Evidence of morphological adaptation to life underwater: sternal keel affects swimming speed in giant water scavenger beetles (Coleoptera: Hydrophilidae: Hydrophilini)

Abstract

Fundamentally, insects evolved on land and secondarily inhabited aquatic environments multiple times. To live underwater, aquatic insects have acquired enormously variable morphological, developmental, physiological, and ecological traits, such as gas exchange systems and swimming-related characteristics. Giant water scavenger beetles of the tribe Hydrophilini (Coleoptera: Hydrophilidae) are characterized by the presence of a sternal keel, which often extends posteriorly. Despite being a conspicuous morphological trait, its function remains unclear. Here, I verified two hypotheses: keel affects (1) submergence time following air replacement and (2) speed and oscillatory movement during forward swimming in Hydrophilus acuminatus Motschulsky, 1854. Submergence time was affected by body mass rather than keel removal; in other words, larger individuals replaced their gas gills more frequently. Keel removal reduced swimming speed by 12.5%. These observations support the second hypothesis, and are also consistent with previous speculations that the sternal keel is a key adaptation for swimming, but the results showed that the degree of oscillation was closely related to body mass but not to keel removal. Further studies are warranted to elucidate precise factors through which the presence of the keel increases swimming speed. Such studies would provide clues into understanding the associations among body size, swimming methods, and morphological traits.