You might be asking yourself, how do the O‘opu get upstream? The answer is in the pelvic fins. 4 of the species of O‘opu have a special characteristic that is unique to the goby family. These fish have their pelvic fins (the paired fins underneath the body) fused to form a sort of sucking disc. This allows the fish to “suck” onto rocks in the stream and to maintain their position in fast flowing waters. It also allows some of them to “climb” to higher elevations within the stream. By clinging onto rocks, these fish can actually climb vertically up a waterfall. Isn’t it amazing? O‘opu Alamo‘o and O‘opu Nopili are the two best climbers.
The O'opu on the left (Akupa) has split pelvic fins, whereas the O'opu on the right has a fused pevic fin also known as a pelvic disc. The Alamo'o, Nopili, Nakea, and Naniha have a fused pelvic fin.
Photo by Yamamoto and Tagawa