Similar to L. concolor, this O‘opu is rare on the island of Oahu, where its numbers have been greatly reduced. However, unlike L. concolor, it remains abundant on all the other major islands. Among the five species of O‘opu, this species is the most elusive and difficult to observe. It is a good climber, and unlike the other species, prefers faster-moving water found in riffles and runs. Morphologically, S. stimpsoni is well adapted to scraping algae from rock surfaces, and is predicted to be strictly herbivorous. Like L. concolor, S. stimpsoni displays sexual dimorphism during the breeding season. This means that there is a visible difference between males and females. When not engaged in courtship behavior, males resemble females, having a yellow-green, brown, or gray base mottled with brown or black. During courtship, however, the male’s body darkens and it develops an iridescent “racing” stripe down its sides. Spawning occurs in crevices or cavities under large rocks. Males have been determined to exhibit territorial defense of spawning sites, larger males having larger territories. These territories are located in typical adult habitat, usually the swiftest parts of streams in depths of less than 1 meter. What is interesting about this species is that larvae are omnivorous whereas adults are herbivorous. In order to accommodate this shift in diet, an incredible metamorphosis of the head and mouth parts occurs, usually within 48 hours of recruits’ entry into streams. During this transformation, the mouth shifts from the terminal to the sub-terminal position. The upper lip also broadens, resulting in a more robust head. Because S. stimpsoni uses its mouth to cling to rocks and climb, young fish must wait for this transformation to occur before attempting to move upstream.
Photo by Yamamoto and Tagawa