Like stationary couch potatoes, corals have long been considered passive organisms relying entirely on ocean currents to deliver nutrients and oxygen directly to them. Even though we know corals have small hair-like appendages, previous work suggested that these cilia only move smoothly and parallel to the coral surface -- which help remove sediment, but aren’t useful for bringing in sustenance. That scenario, however, doesn’t make much sense for colonies living in sheltered parts of a reef or elsewhere with little water movement.
Now, a closer examination shows that tiny coral organisms can use their minuscule appendages to stir water up in turbulent patterns, drawing in nutrients for them to eat. Far from passive, corals are engineers that are quite capable of controlling their environment. The findings are published in Proceedings of the National Academy of Sciences.
Orr Shapiro of the Weizmann Institute of Science in Israel, MIT’s Roman Stocker, and colleagues studied six different coral species in tanks. They used microscopes and high-speed video cameras to observe the microenvironmental processes taking place on the coral surfaces; They also added tracer particles to the water to capture images of movement and flow. “I was expecting that this would be a smooth microworld, there would be not much action except the external flow,” Stocker says in a news release. Instead, within the millimeter closest to the coral surface, “it’s very violent,” he adds.
The arrangement of cilia on the coral’s surface, they found, produce strong swirls of water that filter nutrients towards the corals, while driving potentially toxic waste products away.
Here’s a scanning electron micrograph at 5,000x magnification of the surface of a Pocillopora damicornis branch, showing a dense carpet of cilia. By actively sweeping up water eddies to exchange nutrients and dissolved gases with their environment, corals maintain increased rates of photosynthesis and respiration even when there’s hardly any flow.
“The general thinking has been that corals are completely dependent upon ambient flow, from tides and turbulence, to enable them to overcome diffusion limitation and facilitate the efficient supply of nutrients and the disposal of dissolved waste products,” Shapiro says. All six of the coral species can create turbulent flows around them, and while these findings don’t speak for all corals, “the retention of cilia through 400 million years of evolution suggests that reef corals derive a substantial evolutionary advantage,” he adds.
Besides transforming the way we perceive the surface of reef corals -- from a stagnant boundary layer to a dynamic, actively stirred environment -- the findings could also shed light on cilia in general. These lash-like structures are found in a wide range of hard-to-study environments. In humans, they help move ova along the fallopian tubes and sweep away containments in our airways.
Images: Shapiro et al. via MIT News Office