Claim: Pelvic fin structures can be extremely specialized in actinopterygians, with some fish modifying their pelvic fins into a sucker disk that allow them to adhere to the substrate or climb structures, such as waterfalls.
Evidence: Pelvic fin structures can be extremely specialized in actinopterygians. Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. Gobiids and lumpsuckers modify their pelvic fins into a sucker disk that allow them to adhere to the substrate or climb structures, such as waterfalls.