Species: Sicyopterus stimpsoni

Range: Seemingly inaccessible high pools of Hawaiian streams.

Weird Feature(s): Scales up the slipperiest imaginable waterfalls with its mouth and bizarre pelvic suction cup.

People are regularly astounded when various fish species muscle their way up fast flowing rivers to spawn. And while the salmon is usually the poster fish for this impressive feat, there is one fish that makes those rapid runners look lazy. In order to return to the high breeding pools of Hawaiian mountain streams, Stimpson’s goby actually scales up the slippery rock faces of waterfalls using its mouth as one half of an amazing climbing mechanism.

The goby’s life starts when it wriggles free of its egg in a high pool of a mountain stream. Lacking the strength to fight the stream’s current, it is promptly swept down cascading waterfalls and into the sea. After feeding on plankton for this first stage its life, its mouth eventually changes its shape for a new diet–and a new mode of locomotion. The goby’s mouth shifts downwards to form a sucker which it can use to both cling onto slippery vertical rocks and scrape tasty diatoms off of them as it ascends towards the breeding pools high in the stream. The modified mouth is so good at performing both of these tasks that researchers said in a paper published in PLOS ONE that it remains unclear which purpose the modified mouth initially evolved for. 

The mouth sucker actually works in tandem with a second sucker made of modified pelvic fins on the underside of the fish’s body. Stimpson’s goby switches between using this suction cup and its mouth-sucker as it inches up waterfalls.

A specific set of muscles is responsible for manipulating this pelvic sucker to hold the fish in place when necessary. A separate study published in PLOS ONE examined these muscles in specimens who had made it to varying heights within a stream. What they found was that those individuals who had made it farther up the stream had far more red muscle fiber behind their pelvic suction cups. Prime real estate goes to the muscle heads.

A Stimpson’s goby using both suckers to make it’s way up a smooth surface. (LiveScience)

The gobies can climb waterfalls up to 350 meters tall–that’s 10,000 times the length of their bodies. “For a human to go the equivalent distance based on body size, it’d be like doing a marathon, some 26 miles (42 kilometers) long, except climbing up a vertical cliff-face against rushing water,” said researcher Richard Blob, an evolutionary biomechanist at Clemson University in an interview with LiveScience. Blob was a co-author of both of the aforementioned studies on Stimpson’s gobies.

That’s right, these little fish do waterfall marathons.

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HT: PLOS One, NewScientist, LiveScience

IMAGES: Sucker of a zwartbeck goby by Peter van der Sluijs

VIDEO: Fitness freak fish climbs waterfall by NewScientist, Certain Fish Cam Climb Up Waterfalls by LiveScience