Cilia-based transport systems characterize sponges and placozoans. Cilia are employed in cnidarian gastrovascular systems as well, but typically function in concert with muscular contractions. Previous reports suggest that anthozoans may be an exception to this pattern, utilizing only cilia in their gastrovascular systems. With an inverted microscope and digital image analysis, we used stoloniferan octocoral colonies growing on microscope cover glass to quantitatively describe the movement of fluids in this system for the first time. Flow in stolons (diameter ≈300 μm) is simultaneously bidirectional, with average velocities of 100-200 μm/s in each direction. Velocities are maximal immediately adjacent to the stolon wall and decrease to a minimum in the center of the stolon. Flow velocity is unaffected by stolonal contractions, suggesting that muscular peristalsis is not a factor in propelling the flow. Stolon intersections (diameter ≈500 μm) occur below polyps and serve as traffic roundabouts with unidirectional, circular flow. Such cilia-driven transport may be the plesiomorphic state for the gastrovascular system of cnidarians.

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.

Using microscopy, the gastrovascular systems of four hydroids (Eirene viridula, Cordylophora lacustris, Hydractinia symbiolongicarpus, and Podocoryna carnea) and two distantly related alcyonacean octocorals (Acrossota amboinensis and Sarcothelia sp.) were examined and compared within a phylogenetic framework. Despite a range of stolon widths (means 53–160 μm), the hydroid species exhibited similar patterns of gastrovascular flow: sequentially bidirectional flow in the stolons, driven by myoepithelial contractions emanating from the center of the colony. Unlike the hydroids, the gastrovascular system of A. amboinensis (mean stolon widths for 5 colonies, 0.57–1.21 mm) exhibited simultaneously bidirectional flow with incomplete, medial baffles (width 4–20 μm) separating the flow. Baffles visualized with transmission electron microscopy consisted of endoderm, mesoglea, and occasionally another layer of tissue. Mean flow rates of the gastrovascular fluid for seven stolons ranged from 125 to 275 μm s−1, with maximum rates of 225–700 μm s−1. In Sarcothelia sp., stolons were of comparable width (means for 13 colonies 0.55–1.4 mm) to those of A. amboinensis. These stolons, however, were divided by several partitions (width 8–25 μm), both complete and incomplete, which were spaced every 100.5±5.1 μm (mean±SE; range 27.1–283.7 μm) and appeared structurally similar to baffles. In lanes defined by these partitions, ciliary motion was visible in image sequences, and flow was unidirectional. Within a single stolon, flow moved in different directions in different lanes and changed direction by moving from lane to lane via occasional spaces between the partitions. Mean flow rates for 30 stolons ranged from 75 to 475 μm s−1, with maximum rates of 85–775 μm s−1. For both octocorals, flow rates of the gastrovascular fluid were not correlated with the width of the stolon lumen. While octocoral gastrovascular systems probably exhibit differences based on phylogenetic affinities, in all species studied thus far, gastrovascular flow is entirely driven by cilia, in contrast to the hydroid taxa.

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],

Perturbed colonies of Phenganax parrini and Sarcothelia sp. exhibit migration of symbionts of Symbiodinium spp. into the stolons. Densitometry and visual inspection indicated that polyps bleached while stolons did not. When migration was triggered by temperature, light and confinement, colonies of Sarcothelia sp. decreased rates of oxygen formation in the light (due to the effects of perturbation on photosynthesis and respiration) and increased rates of oxygen uptake in the dark (due to the effects of perturbation on respiration alone). Colonies of P. parrini, by contrast, showed no significant changes in either aspect of oxygen metabolism. When migration was triggered by light and confinement, colonies of Sarcothelia sp. showed decreased rates of oxygen formation in the light and increased rates of oxygen uptake in the dark, while colonies of P. parrini maintained the former and increased the latter. During symbiont migration into their stolons, colonies of both species showed dramatic increases in reactive oxygen species (ROS), as visualized with a fluorescent probe, with stolons of Sarcothelia sp. exhibiting a nearly immediate increase of ROS. Differences in symbiont type may explain the greater sensitivity of colonies of Sarcothelia sp. Using fluorescent probes, direct measurements of migrating symbionts in the stolons of Sarcothelia sp. showed higher levels of reactive nitrogen species and lower levels of ROS than the surrounding host tissue. As measured by native fluorescence, levels of NAD(P)H in the stolons were unaffected by perturbation. Symbiont migration thus correlates with dramatic physiological changes and may serve as a marker for coral condition.