Abstract
We document two profoundly different specialized swimming modes in two ancient lineages of water scavenger beetles (Hydrophilidae): (i) the upside-down swimming using middle legs in the species-poor Amphiops and (ii) the dorsal-up swimming using middle and hind legs in the species-diverse lineage of all other actively swimming taxa, including Berosus analysed here. Both lineages share a unique modification of the mesofurca supporting the leg swimming movements, indicating a single origin of swimming. By behavioural experiments and biomechanical analyses, we reveal that the swimming of Amphiops is optimized for high manoeuvrability, whereas that of Berosus for speed and acceleration. Both swimming modes differ in the form of the meso- and metathoracic skeleton and leg musculature, excluding the possibility that one is derived from the other. Behavioural experiments indicate that both modes are adaptive morpho-functional peaks and that intermediate modes are suboptimal. This aligns with the phylogeny-based model comparison that indicates that both swimming modes have evolved from an ancestral swimming lost in modern beetles. The multi-method approach helps us to reconstruct ancient behaviour and identify trade-offs that shaped the evolution of lifestyles in Mesozoic aquatic beetles.