NIHOA, Northwestern Ha-waiian Islands — Scientists aboard the research ship Hi'ialakai are finding that Hawai'i's isolation may not be as complete as others have thought.

There are biological connections — the existence in these Islands of corals from the South Pacific, fishes from Japan — that may once have seemed impossible. Solving that puzzle is one of the missions of the National Oceanic and Atmospheric Administration ship on its 25-day mission to Nihoa, French Frigate Shoals and Gardner Pinnacles in the Northwest Hawaiian Islands, and remote Johnston Atoll, 500 miles to the south.

Marine ecologist Scott Godwin is fascinated with a golden-brown crab with blue spots that lives inside table corals and can't live anywhere else. It, and the table corals it lives on, are found on several of the Northwestern Hawaiian Islands, and also on Johnston Atoll.

Godwin said researchers are suggesting that table coral larvae made the 500-mile passage from Johnston to French Frigate Shoals and became established, and that sometime later, the crab larvae also made the trip, found the corals they needed and also became established.

Marine researcher Malia Rivera conducted genetic tests on Hapu'upu'u, the immature stage of a grouper species, and found that the genetic diversity of the groupers at French Frigate Shoals is higher than on surrounding islands. One cause could be that it is a population in contact with other populations outside Hawai'i, and Johnston seems the most likely candidate.

"Genetic diversity is a function of population size — more population, higher diversity," said Hawai'i Institute of Marine Biology scientist Brian Bowen. "You could have higher population, or alternatively, you could be having an influence from somewhere else."

That and other previous research seems to suggest a pretty robust connection between Johnston and French Frigate.

Other biological evidence seems to indicate just that, said Randy Kosaki, chief scientist on the Hi'ialakai mission. Perhaps it is a seasonal current that runs north from Johnston, or even a current or wind pattern that happens infrequently, but enough that it occasionally carries Johnston larvae to French Frigate Shoals — the closest point in Hawai'i to Johnston.

Understanding that connection is important from a pure scientific standpoint, but also because an ocean highway for marine organisms might not only bring benign or helpful species. It also could bring disease.

And it may already have, said coral biologist Greta Aeby, of the Hawaii Institute of Marine Biology at Coconut Island and U.S. Geological Survey wildlife veterinarian Thierry Work.

They are studying several marine diseases, including something called white syndrome in corals. It's found at French Frigate and Johnston, but also in the Marshall Islands, American Samoa and Australia. Little is known about it or its cause.

"We're trying to figure out how disease is transmitted across the ocean," Aeby said. "It's just now showing up around the Pacific, but maybe it was there earlier and we just didn't see it."

Johnston is perhaps the most isolated atoll in the world, and a kind of midocean stepping stone for marine life. It has some species of South Pacific corals, fishes and other kinds of life that are not found in Hawai'i, but it also has some that are, several of them found only in the middle of the Hawaiian archipelago but not at the ends. That seems to suggest that some species are comparatively recent immigrants.

There are worrisome issues for the main Hawaiian Islands, Work said, such as the fact that there was a disease of sea urchins at Johnston in 2002 that wiped out large numbers of them. The Johnston populations of urchins are recovering, but reefs in the Caribbean hit by the same disease have not recovered.

"If that got here, it could be a lot of trouble. If you lose your urchins, you get algal overgrowth in your habitats, and you can get major ecological shifts," Work said. "Disease in wildlife is like canaries in coal mines. It behooves you to understand what's going on."

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.