New time-lapse photography has revealed new details about how a coral polyp, a squishy, legless creature, can “walk.” Typically living alone rather than in colonies like reef-building corals, mushroom corals from the family Fungiidae have bodies that resemble shaggy round mushroom caps. While the term “walk” may not accurately describe their movement, coral biologist and microscopist Brett Lewis describes their motion as pulsing and inflating like a jellyfish, allowing them to nudge forward through tiny hops. These findings were reported in a study published in PLOS One on January 22.
The bodies of these mushroom corals are covered in a sticky biofilm that captures small prey. Coral polyps have mouths that open to suck in the film and trap the doomed prey, which is then transported to their stomach filled with wormy filaments. These “worms” have stinging and digestive cells that can move through portholes to escape or even punch out through the body wall. Lewis finds this behavior to be monstrous in inspiration, comparing it to creatures used in Dungeons and Dragons.
Mushroom corals have internal stony skeletons like their reef-building cousins but typically start life on a reef before moving to a sandy ocean bottom. These corals are known to move, with Lewis observing them changing position overnight in his aquariums. By using modern equipment, Lewis captured the movement of these corals on camera, with some individuals covering as much as 36 millimeters in about two hours. The movement involves puffing up the dome-shaped body and making micro-hops by peeling away from the bottom with each inflation.
Young mushroom corals start life on reefs, feeding on tiny prey before eventually migrating off the reef to settle in deeper waters. The corals use color changes in light filtering at different depths to guide them towards deeper waters. Wavelengths towards the blue end of the spectrum penetrate deeper in water, prompting corals in lab tests to move towards blue light and deeper water. Marine naturalist Bert W. Hoeksema has studied how mushroom corals stand up when knocked over, noting that coral mouths can spit out water for jet propulsion. Observing a mushroom coral with three mouths flip over after lying down for hours, Hoeksema suggests that jet propulsion may help them make decisive movements.
