Species evolve extreme adaptations to contend with the rigors of their environment. Microbes thrive in Yellowstone’s boiling springs; kangaroo rats can live without ever taking a sip of water in the scorching U.S. Southwest; monarch butterflies can journey 3,000 miles just to enjoy a bit of Mexican sun. And to colonize new habitats, determined little fish climb their way up Hawaiian waterfalls with their suction-cup mouths, like reverse wall ball toys.
This superhero-like fish, the Nopili rock-climbing goby, are members of the Gobiidae family. Gobies are one of the largest fish families in the world, with around 2,000 species sharing the nomenclature. Many live in streams on volcanic islands like Hawaii, where life isn’t easy for these small, versatile fishes. Their stream homes are often subjected to lava flows, hurricanes and flash floods—and yet still the gobies persist. After a landslide creates vertical obstacles or a storm reworks the flow of water, those tenacious fishes inevitably turn up in the new upstream habitat, even if it’s separated from downstream areas by steep waterfalls.
To achieve this feat, gobies rely upon their singular evolution. All gobies possess an abdominal sucker, formed millennia ago when ancestral gobies’ pelvic fins fused. To scale waterfalls, most of the fishes use so-called “powerburst” climbing. They attach themselves to the slick wall behind a waterfall using their ventral sucker then rapidly undulate their tails, resulting in a shimmying action that slowly propels their flailing, sucking bodies up the waterfall.
The Nopili rock-climbing goby takes wall sucking to another level, however. Its mouth, located on the underside of its head like a cleaner fish, forms a formidable sucker that it uses to inch its way up waterfalls. Like a mountain climber, it takes turns sticking to the wall with its abdominal and oral suckers, slowly but steadily making its way up the sheer, wet rock. In Hawaii, these fish climb waterfalls up to nearly 330 feet tall.
So how did the Nopili rock-climbing goby pull off this super sucker mouth adaptation, while all of its relatives have to make due with just a measly abdominal sucker? The clue likely lies in the species’ feeding strategy. Most other gobies feed on small invertebrates or other fish, but the Nopili rock-climbing goby prefers to scrape tiny bits of algae, called diatoms, off rocks using a mouth-sucking motion mirroring the same movements it uses to climb walls. For an evolutionary scientist, this begs the question of whether the fish’s mouth first evolved for eating and then shifted to wall sucking, or vice versus. Scientists call this kind of evolutionary co-opting “exaptations.”
Researchers from Clemson University and Saint Cloud State University wanted to crack this chicken-or-egg puzzle, so they decided to compare the oral mechanisms of feeding versus climbing in the Nopili rock-climbing goby. If the fish uses its mouth in two very different ways for feeding and climbing, then exaptation likely wouldn’t deserve credit for the interesting adaptation. On the other hand, if the same movements were at play for both activities, then the fish may have simply applied a regular activity (feeding or climbing) to a new role.
The researchers donned snorkeling gear and scooped up several wall-sucking gobies from Hakalau stream on Hawaii Island, in 2005 and 2011. They transferred the fish to a lab, where they observed several of their research subjects’ feeding patterns using high-speed video cameras attached to different sides of the aquarium. In another tank, the researchers also created a faux waterfall using Plexiglas situated at a 62-degree angle. They challenged the remaining fish to climb this wall and filmed those fish during their waterfall-scaling exertions.
From the videos, the researchers identified 23 anatomical landmarks involved in feeding and climbing. After statistically analyzing their results, the researchers report in a PLoS One paper published last week that they found that the Nopili rock-climbing goby’s climbing and feeding movements differed significantly. In other words, the fish are using different movements for feeding and for climbing. The differences, however, were small, and some of the behaviors were so similar they could nearly be superimposed. The puzzling combination of similarities and differences stumped the researchers, and they realized that more investigations would be needed before they could definitively tease out the species’ evolutionary history of feeding and sucking.
“However, strict similarity between feeding and climbing kinematics might not be a fair expectation, even if exaptation had occurred,” they write in their paper. “It may not be reasonable to expect patterns for one behavior to remain completely unchanged after being applied to another function.”
Like most things in science, goby evolution is complex and may not prescribe to a clear-cut explanation. “Exaptation with modification” may have to suffice for elucidating the Nopili rock-climbing goby’s unique talents—at least for now.