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Fish Evolved to Climb Waterfalls with Its Mouth

By Alex B. Berezow

One of the most iconic images of the Pacific Northwest is the salmon run, when salmon return from the ocean, swimming upstream to their place of birth in order to spawn. This requires tremendous physical effort, such as jumping up waterfalls. The salmon which make it all the way home spawn and die. And those are the lucky ones. On the contrary, the journey for the unlucky salmon ends very, very badly.

Salmon aren't the only fish to swim upstream against the raging forces of nature. In Hawaii, Sicyopterus stimpsoni (a type of goby) also makes a journey from the ocean to freshwater, but not for the purpose of spawning. During the trip, it is not uncommon for the fish to scale up 100-meter waterfalls. Unlike salmon, they don't do this by jumping; instead, they use their mouths as a sort of suction device. S. stimpsoni uses oral and pelvic discs to latch on to the underlying rock, and it inches its way upward in this manner. How exactly such a strange method of locomotion evolved was the subject of a recent PLoS ONE paper.

In addition to climbing waterfalls, S. stimpsoni also uses its mouth to scrape algae off rocks. This method of feeding in many ways resembles the mechanism by which the fish climbs waterfalls. So, the authors of the study wanted to determine if the fish coopted its feeding mechanism toward an ability to swim upstream -- an evolutionary process known as exaptation. (Perhaps the most prominent example of exaptation is the evolution of the feather: Birds may have once used them for insulation and only later used them for flight.)

To determine this, the authors compared anatomical and physiological features of the fish as it ate or climbed. (See figure.) If the biomechanical mechanisms strongly resemble each other, then this would imply that S. stimpsoni coopted its feeding mechanism for climbing (or vice versa). If the mechanisms do not resemble each other, then this would imply that exaptation didn't take place.

The results demonstrated many similarities between the feeding and climbing mechanisms used by the fish, but there were some very major differences (e.g., the angle of retraction of the hyoid bone). Thus, the authors were unable to conclude if the fish repurposed its feeding ability for climbing, but suggested "exaptation with modification" was likely. Regardless of how this ability evolved, it is clear that S. stimpsoni has found a unique way to survive in an extreme environment.

Or as Ian Malcolm would say, "Life finds a way."

Source: Cullen JA, Maie T, Schoenfuss HL, Blob RW (2013) Evolutionary Novelty versus Exaptation: Oral Kinematics in Feeding versus Climbing in the Waterfall-Climbing Hawaiian Goby Sicyopterus stimpsoni. PLoS ONE 8(1): e53274. doi:10.1371/journal.pone.0053274

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