There is not a lot out there in the literature about 'o'opu and there are still a lot of aspects of their basic biology and ecology that are largely unknown." What scientists do know, though, is fascinating. Only five native species of 'o'opu have evolved to tackle Hawaii's ragged jumble of freshwater streams and four of them have evolved the remarkable ability and drive to rock climb.ĭespite this, said Grabowski, "Hawaiian freshwater fishes are very understudied. Here, streams are steep and short, and prone to flooding. The volcano acts like a giant wall in the open ocean the only surface against which clouds collide after more than 4,000km of nothing but Pacific heading west from Mexico. This region is the steep backside of majestic Mauna Kea, at 10,211m, the tallest mountain in the world when measured from its underwater base. North of eastern-facing Hilo, it's on the youngest of the main Hawaiian Islands only around 500,000 years have passed since it emerged from the sea. With more than 200cm of annual rainfall, Hawaii Island's verdant Hāmākua Coast drips with waterfalls. And each species appears to prefer a specific habitat along a stream's length, with the most impressive waterfall climbers preferring the most remote and inland pools as adults. These fish are small, unassuming and usually brown and camouflaged, mottled or striped (though the male of one species, 'o'opu 'alamo'o, is a notable exception as it can be half black and half bright orange during spawning season). Scientists say 'o'opu are mostly endemic, understudied and threatened by development around their native stream habitats. Four species of goby and one species of sleeper goby (a family related to gobies but that lacks the fused pectoral fin common to all gobies) comprise the remote island chain's only native true freshwater fish. 'O'opu – the Hawaiian word for fish in the goby family – most often refers to several species of unusual freshwater fish that navigate into inland waterways, some by scaling the islands' waterfalls using their mouths and suction-cup shaped fused pelvic fins. And, yet, upon reaching the stream and looking into the clear brown water, not a single finned creature moved. "'O'opu are pretty easy to detect with a decent pair of polarised sunglasses," Grabowski said.
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