Hawai'i and its sister island to the south, Johnston Atoll, because of their remarkably clear waters, have the world's deepest known photosynthetic corals.

The clarity of the water is important, because clear water lets light travel deeper. Johnston actually has these corals as deep as 500 feet, and in Hawai'i they have been found as deep as 460 feet, to depths where light is just 1 percent of its intensity on the surface.

This is a mysterious part of the ocean — beyond the depth most divers can reach — and much of it is being slowly probed by deep submersible craft.

University of Hawai'i oceanographer Samuel Kahng and U.S. Fish and Wildlife Service coral expert Jim Maragos recently published the results of research that has identified three species of corals in the deep water, all within the genus Leptoseris. One of them may be new to science.

"These Leptoseris corals are low-light specialists," Kahng said.

The ocean's fastest-growing corals are ones that are able to include in their structures single-celled algae called zooxanthellae. The algae use sunlight to make energy, and their waste products are used as food to help the coral polyps grow faster. Most such corals are within skin-diving depths — the corals you see snorkeling on Hawaiian reefs — where there's lots of sunlight.

"Our recent surveys using the Hawai'i Undersea Research Laboratory Pisces V manned submersible have found pristine reefs consisting of plate corals carpeting the bottom at 180 to 270 feet and creating habitat structure for a diverse host of fish and invertebrates," Kahng said. "At the lowest depths, they are stunted, smaller, at the limit of their habitat."

The deep Leptoseris corals are thin and broad and aimed directly at the surface. They are exceedingly fragile, since there is limited wave energy at those depths.

"The common name 'plate' coral refers to their shape, which often resembles a dinner plate, which is oriented to capture maximum downwelling light," Kahng said.

There are corals growing deeper than Leptoseris, but they are slow-growing species that don't have zooxanthellae and thus don't require light.

"This fall we have a HURL deep-water expedition of the 'Au'au Channel (between Maui and Lana'i) scheduled and we hope to gather additional samples of these corals and to learn more about their exceptional biology and ecology," Kahng said.

If you have a question or concern about the Hawaiian environment, drop a note to Jan TenBruggencate at P.O. Box 524, Lihu'e, HI 96766 or jant@honoluluadvertiser.com. Or call him at (808) 245-3074.