Corals May Show Complex, Coordinated Behavior

Xeniid corals, which live throughout the Red Sea and the Indian Ocean, perpetually pulse to circulate gases and nutrients in the surrounding water, thereby improving the health of the entire colony. Since each colony comprises many individuals, coordinated pulsing could increase the efficiency of this process. But is it even possible for a species that lacks a central nervous system to produce such coordinated behavior?

Julia Samson, a PhD candidate in the Biology Department at the University of North Carolina at Chapel Hill, may be the first to have an answer.

Working in the laboratory of Dr. Laura Miller, Samson collected troves of video data on the pulsing behavior of xeniid corals. She focused on a subset of individuals in a colony, each one termed a “polyp,” and recorded the times at which the polyps pulsed. By using a neuroscience technique for identifying neuron firing patterns, Samson found that polyp groups pulsed in predictable half-second triplets. Yet, it was unclear if this behavior was coordinated within the groups.

To address this question, Samson developed computer simulations of potential systems to compare against the real coral data. For example, in one model, pulses occur randomly. In other models, polyps pulse at fixed rates, or at fluctuating rates relative to other polyps.

One particular model—a coupled phase oscillators model—produced simulated data strikingly similar to that observed in real corals. This system is like a set of pendulums in which the swinging of an individual pendulum is influenced by the swinging of other pendulums. In this way, each simulated polyp has its own intrinsic rhythm that can be altered by others, but is not strictly determined by them—a process common in biological synchronization.

Though this simulation suggests that some order underlies the pulsing, it does not identify an actual process used by the polyps to coordinate their behavior.

Still, such a finding is remarkable because it may indicate a novel appearance of organized behavior. Unlike animals with complex nervous systems, or highly social species like bees or ants, xeniid corals do not have hierarchical or top-down control. “How do you coordinate behavior when actors are more independent, and are not actually centrally directed?” wonders Samson.

“There’s definitely some information that is being shared across polyps in a colony,” she continues. “It’s difficult to figure out how they would process the information that they get from the environment, from their neighbors, and how that turns into a decision for each polyp.”

Samson already has some ideas to explore next. The corals could send internal signals through the nerve net, a structure of interconnected neurons that links the polyps together. They may also send chemical messages to one another, or be influenced by external factors such as oxygen levels and local water flow. “As with everything in biology it’s probably a combination of all kinds of things,” she notes.

For now, how coordinated behavior emerges without central control remains an unsolved mystery in biology. Perhaps these answers are held in the pulses of undersea sessile corals.

Samson presented this research at the 2017 annual meeting of the Society for Integrative and Comparative Biology in New Orleans, Louisiana.