Osmotic Shock: Sea urchin eggs are adapted to the high salt concentration of seawater. When placed in fresh water, the lower osmotic pressure causes water to rapidly enter the eggs by osmosis. This influx of water causes the eggs to swell and eventually burst, damaging their cellular structures and causing cell death.
Ionic Imbalance: Seawater contains specific concentrations of various ions, including calcium, potassium, and sodium, which are crucial for maintaining the physiological functions of sea urchin eggs. Fresh water lacks these ions or has them in different concentrations, disrupting the eggs' ionic balance. This imbalance interferes with vital cellular processes, enzyme activities, and membrane transport, leading to cellular dysfunction and ultimately cell death.
Loss of Extracellular Matrix: The eggs possess an extracellular matrix that provides structural support and protection. Fresh water causes the breakdown of this matrix, making the eggs more fragile and susceptible to damage.
Disruption of Fertilization: If the eggs are unfertilized, placing them in fresh water can prevent fertilization from occurring. Even if fertilization has already taken place, the developing embryos are likely to be adversely affected by the changes in osmotic pressure and ionic composition.