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Potential effects of body size changes on locomotor performance in gobiid fishes (family Gobiidae) were studied in two sicydiine species, Sicyopterus stimpsoni from the Hawaiian Islands and Sicydium punctatum from Dominica, West Indies. These taxa both possess a pelvic sucking disc that can be used to adhere to rock surfaces during bouts of waterfall climbing. If this disc scaled isometrically as body size increased, climbing might be impaired in larger fish because body mass increases faster during growth than the area of the sucker that supports body weight. To test whether the pelvic sucker grows in a way that could help maintain climbing performance in large gobies (and that is distinctive compared to other fins), we evaluated the scaling of sucker area, caudal-fin height, and pectoral-Fm length with respect to total body length in Sicyopterus and Sicydium across a range of body sizes. Sucker area scaled isometrically with body length for both species, suggesting that climbing performance might decline as these fishes grow. Although the pectoral fin scaled differently in the two species, caudal-fin height showed strong positive allometry relative to body length in both species. The differing scaling patterns of sucker area and caudal-fin height relative to body length may reflect an increase in the importance of swimming over climbing in sicydiine gobies as they grow larger. Moreover, similarities in the scaling patterns of gobies from geographically distant islands suggest that species in this lineage may be subject to common ontogenetic constraints on performance. [ABSTRACT FROM AUTHOR]