By Liam Warren and Garrett Cleveland
Cover image credit: Aaron Bull/Getty Images
On September 23, the Physics and Astronomy Department hosted Dr. Asja Rajda for her talk titled, “Transport properties of 2D corals and other ways physics can help us understand biology.” Radja holds bachelor’s degrees in physics and biochemistry from the University of Texas, as well as a Ph.D. in physics from the University of Pennsylvania. She has also done postdoctoral work at Harvard University in applied mathematics and as an independent fellow in quantitative biology. Since 2021, Radja has been an assistant professor of physics at Bryn Mawr College. In her research, Radja uses her physics background to understand how certain biological structures are organized and the environmental pressures that cause those traits. In her words, Dr. Radja’s work seeks to answer the question: “what physical principles do biological organisms harness to create diverse morphologies?”
Why Corals?
Dr. Radja was inspired to apply her expertise in physics to study corals after scuba diving in Honduras. As she observed the plethora of diverse organisms, she found herself intrigued by the variation in coral structures. Her focus for this colloquium was on a specific type of coral: soft corals, or octocorallia.
These soft corals are closely related to the better-known scleractinian (hard) corals found in tourist destinations like the Great Barrier Reef, but unlike their famous counterparts, soft corals thrive in high-velocity, bidirectional currents. Even more interesting, while hard corals exhibit bleaching and death because of ocean acidification and climate change, soft corals appear to be proliferating in some areas. Observing this surprising trend, Dr. Radja honed in on a more pressing question: “What physical principles do biological systems harness to survive?”
A Split Genetic Branch
Unlike hard corals, soft corals exhibit a flatter, “quasi-2D” structure. Dr. Radja began by examining the benefits of this structure. Her tedious process of sample collection and curation included searching for diverse soft coral phenotypes in academic journals, encyclopedias, and even images from scuba divers. The background of each of the resulting thousands of images had to be removed for structural analysis of the coral. While some software systems could start the process, each image ultimately had to be “skeletonized” by hand to allow for mathematical analysis (see Figures 2 and 3). In Dr. Radja’s words, “We bring wine on a Friday, and we sit down and we trace.”
After she and her grad students developed a database of coral images, they created graphs to count “edges” representing branches, and “nodes” representing branching and reconnection sites. Based on these graphs and their analysis, Dr. Radja and her team found two distinct morphologies: tree-like and mesh-like. The tree-like corals are more centralized, with one primary “trunk” from which branches split off but rarely reconnect under normal circumstances. Conversely, mesh-like corals “[leverage] redundancy and decentralization” with a complex web of branches and reconnections. Armed with this information, Radja went on to explore the biological utility of each phenotype. What environmental conditions selected for these distinct structures, and how do they confer advantages to coral colonies? Additionally, is one morphology more widespread than the other?
Diverging Paths
To dig deeper into the evolutionary mechanisms that brought about these morphologies, Dr. Radja looked back at the evolutionary tree. With a combination of recursion analysis and Bayesian statistics to limit transformation bias, her team found a 50-50 split in the abundance of each structure, suggesting neither had an evolutionary advantage.
However, each type of coral thrived in a different environment. Radja’s team performed a Ricci curvature analysis on the soft corals to examine their circulatory potential. A positive Ricci curvature in a coral network indicates high connectivity, while negative curvature shows low connectivity and thus bottlenecks. Tree-like corals exhibited higher degrees of positive curvature, whereas mesh-like corals exhibited higher degrees of negative curvature. In soft corals, these differing types of networks mean that tree-like corals are more specialized and found in high-flow environments compared to mesh-like corals, which thrive in low-flow, low-nutrient environments.
Despite their structural differences, tree-like corals can adapt to environmental changes by undergoing a process called anastomosis. By forming temporary connections between branches, the curvature in tree-like corals becomes more negative as they adopt a more mesh-like structure in response to low-nutrient availability or low-flow regions.
Corals for Conservation
By understanding the mechanisms that make some species of coral more resilient than others, scientists can learn which species can feasibly be protected. Conservation efforts can accordingly be specialized, with those aimed at diminishing coral bleaching focused on hard corals and increased pollution mitigation efforts focused on soft corals. Since soft corals are the more resilient species, they could be easier to protect. Scientific insights like these are crucial to effectively distribute conservation efforts and maximize the success of ecosystem preservation and protection efforts.
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