Symbiodinium migration mitigates bleaching in three octocoral species

• Octocoral colonies of three species were experimentally perturbed.
• In each species, symbiont migration into the coenenchyme occurred.
• Symbiont migration mitigates bleaching.
• Mitigation may depend on genetic affinities of host/Symbiodinium combinations.
• Morphological features of the coenenchyme may also affect mitigation.

Coral reefs are increasingly threatened by bleaching, a breakdown of the mutualism between coral hosts and symbionts (Symbiodinium spp.). Symbiont movement within a host may mitigate the effect of environmental stressors that trigger bleaching. Octocorals represent an important component of reef ecosystems, and the alcyonacean taxa Phenganax parrini, Sarcothelia sp., and Sympodium sp. were experimentally perturbed. In colonies subject to elevated temperature (incubated at 30–32 °C to a maximum temperature of 31.5–33.5 °C) and illumination, the number of Symbiodinium decreased in the tissue but increased in the gastrovascular system, with only a small proportion of symbionts expelled. Following within-colony symbiont migration, the three octocoral species retained high densities of symbionts in the coenenchyme. Nevertheless, variable mortality and retention occurred (≈ 85, 0, and 53% of the initial number of Symbiodinium were calculated to have died and ≈ 15, 100, and 45% were calculated to have been retained by P. parrini [maximum 33.5 °C, 24 h], Sarcothelia sp. [maximum 31.5 °C, 16 h], and Sympodium sp. [maximum 31.5 °C, 24 h], respectively). The perturbation responses could be due to genetic differences between the hosts or symbionts, or to a host–symbiont interaction. Mitochondrial (mtMutS and COI) and nuclear (28S rDNA) gene sequences demonstrated that Sympodium sp. and Sarcothelia sp. are closely related xeniids but are only distantly related to P. parrini. Conversely, internal transcribed spacer 2 ribosomal DNA regions showed that P. parrini and Sarcothelia sp. hosted Symbiodinium types D4-5 and D4-5-9, respectively, while Sympodium sp. had a type C1 variant. Architectural features of the host colonies may also be relevant, with runner-like features of P. parrini possibly limiting symbiont retention due to high surface/low volume coenenchyme. Despite these differences, all three alcyonaceans exhibited symbiont migration and symbiont retention when perturbed. Further research on additional taxa will be necessary to determine the relative role of host–Symbiodinium interactions and host architectural features in modulating the mitigation of bleaching via migration.