Seasonal aspects of sodium, potassium, and water balance in the predaceous diving beetle Dytiscus verticalis

Abstract

A previous study showed that food availability i the laboratory can dramatically affect sodium balance of the predaceous diving beetle Dytiscus verticalis. Because this species inhabits small ponds that dry up in the summer, it seemed likely that wild beetles would undergo annual changes in ody ion content. Seasonally collected predaceous diving beetles were analyzed for dry body mass, body water, body sodium and potassium, hemolymph sodium concentration, and hemolymph osmolality. Beetles varied in all characteristics over a 16-month period, and variation was seasonally cyclic for body water, body sodium, hemolymph sodium, and hemolymph osmolality, but not for dry mass and body potassium. A laboratory experiment and two field-enclosure experiments determined possible mechanisms underlying these cycles. Beetles in enclosures with food had greater dry mass than nonfed beetles in enclosures, but not greater than that of wild beetles. The number of tadpoles killed by beetles in enclosures correlated well with beetle dry mass. Because food availability directly affects dry mass, wild beetles must not suffer seasonal periods of food limitation. Body sodium and potassium levels also appear related to food availability, but not closely enough to explain the seasonal variation in sodium, the nonseasonal variation in potassium, and the nonparallel variation in the two cations. Food intake can be important for increasing both hemolymph sodium concentration and hemolymph osmolality, but these characteristics can be regulated independently of food intake and of each other. Seasonal variation in hemolymph sodium was out of phase with body sodium variation, suggesting that sodium supply is not the cause of hemolymph sodium cycles. Hemolymph osmolality was greatly increased in winter months, perhaps reflecting elevated levels of free amino acids in the hemolymph.