Using the whole body as a sucker: combining respiration and feeding with an attached lifestyle in hill stream loaches (Balitoridae, Cypriniformes).

Abstract

Small fishes living in fast-flowing rivers face a harsh environment as they can easily be swept away by the rapid currents. To survive such circumstances, teleosts evolved a wide variety of attachment mechanisms, based on friction, negative pressure or both. Balitorinae (Balitoridae, Cypriniformes) are exceptional in using their whole body as an adhesive apparatus. We investigated the morphological adaptations of Balitorinae by studying the osteology and myology of four species (Beaufortia leveretti, Sewellia lineolata, Pseudogastromyzon myersi, and Gastromyzon punctulatus) using clearing and staining, serial cross-sections and CT-scanning. A kinematic analysis was performed to study the respiration and feeding mechanisms and to identify key structures in these mechanisms. Our research showed that the whole body of Balitorinae acts as a suction disc, with friction-enhancing structures (unculi) on the thickened anterior rays of the paired fins. The abruptly rising head profile, supported by the extremely enlarged lacrimal bone and the flat ventral body surface facilitate effective substrate attachment. During attachment, the pelvic girdle is pulled anterodorsally, suggesting the formation of a negative pressure underneath the body. Detachment by water inflow underneath the body is prevented by three mechanisms. 1) Barbels control the water inflow by detachment and reattachment to the substrate. 2) Most water present underneath the body is removed during inspiration. 3) Excess water is regularly removed by movements of the posterior pectoral fin rays. The balitorine body is thus modified as such that it allows effective attachment, while not impairing respiration. Comparison with other teleosts living in similar environments shows that most species use more locally concentrated modifications of the paired fins and/or the mouth for attachment. The high diversity in teleostean adhesive apparatuses and associated myological modifications suggest a substantial functional convergent evolution, without necessarily highly convergent anatomical adaptations.