Climb every waterfall! To reach competitor-free fish nirvana, Hawaiian gobies scale sheer cliffs to reach pools 2,000 feet above the sea

Natural History, Oct, 2004 by Peter T. Sherman, Perri K. Eason

Hawaii is renowned for its coral reefs teeming with rainbow-hued fishes, but one of the state's most remarkable aquatic species lives in freshwater mountain streams on the five largest islands. On the northern side of the Big Island (Hawai'i), for instance, the waters of Hi'ilawe Falls cascade down a cliff more than a thousand feet high. Some day, if you make your way to the top of the falls, take a look in the pool above the precipice. There you will see only one species of fish, a mottled brown goby that grows, on average, to about two and a half inches long. The Hawaiians call it the 'o'opu alamo'o, "the local fish with a lizard-like head." If (apart from its Hawaiian name) this fish doesn't seem very impressive at first, think about the following questions: How did a freshwater fish get there, 2,500 miles from the nearest continental landmass? And how did it get to the top of the Big Island's highest waterfall?

The main Hawaiian Islands began to form about 5.5 million years ago, as the Pacific tectonic plate moved over a "hotspot" in the Earth's mantle that spewed enough lava onto the ocean floor to build islands up from the depths. The five largest islands formed in order--Kauai, Oahu, then Molokai and Maui (originally joined), and the Big Island--as the ocean floor slowly inched across the hotspot and toward the northwest. The Big Island still sits over part of the hotspot, growing daily thanks to Kilauea, one of the most active volcanoes on Earth. None of the island's surface lavas are more than 700,000 years old.

Although lush with life today, the islands were all once as barren as the freshly brewed volcanic rock that is still cooling along the southern coast of the Big Island. Plant spores and seeds that drifted on currents of air or ocean were the first to colonize the newly formed landscape. Animal life followed more slowly. Birds and arthropods were the first to arrive, and with the land to themselves, they evolved into thousands of species that occur only in these islands. No amphibian or terrestrial reptile, and only one terrestrial mammal, the hoary bat, completed the journey on its own. People were the late-comers: not until sometime between 1,000 and 1,500 years ago did they first reach the islands. The original settlers were Polynesians, and with them began the rapid introduction, both intentional and inadvertent, of exotic fauna and flora.

With a fertile imagination, you might think that the ancestors of the goby that now lives at the top of Hi'ilawe Falls had somehow been conveyed there, along with the pool itself. Perhaps, for instance, in the course of geologic time, the 'o'opu alamo'o's ancestors frequented a tidal pool that slowly became isolated from the ocean and then was uplifted by the volcanic forces of island building. But the Hawaiian islands have grown as lava was added to their upper surface; any pool containing fish would be boiled away rather than elevated. The true story of the 'o'opu alamo'o is even more fantastic.

The ancestors of this goby not only had to be athletic enough to make their way to dizzying heights. They also had to overcome what, to most aquatic creatures, is an even more formidable obstacle: the invisible but fundamental barrier between freshwater and saltwater. There are two ways they could have reached their present habitat, and each entails that transition. Either they must have originated as freshwater fish on the mainland or some other island, and then have found a way across open ocean waters. Or, if they were saltwater fish, they had to make a transition from saltwater to freshwater life.

But there's more. It turns out that not only the ancestors, but each new generation of 'o'opu alamo'os has to overcome the same two hurdles. Although the young hatch in the upper streams and pools, they are soon swept downstream and over waterfalls to the sea. There they fend for themselves, growing from hatchling to fry in a marine environment. If they make it through that hurdle, they find their way to an estuary, make the transition to freshwater, and finally undertake the arduous climb to the tops of the waterfalls. They accomplish this journey through some remarkable adaptations as well as single-minded effort. But it's all worth the trouble: in contrast to the return of spawning Pacific salmon to the stream of their birth, a goby's arrival brings not death from exhaustion, but a quiet adolescence and adult life.

The 'o' opu alamo'o (Lentipes a con-color) is one of just five species of freshwater fish native to the Hawaiian Islands. Three of the others are also gobies, and the fifth is a close goby relative called a sleeper, because of its sluggish behavior. That distribution is consistent with the pattern on most of the world's oceanic islands. Gobies are the fishes most often encountered in island streams, and in Pacific island streams, they are typically joined by sleepers. Yet most gobies and many sleepers in the world are marine. The most likely explanation is that the ancestors of the Pacific islands' native freshwater fishes were all saltwater fishes.