Fecal pellets of giant clams as a route for transporting Symbiodiniaceae to corals

Abstract

Because more than 80% of species of gamete-spawning corals, including most Acroporidae species, do not inherit Symbiodiniaceae from their parents, they must acquire symbiont cells from sources in their environment. To determine whether photosynthetically competent Symbiodiniaceae expelled as fecal pellets from giant clams are capable of colonizing corals, we conducted laboratory experiments in which planula larvae of Acropora tenuis were inoculated with the cells in fecal pellets obtained from Tridacna crocea. T. crocea fecal pellets were administered once a day, and three days later, cells of Symbiodiniaceae from the fecal pellets had been taken up by the coral larvae. T. crocea fecal pellets were not supplied from the 4th day until the 8th day, and the cell densities in the larvae increased until the 8th day, which indicated the successful colonization by Symbiodiniaceae. The control group exhibited the highest mean percentage of larvae (100%) that were successfully colonized by culture strains of Symbiodiniaceae, and larvae inoculated with fecal pellets reached a colonization percentage of 66.7 ~ 96.7% on the 8th day. The highest colonization rate was achieved with the fecal pellets containing cells with high photosynthetic competency (Fv/Fm). Interestingly, the genetic composition of Symbiodiniaceae in the larvae retrieved on the 8th day differed from that in the fecal pellets and showed exclusive domination of the genus Symbiodinium. A minor but significant population of the genus Cladocopium in the fecal pellets was not inherited by the larvae. These experiments provided the first demonstration that the Symbiodiniaceae from tridacnine clams provided via fecal pellets can colonize and even proliferate in coral larvae.

Fish predation on corals promotes the dispersal of coral symbionts

“…Our findings suggest that the feces of obligate corallivores and at least some facultative corallivores constitute significant but underexplored environmental 'hotspots' of Symbiodiniaceae on coral reefs; such feces may supply Symbiodiniaceae cells to potential hosts directly, or to other environmental reservoirs as they disintegrate [14,45]. Corals have been shown to take up Symbiodiniaceae from sediments and seawater, as well as from the feces of giant clams [15,34,45,63]. The sea anemone Aiptasia pulchella has also previously been shown to incorporate Symbiodiniaceae from fish feces as symbionts [43].…”

Light-Dependent Phenomena and Related Molecular Mechanisms in Giant Clam-Dinoflagellate Associations: A Review

“…Z-tubule, zooxanthellal tubule. (Griffiths and Klumpp, 1996), which can serve as large reservoirs of symbiotic dinoflagellates in the reef ecosystem (Umeki et al, 2020). In general, giant clams are associated with three genera of phototrophic dinoflagellates, Symbiodinium, Cladocopium, and Durusdinium (Hernawan, 2008;DeBoer et al, 2012;Ikeda et al, 2017;LaJeunesse et al, 2018;Lim et al, 2019).…”

“…While scleractinian corals expel degraded dinoflagellates (Fujise et al, 2014), those expelled by giant clams are intact, viable and competently phototrophic (Morishima et al, 2019). Importantly, healthy dinoflagellates expelled by giant clams can repopulate bleached Symbiodiniaceae-bearing hosts (Morishima et al, 2019) including scleractinian corals (Umeki et al, 2020). However, as a member of the reef ecosystem, the survival of giant clams is also affected negatively by various anthropogenic activities and global warming (Van Wynsberge et al, 2016;Neo et al, 2017).…”

Fish predation on corals promotes the dispersal of coral symbionts

“…Our ndings suggest that the feces of obligate corallivores and at least some facultative corallivores constitute signi cant but underexplored environmental 'hotspots' of Symbiodiniaceae on coral reefs; such feces may supply Symbiodiniaceae cells to potential hosts directly, or to other environmental reservoirs as they disintegrate (Castro-Sanguino and Sánchez, 2012;Nitschke et al, 2016). Corals have been shown to take up Symbiodiniaceae from sediments and seawater, as well as from the feces of giant clams (Coffroth et al, 2006;Lewis and Coffroth, 2004;Nitschke et al, 2016;Umeki et al, 2020). The sea anemone Aiptasia pulchella has also previously been shown to incorporate Symbiodiniaceae from sh feces as symbionts (Muller Parker, 1984).…”