Fishes exhibit a wide variety of shapes and sizes that allow them to perform an impressive range of behaviours, from swimming to gliding and even rock climbing. While a rock-climbing fish may sound odd, numerous fishes do so using climbing gear built into their bodies. Climbing can be beneficial – to avoid predators – but can be risky too, because one slip could be fatal. Consequently, climbers must attach themselves securely to the climbing surface. Some fishes climb with little more than their fins and a dab of mucus, but what makes some species stickier than others? Adhityo Wicaksono, from Abo Akademi University, Finland, and colleagues from several international institutions compared climbing and non-climbing species of mudskipper fishes to evaluate which features might help them ascend trees and rocks in their natural habitats.
Mudskippers are highly terrestrial fish and have numerous adaptations for moving on land, including pelvic fins, which can differ significantly between climbing and non-climbing species. For example, Boleophthalmus boddarti does not climb, yet it has pelvic fins that are fused into a suction disk, whereas Periophthalmus variabilis can climb and has unfused pelvic fins. Pelvic fins can be particularly important climbing structures because mudskippers and other gobies use them to attach to surfaces in between movements. To test the hypothesis that unfused pelvic fins may aid climbing in mudskippers, Wicaksono and colleagues integrated experimental techniques with computer simulations to compare how pelvic fin structure or the mucous secreted by the fins related to the fishes’ climbing abilities. The team measured how well the pelvic fins resisted being dislodged from a surface in different directions, evaluated whether the chemical composition of the mucous surrounding the pelvic fins improved adhesion, and then estimated how much weight the fins could bear.
The authors found that the climbing mudskipper had more flexible pelvic fins than the non-climbing species, which could increase contact and adhesion during an ascent. The chemical composition of the mucous secreted by the pelvic fins was relatively similar across the mudskippers and particularly good at sticking to surfaces that the fish climb in nature, such as rocks. Wicaksono and his team also found that the unfused fins of the climbing species better resisted disturbances parallel to the line of movement, which might help reduce slippage while climbing. In contrast, they found that the non-climbing species was better at resisting disturbances perpendicular to its line of movement, potentially improving its ability to hold on tight to rock surfaces in the surf. Although the climbing species was smaller, it had larger pelvic fins relative to its body size. Smaller bodies can make climbing easier by reducing the weight that the pelvic fins have to support, and may provide additional benefits on land, such as making it easier to maintain their body temperature and prevent dehydration from excessive water loss through the skin.
Together, these results broaden our perspective of how interactions between organisms and their environment can relate to the form and function of physical characteristics. The study also shows that integrating various approaches provides a finer lens for studying the mechanistic causes of biodiversity. And when it comes to climbing with mudskipper fins, it may pay to be more flexible.