Osmoregulation in the Antarctic nematode Panagrolaimus davidi

A technique for determining the internal osmotic concentration of a small nematode using a modified nanolitre osmometer is described and used to investigate osmoregulation in the Antarctic nematode Panagrolaimus davidi. This technique enables the osmotic concentration to the measured with an accuracy of +/-12 mmol kg(-1). The pattern of melting in the nematode's different body compartments suggests that it is the osmolality of its pseudocoelomic fluid that is being measured. Panagrolaimus davidi maintains its internal osmotic concentration above that of the external medium and is thus an hyperosmotic regulator. The nematode achieves regulation under hyposmotic stress more rapidly than under hyperosmotic stress.