Measurement of hydrostatic pressure in salmonid eggs

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Abstract
A method is described, based on applanation tonometry of the human eye, for measurement of hydrostatic (internal, turgor) pressure in salmonid eggs. The area of an egg flattened by an external applanating force is related to the volume of the egg displaced by applanation. The hydrostatic pressure in steelhead trout eggs (Salmo gairdneri) (1248 h postfertilization, 5 °C) was 53 (−7, +8) mmHg (1 mmHg = 133.322 Pa). Unfertilized, flaccid eggs of the species, with an assumed zero internal pressure, had a measured pressure of 1.1 (−0.6, +1.1) mmHg, taken as equivalent to the force required to bend the relatively thick capsule (zona radiata) wall. Hence, the corrected internal pressure is estimated as 51.9 mmHg. Internal pressure in fertilized eggs of three species of Pacific salmon (Oncorhynchus nerka (sockeye), O. gorbuscha (pink), and O. tshawytscha (chinook)) increased during incubation at 10 °C. Between 2 and 8 h after fertilization, hydrostatic pressure was approximately 15 mmHg for sockeye, 25 mmHg for pink, and 40 mmHg for chinook. After 1000 h of incubation, the pressures (millimetres Hg) were approximately 76 for sockeye, 47 for pink, and 51 for chinook. Possible relations between hydrostatic and osmotic pressures are discussed with reference to effective filtration pressure, as well as the potential influence of the latter on oxygen transport across the capsule.