The radiate membrane ("capsule") of chum salmon (Oncorhynchus keta) eggs has integrated with it another, outermost, non-chitinous membrane which prevents the digestion of the radiate membrane by "hatching enzyme" of chum salmon or by pronase applied externally but not internally. Crude chum salmon hatching enzyme(s) has an optimum pH of 7.5–8.0 for the release of soluble peptides from the radiate membrane, requires metal ions for activity, and can also decompose the radiate membranes of coho salmon (Oncorhynchus kisutch) and steelhead (Salmo gairdneri) eggs.