Correlations between the waving motions of individual coral polyps have been observed – a discovery that could boost our understanding of coral reefs. Through a carefully-controlled experiment, Shuaifeng Li and Jinkyu Yang at the University of Washington, Seattle and colleagues found that the random swaying of the animals became more synchronized at cooler temperatures, and under red light. This insight enabled the team to build predictive models of the polyps’ behaviour that could eventually inform vital conservation efforts for threatened coral reefs.
The vibrant and diverse structures found in coral reefs are built up by colonies of animals called coral polyps. These roughly cylindrical creatures are attached at their bottom ends to a hard exoskeleton, excreted by previous generations of polyps – sometimes over thousands of years. At its top end, a polyp’s mouth is surrounded by a crown of tentacles, which move in swaying motions to feed, reproduce, and defend themselves from predators.
These motions are remarkably diverse: while some polyp species exhibit unique rhythmic pulsations, the movements of other species are barely perceptible. To better protect corals from the myriad threats they face, researchers are now attempting to quantify and characterize these different types of motion. However, since the movements appear to be inherently random and complex, this level of analysis is exceptionally challenging.
In the study of some biological systems, mathematical models and tracking techniques are already providing a better awareness of how organisms behave. For example, random Brownian motion is now known to be essential to understanding the motions taking place within systems of microbes, like bacteria. Yet for animals like polyps, which exist in extensive, interconnected colonies, it is still unclear whether similar mathematical rules can be applied.
To better understand coral swaying, the researchers took a fragment of coral containing a small number of polyps, and placed it in a 3D-printed octagonal tank – with each side housing an individually-controllable valve. This allowed them to precisely alter the water flowing around the polyps – ensuring that the motions they observed were generated by the polyps themselves, and not from the impact of flowing water.
The team analysed changes to the fragment when they varied the temperature and light conditions inside the tank. The researchers then applied numerical analysis to the motions they observed – searching for mathematical relationships between the swaying of the polyps.
They discovered that when the fragment was exposed to both cooler temperatures and red light, the polyps’ swaying became more synchronized. This correlated motion disappeared at higher temperatures, and under blue light. Building on this insight, Li and colleagues then constructed a mathematical model of these motions, allowing them to predict the responses of the polyps in response to varying conditions.
Hopes rise for some coral survival
Although the team cannot yet explain the biological significance of their findings, the results could lead to a better understanding of how wild corals behave – particularly in response to factors including rising temperatures and ocean acidification. In turn, their insights could help conservationists to better preserve coral reefs, and protect the rich, diverse ecosystems they support.
The research is described in Physical Review Applied.