Adaptations of Coleoptera to the marine environment. II. Observations on rove beetles (Staphylinidae) from rocky shores

Two common rove beetles inhabit the intertidal rocky shores of British Columbia: Liparocephalus cordicollis Le Conte and Diaulota densissima Casey. Liparocephalus cordicollis, the more abundant of the two, is a predator that feeds preferentially on small chironomid larvae. Adults and larvae of this species can withstand submergence in seawater at 10 °C for more than 2 weeks. Thus they are able to survive continuous inundation from one spring tide to the next, just above lowest tide levels. Adults of L. cordicollis are osmotic regulators and can stabilize their body weight at salinities varying from 2 to 45‰. Both of these staphylinid species respire above and below water. Oxygen consumption of L. cordicollis in air at 10 °C was 376 ± 59 μL O2∙g−1∙h−1, but when the beetles were immersed in seawater at the same temperature the respiration rate was reduced to 45 ± 8 μL O2∙g−1∙h−1. During submersion, temperature-dependent reactions were observed (Q10 = 2). Respiration was constant within the salinity range of 2–30‰, but at higher levels (45‰) oxygen consumption increased to 55 ± 5 μL∙g−1∙h−1. When L. cordicollis was exposed to warm, dry conditions (simulating low tide on a warm summer day) the rate of evaporation from the cuticle was very high, calculated at 175 μg H2O loss per cm2 body surface per hour per mm Hg saturation deficit. The LT50 (mean lethal time) for a laboratory population was 2.5 h at a vapor pressure deficit of 7 mm Hg. In the larvae of beetles of the two species, asynchrony of growth in the population occurs because submerged larvae stop feeding and growing. Consequently, the development of individuals living near the low tide line is protracted compared with that of individuals living in the midlittoral zone.