Subitaneous eggs from an euryhaline calanoid copepod Acartia tonsa were challenged by changes in salinity within the range from full strength salinity, down to zero and up to >70 psu. Egg volume changed immediately, increasing from 2.8 × 10(5) μm(3) at full strength salinity (35 psu) to 3.8 × 10(5) μm(3) at 0 psu and back to its initial volume when gradually being returned to full strength salinity. Egg osmolality followed the molality of the surrounding water when challenged within a salinity range from 2 to 50 psu. Egg respiration was not affected when eggs kept at 35 psu was exposed to low salinity (2 psu). These results suggest that eggs are unable to regulate their volume or osmolality when challenged with changes in salinity. Gradual changes in salinity from 35 to 2 psu and back did not harm the eggs (embryos), since the hatching success remained unaffected by such changes in salinity. In contrast, extreme hyper-saline conditions (76 psu) made the eggs implode and killed the embryo. We propose that the embryo is protected from salinity stress by its plasma membrane and that water exchange driven by osmosis is restricted to the perivitelline space of the egg, which acts as a perfect osmometer in the salinity range of 5-35 psu. We hypothesize further that the embryo is able to keep its volume and osmolality constant due to the impermeability of the inner plasma membrane of the egg or by a combination of osmoregulation and reduced permeability of the inner plasma membrane.