One of Hawai'i's most prolific producers of calcium carbonate — the stuff coral reefs and sand beaches are made of — is something you might not expect, a green seaweed called Halimeda kanaloana.

It is one of seven known species of Halimeda in the Islands, but this particular one is found nowhere else in the world. It is common on sand fields in the waters around the islands of Maui County, from near the beach to deep water. It has also been found in deep water around O'ahu, but has not been found around the other islands.

It's a unique plant in a number of ways, but one that's quite interesting is its production of rock.

University of Hawai'i marine botanist Heather Spalding, who specializes in the marine life of the deep waters around the Islands, said the plant grows to be about a foot tall with numerous branches made of tiny leaflets strung end to end like a chain. It has a skeleton made of calcium carbonate, and when a piece is broken off, it soon turns into a tiny white flake that in calm water can be part of a field of flaky sand.

It grows in great underwater meadows on the sand slopes around Maui County, creating habitat for all kinds of marine life, including shrimps, urchins, small fishes and more. Spalding said she sees predatory fishes cruising over the Halimeda meadows looking for a meal of the smaller fish hiding in the algae patch.

Another odd feature of this plant and its relatives is that every plant is made up of just one cell — it has no cell walls.

Hawaiian sand beaches are made up of all kinds of bits of material, including broken seashells, the shells of one-celled animals called foraminifera, chunks of broken coral, bits of crusting coralline algae. But it turns out they don't have much Halimeda in them, said Kaua'i geologist Chuck Blay, who has made a study of beach sand around the Pacific.

Blay said that once it starts rolling around in waves or bouncing on the bottom in a current, the Halimeda segments quickly break down to a fine sediment.

"The plant is incredibly productive, but it just doesn't hold up to abrasion," Blay said. Much of the muddy sediment in calm waters around Florida and other parts of the world is made up of Halimeda dust, he said. In some quiet parts of the ocean, Halimeda fields create great mounds of material as the leaves break off and plants die. But in Hawai'i's high-energy environment, less so, he said.

Its value in the Hawaiian environment may be less its production of calcium carbonate, and more the habitat it creates for other species.

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.