Octocorals compose a major part of cnidarian diversity. As with other symbiont-containing cnidarians, octocorals are susceptible to a stress response and subsequent "bleaching," which typically involves the loss of photosynthetic dinoflagellate symbionts. Studies of bleaching often focus on hexacorals, including sea anemones and scleractinians. The extent to which these results can be generalized to octocorals remains unclear. Bleaching was examined using two representative species of the Holaxonia-Alcyoniina clade of alcyonacean octocorals, Phenganax parrini and Sarcothelia sp. Remarkably, colonies of both species showed the same pattern in response to perturbation: symbionts in the polyps detach or die, leaving the polyps entirely bleached, yet at the same time large numbers of symbionts accumulate in the stolons. These symbionts are contained in host cells, many of which appear to attach to the stolon tissue. A comparison of living and fixed specimens suggests that these cells are loosely bound to, but not actually in, the stolonal tissue. Since gastrovascular fluid in the stolons is driven by cilia, these accumulating cells may lower fluid velocities. The accumulation of symbionts in the stolons during perturbation may have considerable relevance to how octocoral colonies recover from bleaching.

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],