|The Nopoli Rock-Climbing Goby. Image credit: msnbc.com|
Most gobies are 'amphidromous', meaning that they migrate between the ocean and freshwater. Adult fish mate and lays eggs in freshwater streams; after the eggs hatch, larvae are swept downstream into the ocean where the larval gobies mature to the juvenile stage before returning to freshwater to continue the cycle. Traveling upstream is challenging for gobies, however, as they are often required to scale large waterfalls before reaching freshwater.
Fortunately, evolution is on their side: gobies have a sucker on their bellies which is formed by the fusion of pelvic fins and facilitates gobies' extraordinary vertical mobility. The pelvic sucker is used to attach the fish to a rock face. While climbing, gobies move their pectoral fins towards their body and undulate their bodies to propel themselves upwards before reattaching to the rock with their pelvic sucker. This series of movements- known as 'powerburst' climbing- is repeated until the gobies reach the top of the waterfall.
|Gratuitous picture of a waterfall. Image credit: travelblog.org|
Goby species of the genus Sicyopterus are unique in that they possess an extra mouth sucker which they employ when scaling waterfalls during their journey upstream. These species use a different form of locomotion known as 'inching' when climbing up waterfalls, alternately using their pelvic and mouth suckers to 'inch' their way up a waterfall while remaining firmly attached to a rock face. Although their progress is slow, inching accounts for S. stimpsoni's remarkable ability to climb waterfalls up to 300 feet in height.
Footage of the Rock-Climbing Goby doin' its thing (but not breaking any records for speed) Video credit: Live Science
But the differences don't end there. Species of the genus Sicyopterus also have a feeding strategy which differs from other gobies. While in the ocean, Sicyopterus larvae subsist on a diet of plankton which they capture in their forward-facing mouth suckers in a manner similar to other gobies. As they mature, however, the mouth sucker is re-positioned on their bellies and the Sicyopterus species switch to a 'benthic' feeding mode where the mouth sucker is used to scrape algae off of rocks. The transition to benthic feeding occurs at approximately the same time the goby begins travelling upstream.
Researchers led by Richard Blob of Clemson University hypothesized that these two characteristics, unique to Sicyopterus gobies, may be related. Blob and his team studied the movements of the oral sucker in Sicyopterus stimsoni during feeding and climbing, and their results were published yesterday in the journal PLOS One.
The jaw muscle movements of S. stimsoni while feeding and climbing were overall quite similar, they found. Their results bring us to a chicken-and-egg conundrum: which came first, the feeding or the inching?
Let's take a moment to marvel at what these fish- and evolution, for that matter- are capable of. Blob was quoted in an article as stating that "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". Impressive, to say the least.
For S. Stimsoni, found throughout the Hawaiian islands, navigating freshwater streams in volcanic islands poses a unique challenge to gobies, as there is the constant threat of hurricanes, floods, and volcanic lava flow to be reckoned with- all events that could easily wipe out an entire population of fish. The gobies' life cycle helps them to avoid a catastrophic blow to their numbers because they depart from freshwater long enough to grow and mature in the ocean before returning. Even if a goby population is injured by a catastrophic event upstream, it is easily replenished by young gobies after their brief stint in the ocean.
Extreme environments such as those inhabited by gobies place an intense selective pressure on an organism for adaptations which help them survive the stresses and challenges of living in an unstable environment. Gobies have managed to thrive in these areas thanks to such adaptations- namely, their life cycles and their pelvic suckers. So where does that leave the Sicyopterus gobies?
According to Blob and his team, the mouth sucker on Sicyopterus gobies such as S. stimsoni may be an example of an evolutionary phenomenon known as exaptation. Unlike an adaptation, which is a trait that increases the fitness of an organism, an exaptation is something of an evolutionary afterthought. The term exaptation was first coined by Stephen Jay Gould and Elisabeth Vrba in a 1982 publication on the subject. Gould and Vrba define exaptations as "characters evolved for other usages (or no function at all)...later coopted for their current role" (Gould and Vrba, 1982).
In the PLOS One article, the authors mention the difficulty of proving a trait as being an 'exaptation' and point to criteria established for formally identifying exaptations. "The first step in such an assessment", the authors write, "is to evaluate whether oral movements for climbing and feeding should be considered as the same trait, based on the extent of their similarity".
While the researchers did observe several differences between the two activities, they speculate that they may have more to do with the directionality of the goby's movement or had little effect overall on the general motion of the goby while feeding or climbing. Their findings ultimately seem to suggest exaptation- or "exaptation with modification" of activity between feeding and climbing. The authors take care to point out that "given the strength of selection that appears to operate on both feeding and climbing performance in climbing gobiids, it may not be reasonable to expect patterns for one behavior to remain completely unchanged after being applied to another function"(Cullen et. al., 2012).
At this point, it would be hard to determine which function- feeding or climbing- was co-opted for the other, as there is no instance of the oral sucker being used for one activity and not the other among the Sicyopterus gobies. Nonetheless, these findings help to establish the possibility that feeding and climbing in Sicyopterus gobies may be the result of an evolutionary phenomenon which is often downplayed.
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
Gould SJ, Vrba ES (1982) Exaptation- A Missing Term In The Science of Form. Paleobiology, Vol. 8, No. 1, 4-15.