By Noah Bressman
Spending time playing around in tropical island waterfalls sounds like a dream vacation for many of us. However, if you are an inch-long fish that needs to climb those waterfalls to reach their breeding grounds, then they become less alluring. Juveniles of several goby species around the world migrate from the oceans to climb hundreds of feet up vertical waterfalls and reach isolated stretches of river that lack predators, allowing them to safely mature and reproduce. According to Dr. Rick Blob’s lab at Clemson University, these gobies achieve this incredible feat through two methods: “powerburst” climbing and “inching”. Powerburst climbers use their pectoral fins and tails to move up waterfalls in short bursts, attaching to the rock face between bursts using pelvic fins that have fused into suction cups. Inchers also have pelvic suction cups, as well as modified mouths that act as suction cups. This allows them to “inch” up the waterfalls by alternating which suction cups are attached.
While several closely-related groups of gobies have evolved the powerburst method, only two species within the genus Sicyopterus have evolved the inching method. Furthermore, these are phylogenetically nested within the powerburst groups, meaning the inching method has evolved more recently from powerburst climbers. Previous work by the Blob lab has shown that there is a wide range of performance for powerburst climbing, but it was unknown how performance varies for inching, the evolutionarily-newer waterfall climbing method. Do more recently evolved traits show less functional diversity than older traits?
To test this, Blob and his team went to the island of La Réunion in the Indian Ocean to study the inching climber S. lagocephalus and the powerburst climber Cotlyopus acutipinnis. They collected these fish, placed them on artificial waterfalls, and filmed them to describe their performance and kinematics –the motions the fish use to move up waterfalls. Blob and his team then compared this data to data they had previously collected on Hawaiian and Caribbean waterfall-climbing gobies.
Overall, Blob and his team found that the Indian Ocean incher and powerburst climber had similar kinematics to the Hawaiian and Caribbean incher and power burst climbers, respectively. The inchers also had similar net performance to each other, climbing up waterfalls at similar speeds, while the powerburst climbers had more variable performance between species. On the surface, this supported Blob’s hypothesis that newer traits would have less time to accumulate evolutionary changes, resulting in less variability than older traits.
However, after looking more closely, Blob and his team found that while both inchers achieved similar net climbing speed, they achieved it through different means. The Hawiian incher used longer durations of slower movement, while the La Réunion species used shorter durations of faster movement. Despite these differences, Blob found that net performance of both of these species were similar to each other and to most of the powerburst climbers. This suggests a many-to-one mapping of structure to function, meaning waterfall climbing gobies use multiple different waterfall climbing techniques to achieve similar performance, similar to how birds and bats can both fly well using different types of wings. Regardless, I’m just glad that I have hands and feet to climb waterfalls instead of having to use my mouth and belly.
Noah Bressman is a PhD Candidate at Wake Forest University studying fish functional morphology, biomechanics, and behavior, with a special focus on amphibious fishes. You can find more at NoahBressman.wixsite.com/Noah or @NoahwithFish, or contact him at NoahBressman@gmail.com.