Within-Colony Migration of Symbionts During Bleaching of Octocorals

Abstract

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.

Physiological correlates of symbiont migration during bleaching of two octocoral species

“…nicotinamide adenine dinucleotide [phosphate] (NAD[P]H), RNS and ROS} were investigated in the host tissues and in individual migrating symbionts using fluorescence microscopy. As suggested by previous results (Parrin et al, 2012), symbiont migration and related physiological changes occur at different rates in the two study species, with colonies of Sarcothelia sp. exhibiting greater sensitivity.…”

“…All of the experiments reported were conducted during time periods and conditions when symbionts migrate (Parrin et al, 2012). While symbiont migration has not been widely examined, it is implicit in many discussions of bleaching even in hexacorals (e.g.…”

“…Such migrating symbionts were consistently contained within host cells (Parrin et al, 2010;Parrin et al, 2012). This results in a pattern of bleaching in which the polyps are completely bleached, while the stolons are not (Fig.…”

“…In this context, respiration associated with increased metabolic demand may be a relevant factor. Symbiont migration is associated with decreased velocity of gastrovascular fluid in P. parrini (Parrin et al, 2012) and in Sarcothelia sp. (K. L. Harmata and N.W.B., unpublished).…”

“…Additionally, in two species of these octocorals, Phenganax parrini (Alderslade & McFadden) and Sarcothelia sp. (see McFadden et al, 2006), following perturbation involving light, temperature or confinement in small volumes of seawater, symbionts were recently found to migrate from the polyps to the stolons (Parrin et al, 2012).…”