Egg coats of the rock shrimp Rhynchocinetes typus: ultrastructural characterization and their function during fertilization

The aim of the present work was to characterize structurally and ultrastructurally the egg coats of the rock shrimp, Rhynchocinetes typus, and to describe their functional roles during fertilization. Oocytes fixed directly from the ovary, have a total diameter of 549 microm and are covered by a 10- microm-thick transparent envelope. Electron microscope sections (dehydrated) of the egg envelope revealed an electron-dense external coat of 0.4 microm covered by filamentous processes, and a granular inner coat of 4- microm thickness. Oocytes placed for 5 min in seawater had a significantly larger diameter (573 microm), because of the increase in the thickness of the egg coats (32 microm) and the formation of a 16- microm perivitelline space. The diameter of the egg proper was reduced by the same extent as the size of the perivitelline space. All these changes were associated to the loss of the egg fertilizability. SDS-PAGE of isolated and solubilized egg coats with 20% beta-mercaptoethanol or 25 mM dithiothreitol (DTT) showed bands between 58 and 105 kDa and between 44 and 103 kDa, respectively. During normal fertilization, the sperm undergoes a drastic change in shape after first contact with the egg. We observed a similar change when solubilized egg coats were placed with vas deferens sperms. When the solubilized egg coat proteins were ultrafiltrated with a membrane of 10,000 MWCO (pore size) and then assayed for their effect on fertilization, an inhibitory effect of 30%, 41%, and 59% was found when oocytes were incubated with spermatozoa pre-treated with 30, 60, and 120 microg/ml of proteins solubilized with beta-mercaptoethanol. A similar inhibitory effect was found when egg coat proteins solubilized with 25 mM DTT were used. Our results suggest that, in the shrimp, the egg coats play an active role during the morphological changes of the sperm during their passage through them.