Evolutionary optimization of an anatomical suction cup: Lip collagen content and its correlation with flow and substrate in Neotropical suckermouth catfishes (Loricarioidei)

In riverine ecosystems, downstream drag caused by fast-flowing water poses a significant challenge to rheophilic organisms. In neotropical rivers, many members of a diverse radiation of suckermouth catfishes (Loricarioidei) resist drag in part by using modified lips that form an oral suction cup composed of thick flesh. Histological composition and morphology of this cup are interspecifically highly variable. Through an examination of 23 loricarioid species, we determined that the tissue most responsible for lip fleshiness is collagen. We hypothesized that lip collagen content is interspecifically correlated with substrate and flow so that fishes living on rocky substrates in high-flow environments have the largest, most collagenous lips. By mapping the amount and distribution of lip collagen onto a phylogeny and conducting ANOVA tests, we found support for this hypothesis. Moreover, these traits evolved multiple times in correlation with substrate and flow, suggesting they are an effective means for improving suction-based attachment. We hypothesize that collagen functions to reinforce oral suction cups, reducing the likelihood of slipping, buckling, and failure under high-flow, high-drag conditions. Macroevolutionary patterns among loricarioid catfishes suggest that for maximum performance, biomimetic suction cups should vary in material density according to drag and substrate requirements.