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.
Highlights
Coral reefs are habitats with high biodiversity in which approximately one-quarter to onethird of all marine species are found, while coral reefs cover only 0.2% of the ocean’s surface [1,2,3]
Inoculation coral larvae with the symbiont cells
We found that A. tenuis larvae acquired symbiont cells derived from T. crocea fecal pellets and allowed their proliferation, and these fecal pellets might represent one of the symbiont sources in the environment
Summary
Coral reefs are habitats with high biodiversity in which approximately one-quarter to onethird of all marine species are found, while coral reefs cover only 0.2% of the ocean’s surface [1,2,3]. Environmental pools of the Symbiodiniaceae population could play elemental roles to supply symbionts for juveniles and/or adults of these host animals [8] Consistent with this view, several studies have addressed the genetic diversity of Symbiodiniaceae in the environment [8,9,10,11,12,13,14], their abundances [13, 15] and the similarity or dissimilarity between coral juveniles and host-free populations in sediments [16]. Some cnidarian hosts expel Symbiodiniaceae that undergo cell division [22] or maintain competency in the photosystem [17, 18], and a previous study showed that significant percentage of the discharged cells from a coral are degraded or damaged [21]
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