Environmental constraints on the mode of symbiont transmission in corals

Abstract

Theory predicts that passing symbionts directly from parent to offspring should benefit the host by reducing conflict between diverse symbionts and benefit the symbiont by guaranteeing transmission. However, in some symbiotic associations, such as those between corals and algae of the family Symbiodiniaceae, vertical transmission is rare, suggesting a substantial cost. Here, we show that coral larvae which contained symbionts had lower survivorship than larvae of the same species that lacked symbionts when exposed to natural sunlight. Higher activity of antioxidant defenses and higher levels of oxidative cellular damage observed in symbiotic larvae suggest that reactive oxygen species, produced by stress on the photosynthetic apparatus of the symbionts, are responsible for these higher rates of mortality under solar radiation. We hypothesize that symbionts are a burden in the early life history of broadcast spawning corals under conditions typically experienced on the ocean surface during dispersal. This is one possible reason why species with an obligate pre-motile period of between 12 and 36 h very rarely have vertical transmission of symbionts, in contrast to species whose larvae are small or swim on release that often have vertical transmission of symbionts.