Voyaging canoe Hokule'a anchored at dawn off Laysan Island during its voyage through the Northwestern Hawaiian Islands. Escort vessel Kama Hele is at right, with a dinghy between them. Jan TenBruggencate • The Honolulu Advertiser

University of Hawai'i botany professor Isabella Abbott said it may have been halimeda, which, when it dies and breaks up, is responsible for making much of the sand on the islands' beaches.

At Kure, there was a purple-gray bottom of coralline algae, with only a few coral heads. The reef formed long sand-bottomed channels, caves and arches that provided habitat for hundreds of fish.

The plant life of the northwestern islands also includes what UH-Hilo marine science professor Karla McDermid called "nifty sea grasses." These are grasses and not seaweeds. There are two kinds, Halophila hawaiiana, found only in the Hawaiian archipelago, and Halophila decipiens, found all over the world.

"They are flowering plants. Their flowers look like little crocuses, the size of the fingernail on your little finger," Abbott said.

"To me, they look more like tulips," McDermid said.

The flowers produce pollen, and the pollen floats in the ocean.

The tiny leaves of these seagrasses are only an inch or so long and are shaped like canoe paddles.

"They are growing in meadows," McDermid said. "The turtles love them."

Most of the species of plants at snorkeling depths in the Northwestern Hawaiian Islands are the same ones you might find in the main islands, but as researchers have gone deeper, they've found strangeness.

"There are lots of species that have very interesting distributions," McDermid said.

She and Abbott found that the range of seaweeds is unlike anything found in any similar-size island group.

Some are related to seaweeds from Japan, others to those from the Americas, and there's one very odd seaweed whose closest relatives live in Southern Australia waters.

The Japan connection may be explained by the powerful currents that sweep the waters of the Pacific Ocean around, McDermid said. The north-flowing Kuroshio current, which runs along the coast of Japan, has vast eddies that can reach out as far as the Northwestern Hawaiian Islands.

Eragrostis grass provides habitat for several species of wildlife on Laysan Island. The salty central lake is visible in the background. Jan TenBruggencate • The Honolulu Advertiser

Abbott said there is no easy explanation for finding Australian seaweeds in the Northwestern Hawaiian Islands.

"How it goes from south to north, I have no idea," Abbott said.

"But we haven't collected much from the islands in between. It is possible there is a continuous stream of these seaweeds in between."

One limu, with beautiful red leafy blades, known as Kallymenia, is found in the cold waters of Japan and in the Northwestern Hawaiian Islands, McDermid said.

That's one characteristic of the algae of these islands: There are representatives of cold-water species as well as representatives of warm-water families.

"It's an unusual mixture of tropical seaweeds and temperate species. It is right at the edge of the tropics, so you get a rich diversity," McDermid said.

There are also many endemic species, seaweeds that have evolved in these islands into new forms.

"There are a lot of species that are unique to the Northwestern Hawaiian Islands," McDermid said.