Coral bleaching is one of the most important ecological problems facing humankind today. In this process, the coral loses its symbiotic dinoflagellates and appears white or pale. Since the symbionts provide most of the corals’ nutrition, bleached colonies typically regress or even die. Insight into this process can be gained by using laboratory model systems. Indeed, the physiology of a colonial organism cannot be endlessly extrapolated from a single polyp anemone, which is the current model system. In order to understand the basic physiology of the coral gastrovascular system, and thus a major component of bleaching, colonies of an as-yet undescribed colonial octocoral (hereafter referred to as clavulariid species A) were surgically grown on microscope cover glass and filmed using an inverted microscope. From this, we were able to glean some insight into the inner workings of a colonial coral. These corals experienced symbiont velocities in the gastrovascular lumen of an average of 400–500 µm/sec, and they demonstrate simultaneous bidirectional flow within the lumen. Building on this, colonies of clavulariid species A were thermally perturbed by placing them in incubators at 32°C for 12 hours. Several stolons of each colony were then filmed and the numbers of symbionts in the tissue and in the gastrovascular flow were measured. As well, velocities were taken of symbionts in the gastrovascular flow for each stolon. Colonies were then returned to normal culture conditions at 27°C. After 36 hours of recovery from thermal perturbation, 42% of the variance in colony growth was explained by three factors: the numbers of symbionts in the tissue, the numbers of symbionts in the lumen, and the velocities of symbionts in the lumen. These data suggest a mechanism (gastrovascular velocity) by which coral colonies can regulate their symbionts population. Such a mechanism may be relevant to patterns of bleaching observed in the field.