FORMATION OF THE ENVELOPING LAYER OF THE CHUM SALMON EGG IN ISOTONIC SALT-SOLUTIONS

After stimulation in a hypotonic solution (9.4 mOsm kg(-1)), inseminat ed eggs of the chum salmon Oncorhynchus keta initiate cleavages in iso tonic salmon Ringer's solution (267.3 mOsm kg(-1)) containing 3.2 mM C a2+ ions. Blastomeres oil these eggs, however, separate from each othe r and the enveloping layer is not observed at the blastula stage. An i ncrease in external divalent cations rescues the separation; the conce ntration of CaCl2 in the external medium should be 25 mM or more to in duce close contact of blastomeres and the formation of an enveloping l ayer in isotonic salt solutions. The effectiveness of Ca2+ ions can be substituted by Mg2+, Sr2+ and Zn2+ ions; the same results are obtaine d in isotonic MgCl2 and SrCl2 solutions (100 mM) or in isotonic sah-no n Ringer's solution containing Zn ions (6.2 mM). The close contact of blastomeres and the formation of an enveloping layer are also observed in a low Ca2+ concentration (<0.1 mM) in a hypotonic salt solution (9 .4 mOsm kg(-1)). The Ca2+ level in the external medium to induce the e nveloping layer formation seems to be correlated with the salinity of the incubation medium. It is suggested that adhesion molecules on the surface of blastomeres in the chum salmon eggs are different in proper ties from those found in sea urchin and other fish species. (C) 1996 T he Fisheries Society of the British Isles