When the Nopili goby (Sicyopterus stimpsoni) moves from salt water to fresh water, it turns its frown upside down. Over the course of about 2
days, the finger-sized fish's mouth migrates from the tip of the head to its chin. That new
orientation lets the goby do two things: scrape food from rocks and climb up waterfalls. That's right—to get to the upper parts of streams where they
mate, the fish have to drag their bodies up walls of wet rocks. The goby uses its suctioning mouth and a sucker on its stomach to inch upward like a
caterpillar. High-speed video revealed that the fish feed quite differently than other gobies when they scrape diatoms from the rocks, extending their top
jaw way out and not pulling the lower jaw back as much as other species do. The fish's jaw movements were also pretty similar when eating and climbing,
researchers report online today in PLOS ONE. The scientists conclude that this is probably an example of exaptation—when a structure that was
meant for one function is co-opted for another. (For example, feathers might have originally been for insulation, then just happened to come in handy for
flight.) But scientists don't know which function came first, the eating-by-scraping or the climbing-by-sucking.
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