OSMOTIC HATCHING IN THE EGGS OF SOME FRESH-WATER COPEPODS

¹ Dcpartment of Biology, Western Reserve University, Cleveland 6, Ohio

1. The hatching process is described for the fresh-water copepods Diaptomus ashlandi, D. siciloides, D. oregonensis, Cyclops bicuspidatus, and Mesocyclops edax. In all of these species the inner membrane expands by the osmotic entry of water. The internal pressure thus produced ruptures the outer membrane, and the inner membrane containing the nauplius is extruded, forming a sphere whose volume is more than 2 times that of the original egg. Subsequently the inner membrane bursts and the nauplius is thrown out.

2. It is shown that the osmotic pressure of the fluid within the expanded inner membrane is equivalent to that of a 0.03 to 0.04 M sucrose solution.

3. The inner membrane remains impermeable to water until the egg is ready to hatch. Thereupon the membrane changes its permeability within a short period of time. Hatching can be prevented indefinitely in eggs that are ready to hatch by immersing them in sufficiently concentrated sucrose solution.

4. Although during normal hatching the nauplius is active for a period of approximately a minute before hatching, this activity is not necessary for the completion of the hatching act. Nauplii hatched, even though they had been completely immobilized.

5. Attempts to demonstrate the presence of a hatching enzyme were unsuccessful.

6. It is suggested that the pre-hatching change in permeability of the membrane is caused by the action of chemicals produced by the larva. It is further suggested that the greater osmotic pressure of the fluid within the inner membrane is caused by external metabolites of the larva—either excretory or secretory.