Ancient divergent evolution of specialized swimming modes in aquatic beetles

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 <i>Amphiops</i> and (ii) the dorsal-up swimming using middle and hind legs in the species-diverse lineage of all other actively swimming taxa, including <i>Berosus</i> 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 <i>Amphiops</i> is optimized for high manoeuvrability, whereas that of <i>Berosus</i> 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.