Deep in the rock of the Islands, in dark crevices where groundwater flows toward the sea, life is evolving into new forms.
(Photo: These 'ōpae 'ula from Kalaeloa on O'ahu are not as deeply red-colored as ones from Kona. Scott Santos photo.)
Among them are tiny red shrimps, which come into the light only in the rare ponds and anchialine pools where caves and cracks open up to the sunlight. But they are also found in wells dug around the islands, indicating that they inhabit the dark areas as well.
These little crustaceans, less than a half-inch long, are known as 'ōpae 'ula in Hawaiian and Halocaridina rubra to science. They graze on the bacteria and algae that line the walls of their watery habitats. They have been mainly found in lava areas, but have also been located in pools in limestone, like the once-common sinkholes of the 'Ewa plain.
They are generally found near the coast, living in waters that are close enough to the ocean to sometimes have some salt content. The shrimps seem to tolerate a range of salinities.
Researchers say Hawai'i is the perfect place to study these creatures, which some folks keep as aquarium pets, because it has the most extensive anchialine habitats in the world. Auburn University shrimp researcher Scott Santos said he and his team are looking into the origins of some of the shrimp being sold on the internet and in stores as aquarium curiosities.
Earlier this year, researchers Jennifer Ivey and Santos of Auburn University, reported the complete mitochondrial DNA of the shrimps. They reported, in the genetics journal Gene, that this is a step in making it easier to study the population structure of the shrimps and for planning for their conservation.
More recently, researchers looking into the genetics of the shrimp have found that while they are located throughout the Hawaiian underground world, the 'ōpae 'ula do not appear to mix much once they're established.
A study, to be published in 2008 in the journal Limnology and Oceanography, looked at the genetics of 573 shrimp found at 34 different sites on three islands—Hawai'i, Maui and O'ahu.
As you might expect, they found that the ones nearest to each other were likely to be closely related, and those on different islands or far away on an individual island are likely more distantly related. They appear to form geographically isolated breeding groups.
The research found 13 genetic groupings, which were associated with eight “lineages.”
On the Big Island, the shrimp of west Hawai'i are distinct from the ones on the east side of the island, and there's another group in east Ka'u. On Maui, the ones at south Maui and different from the ones near Hāna. On O'ahu, the windward side's shrimps are distinct from those off Wai'anae and the ones at 'Ewa are different still.
The only multi-island lineage was that the shrimp of west Hawai'i are closely related to shrimp found at O'ahu's Kapapa Island in Kāne'ohe Bay.
Clearly, in order to take up residence on a new island, the shrimps must have some ability to disperse across the ocean. But they apparently don't do so often, and the ocean seems to represent a significant barrier. Metaphorically speaking, the ocean for them is a wall you might be able to climb in extraordinary circumstances, but which you seldom do.
Researchers suggest that the Kapapa Island 'ōpae 'ula manage to remain genetically distinct from the ones nearby on O'ahu because they are protected by the ocean barrier.
Within an island, the researchers identified mechanical barriers to transport—essentially features in the rock of the islands that prevent shrimps from one region from communicating with shrimps from another.
The 'ōpae 'ula, thus, are trapped between oceanic and geological barriers, and left to evolve independently in their subterranean domain.
An extensive team of researchers has been studying these cryptic critters. The co-authors of the Limnology and Oceanography article will include among them Jonathan Craft of Auburn University, Atlantis Russ of the University of Hawai'i at Hilo Tropical Conservation Biology and Environmental Sciences Program, Mike Yamamoto of the state Division of Aquatic Resources (DAR), Thomas Iwai Jr. of DAR's Anuenue Fisheries Research Center, Skippy Hau of DAR, John Kahiapo of DAR, Charlie Chong of Lanakila Learning Center in Hilo, Sharon Ziegler-Chong of UH-Hilo's Pacific Aquaculture and Coastal Resources Center, Cam Muir of UH's Biology Department, Yoshihisa Fujita of the University of the Ryukyus, Dan Polhemus formerly of Bishop Museum and now head of DAR, Robert Kinzie III of the Hawai'i Institute of Marine Biology and Santos of Auburn University.
© 2007 Jan W. TenBruggencate