New research published in the journal Coral Reefs revealed unexpectedly high growth rates for deep water photosynthetic coral. The study measured growth rates of deep water Leptoseris corals in Hawaii and found growth rates between 1in – 0.3in or (25mm – 7.5mm) per year.
Very little is known about coral growth rates in the mesophotic zone, an area in the ocean below 130 feet (40m) to 500 feet (110m). At the lower end of their depth range, the sunlight available to the Leptoseris species from this study is less than 0.2% percent of surface light levels.
The new study, led by Samuel Kohng, from the University of Hawaii Department of Oceanography, alters the assumption that deep corals grow extremely slowly. Until recently a single species of Leptoseris fragilis collected at 60m in the Red Sea was used at the growth standard for deepwater photosynthetic coral species, recorded at 0.04 inches (1mm) of growth per year.
Accessing these depths requires technical diving equipment, submersibles, or scientific ROV’s, making research and observations scarce. For this study the team collected four Letptoseris colonies in the Au’au Channel, Hawai’i using HURL’s Pisces IV/V submersibles.
They found one species, in particular, Leptoseris hawaiiensis had growth rates between 1 inch (25mm) per year at 225 feet (68m) and 0.3 inches (7.5mm) per year at 360 feet (110m) depth.
Maximizing light absorption
The research team found that these low light, deep water specialists employ an interesting strategy to dominate their preferred habitat. Their thin skeletons and plate-like shape allow for an efficient use of calcium carbonate to maximize surface area for light absorption while using minimal resources to form their skeleton. These thin corals only grow radially outward, not upward, and do not thicken over time like encrusting or massive corals.
“Additionally, the optical geometry of their thin, flat, white skeletons form fine parallel ridges that grow outward from a central origin,” said Kahng. “In some cases, these ridges form convex spaces between them which effectively trap light in reflective chambers and cause light to pass repeatedly through the coral tissue until it is absorbed by the photosynthetic machinery.” [SOEST]
Richard Pyle and researchers dive deep to study coral growth over 250 feet down. Published March 2011 – READ MORE
Researchers try to understand how fields of Leptoseris can grow at 300 feet deep. Published May 2011 – READ MORE