-0453 Ishigaki, Okinawa, Japan;(3) Extremobiosphere Research Center, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 237-0061 Yokosuka, Kanagawa, Japan |
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Abstract: | Distribution and morphology of zooxanthellae were investigated histologically and ultrastructurally in veligers and juveniles
of three giant clam species, Tridacna crocea, T. derassa, and T. squamosa. No zooxanthellal cells were associated with gametes. In veliger larvae, zooxanthellae were ingested and digested in the
stomach. Within several days after metamorphosis from veliger to a juvenile clam, the zooxanthellal tube, in which zooxanthellae
were packed, elongated from the stomach toward the mantle. Zooxanthellae in the tube appeared in a line, and we designated
the appearance of the lined zooxanthellae in the mantle of juvenile clams as the first sign of the establishment of symbiosis.
The zooxanthellal tubular system developed as the clams grew, particularly in the mantle margin, and then hypertrophied siphonal
tissue formed. In zooxanthellal tubes, zooxanthellae usually had intact ultrastructures suggesting that they were photosynthetically
active, while the stomach always contained degraded zooxanthellae that were probably discharged from the zooxanthellal tube.
Giant clams probably digest zooxanthellae directly, and ingest the secreted photosynthates from zooxanthellae. There may be
a selection mechanism to discharge unhealthy zooxanthellae from the mantle into the stomach. In addition to zooxanthellae,
digested diatoms and other unidentified digested materials in the stomach suggest that filter-feeding also contributes to
giant clam nutrition. |
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