Osmolality of rectal fluid from two species of osmoregulating brine fly larvae (Diptera: Ephydridae)

Abstract

Two sympatric species of saltwater-dwelling Ephydrella larvae inhabit saline waters ranging between 0.9 and 2.0 mol l-1 NaCl. When larvae with anal microcannulae were exposed to media of varying salinity, both species were found to be capable of excreting rectal fluid (urine) which was hyperosmotic or isosmotic to the media whilst maintaining the haemolymph osmotic concentration at a relatively constant value. Over environmental osmotic concentrations ranging from 43-5850 mOsm kg-1, the osmotic concentration of rectal fluids from E. breviseta varied between 36 and 4800 mOsm kg-1 and was relatively close to isosmoticity with the external medium. The fluid from E. marshalli varied between 180 and 8700 mOsm kg-1 and was hyperosmotic to the medium. The osmotic concentration of rectal fluid in both species is almost entirely accounted for by Na+ and the assumed Cl- counterion. K+ was in relatively low concentration in the rectal fluid but was considerably higher than in the haemolymph. In external media of low NaCl concentrations the rectal fluid from both species was hyposmotic to the haemolymph and at high concentrations the rectal fluids were hyperosmotic to the haemolymph. A consideration of drinking rates and Na+ excretion indicates that E. marshalli is in sodium balance in a medium of 1 mol l-1 NaCl but not in 0 or 3 mol l-1 NaCl. This coincides with a significantly higher drinking rate in 1 mol l-1 NaCl. Show more