When Aesop penned his famous tale of the tortoise and the hare, he probably wasn't thinking about the waterfall-climbing gobies of Reunion Island. Not one, but two species of Sicydiinae gobies call this place home, embarking on an arduous journey upstream (literally!) to find safe, freshwater pools in which to spawn. Slow and steady, Sicyopterus lagocephalus inches its way up, secured to the slick rocks by sticky suckers on its belly and mouth. In contrast, Cotylopus acutipinnis leaps ahead by flicking its tail and pectoral fins, much like Aesop's hare. Unfettered by its oral sucker, C. acutipinnis covers a lot of ground in a short time, but takes long rest periods between leaps. Considering these radically different approaches to life's obstacles, Raphaël Lagarde, currently a postdoctoral fellow at the University of Perpignan, France, and colleagues from Reunion Island, Madagascar and Canada wanted to know which goby was the better mountaineer and why.
Racing requires a racetrack, so the researchers set a 2.4 m ramp at a 45 deg angle, lined it with sandpaper and pumped water over it. Both gobies managed to climb the simulated waterfall, but overall, the tortoise-like S. lagocephalus climbed three times faster than the hare-like C. acutipinnis. Though C. acutipinnis could easily out-sprint S. lagocephalus, it wasn't enough to make up for the extra time spent resting: C. acutipinnis spent over 90% of its time sitting motionless, whereas S. lagocephalus only spent about two-thirds of its time on breaks.
Next, the researchers focused on the muscle powering their gobies’ movements, hypothesizing that their physiology would match their climbing style. They predicted that slow and sustained exercise, like the steady inching of S. lagocephalus, would be powered by aerobic pathways. In contrast, the fast and explosive ‘power burst’ style of C. acutipinnis should rely on anaerobic pathways. Supporting these predictions, the slowly inching S. lagocephalus had higher activities of enzymes involved in carbohydrate metabolism – citrate synthase, pyruvate kinase and lactate dehydrogenase – than C. acutipinnis. The muscles of S. lagocephalus had bigger metabolic engines for aerobic activity – exactly what they needed to keep on keeping on.
Finally, the researchers examined the fuels needed to power the muscle's metabolic engine: glycogen and fats, as well as lactate, which is produced when muscles dip into anaerobic pathway. Both gobies had similar amounts of glycogen and fats in their muscle, suggesting that running out of oxygen didn't explain the lengthier pauses of C. acutipinnis. Both species also kept lactate levels low, even after an intense climb, suggesting that their chosen rest periods were enough to avoid overexerting their muscle. The data also hinted at an interesting aspect of goby life history – neither species had impressive fuel reserves, suggesting that wild gobies probably snacked on their way to the top.
Slow and steady wins the race, whether you are a tortoise or a waterfall-climbing goby. Sicyopterus lagocephalus crawled across the finish line ahead of C. acutipinnis, supported by a more sustainable form of locomotion and more powerful metabolic machinery. As both species search for breeding sites throughout the year, the quick pace of S. lagocephalus may lead to competitive advantage. Like another old proverb, perhaps the early fish gets the pool.