How animals survive in the wild and defend themselves from natural predators is one of the most fascinating and essential mechanisms that researchers can study. Because animals cannot rely on the same mental strategies that humans can to extract themselves from dangerous situations, the animals often need to take advantage of innate physical capabilities. Photonics, in conjunction with other techniques, can help to uncover these complexities of animals’ physical structure and the functioning that enables their built-in defenses. To unlock the mystery behind these innate safeguards, Ahmet Karabulut, a pre-doctoral researcher in the lab of Matt Gibson at the Stowers Institute for Medical Research, examined electric-looking (thanks to fluorescent tagging) sea creatures — in this case, sea anemones. Through this examination, they came to understand the structure and operating mechanism of the stinging organelles that they and their jellyfish cousins use to keep natural enemies at bay. A fluorescent microscopy image of nematocysts — or the stinging organelles of the starlet sea anemone, Nematostella vectensis. Courtesy of the Stowers Institute for Medical Research. Karabulut used superresolution imaging, 3D electron microscopy, and genetic perturbations to investigate the stinging cells, or nematocysts, of sea anemones to unveil their precise mechanics. He wanted to establish the means by which a painful sting can occur in just a few thousandths of a second, a speed that is one of the fastest biological processes to occur in nature. The nematocysts, he found, release in three phases and consist of pressurized capsules with a coiled, harpoon-like thread inside. When triggered by prey, a threat, or an innocent beachgoer, the capsule discharges in an explosion, ejecting the coiled thread and puncturing the target. The first phase of release involves a projectile-like discharge of densely coiled threads from the capsule. In the second phase, the threads rapidly elongate by turning inside out in a process called eversion. When the shaft turns inside out, it forms a triple helical structure that surrounds a fragile inner tubule covered with toxic barbs. Finally, the tubule then begins its own eversion process to elongate into the soft tissue of the target while releasing neurotoxins. “The explosive discharge of the nematocyst is extremely fast and very hard to capture in detail,” Karabulut said. “For this reason, the motion of the thread during [the] early phase remained elusive.” Karabulut used a reagent that both induced the discharge of the nematocysts and fixed the samples, which allowed him to analyze their inner workings with the aid of the fluorescence captured under a microscope. Composed of labeled tentacles with partially discharged nematocysts, the samples were treated and placed on a glass slide and analyzed using a ZEISS LSM 780 confocal microscope. “I saw a stunning snapshot of nematocysts in the earliest phases of the discharge sequence in the tentacle sample,” Karabulut said. “It was fascinating. I could see a fireworks-like show of nematocysts in various stages of their firing sequence frozen in time.” The geometric transformations, by Karabulut’s account, involve a painful — to prey and beachgoers — but beautifully orchestrated process. “I am in awe at the beauty found in nature that can only be seen under the microscope,” he said. In the future, the team believes, the nematocytes’ elaborate natural process of eversion could provide the inspiration for devices in both medicine and materials science.
