Deep-sea corals grow faster and are more resilient than expected

Deep-sea Leptoseris corals are growing at unexpectedly high rates despite extremely low light conditions, according to new research. The findings contradict the assumption that deep corals grow very slowly with less light available for photosynthesis.

Leptoseris is a type of coral species that lives in the deep waters of the Indo-Pacific where sunlight is scarce. The corals depend on microalgae called zooxanthellae for oxygen, energy,  and waste removal. 

A team of experts led by Samuel Kahng of the University of Hawaii at Mānoa used  submersible vehicles to collect Leptoseris colonies at depths between 225 and 360 feet in the Au’au Channel. 

The researchers used advanced dating methods to establish the age of each coral skeleton at multiple points along its radial growth axis.

At the deepest end of their range, the Leptoseris species were found to have less than 0.2 percent of the sunlight found at surface levels. 

Less sunlight usually means slower growth for photosynthetic species, and previous studies found evidence that one particular Leptoseris species grows only 0.04 inch per year. In the new study, however, the team discovered that Leptoseris corals are actually growing much faster. 

“Considering the low light environment, the previous assumption was that large corals at these extreme depths should be very old due to extremely slow growth rates,” said Professor Kahng. 

“Surprisingly, the corals were found to be relatively young with growth rates comparable to that of many non-branching shallow water corals. Growth rates were measured to be between nearly 1 inch per year at 225 feet depth and 0.3 inches per year at 360 feet depth.”

The researchers discovered that these deep-sea corals have developed a strategy to dominate their habitats. The size and shape of their skeletons maximize the surface area for light absorption, while minimizing the resources needed to form the skeleton. The thin corals only grow outward, not upward, and do not thicken over time like massive corals.

“Additionally, the optical geometry of their thin, flat, white skeletons form fine parallel ridges that grow outward from a central origin,” said Professor 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.”

The enhanced growth of Leptoseris in low light conditions has important implications for its ability to compete for space and dominate organisms that grow more slowly. 

“It also illustrates the flexibility of reef building corals and suggests that these communities may be able to develop and recover from mortality events much faster than previously thought,” said Professor Kahng.

The study is published in the journal Coral Reefs.

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By Chrissy Sexton, Earth.com Staff Writer