Sodium and water balance in larvae of the predaceous diving beetle, Dytiscus verticalis: An air-breather resistant to acid-induced sodium loss

Abstract

1. 1. Sodium influx and efflux in artificial soft water were 0.05 and 3.15μmol/100g wet body mass·hr (μ mol/100gwm·hr) respectively, for larval Dytiscus verticalis. These rates are low in comparison with other freshwater animals, but are similar to rates measured for adult beetles. 2. 2. Sodium influx increased when larvae were put in successively more concentrated sodium solutions, but the change in influx failed to show saturation kinetics typical of active transport systems. 3. 3. Additionally, larvae failed to halt sodium loss even after body sodium levels declined by 9% and bath sodium concentration reached 68 μM. Thus larval D. verticalis, like the adults, are unable to utilize sodium from water to balance sodium losses. 4. 4. Tritium-labelled water influx into larvae (4.91 ml/100gwm · hr) was roughly half that of influx into a dragonfly naiad possessing rectal tracheal gills (9.31 ml/100 gwm · hr). Thus this air-breathing beetle larva is less permeable to water than an insect that respires aquatically. 5. 5. Sodium balance of beetle larvae was not affected by exposure to pH 3.0. In dragonfly naiads a similar exposure significantly decreased sodium influx and increased sodium efflux. 6. 6. Thus disruption in sodium balance of aquatic insects is likely to provide a useful bioassay for effects of acidic pollution only in forms using aquatic respiration. Air breathing forms are predicted to be more tolerant to acidic and metallic pollutants that specifically inhibit sodium balance.