Relatively little is known about egg activation in the Crustacea, with the exception of morphological observations on the resumption of meiosis and the cortical reaction. Morphological analyses have demonstrated that the resumption of meiotic maturation is initiated by seawater contact in some decapod species. In the prawn Palaemon serratus, meiotic resumption of oocytes which are spawned at first meiotic metaphase, depends on the presence of external Mg2+ but not on external Ca2+ (Goudeau and Goudeau 1986). In Penaeid shrimp, meiotic resumption of oocytes, which are spawned at late prophase or early metaphase, requires Mg2+ when the gametes are fertilized and both Mg2+ and Ca2+ if unfertilized (Pillai and Clark 1987). In crabs, spawned oocytes are in first meiotic metaphase and also resume meiosis upon exposure to seawater. The process requires only 30–50 µM Ca2+ in Mg2+ free artificial seawater (ASW) (unpublished results). With respect to the cortical reaction, cytological observations have permitted the detection of a sperm-dependent cortical vesicle exocytosis in barnacle eggs (Klepal et al. 1979). In Penaeid oocytes, a specific release of jelly components, initiated by spawning and originating from extracellular crypts formed by invaginations of the plasma membrane, has been observed. Jelly components undergo an enzyme-mediated transition from a heterogeneous to homogeneous state, which is dependent on both Mg2+ in seawater and a protease (Clark et al. 1974; Clark and Lynn 1977; Clark et al. 1980; Clark et al. 1985; Lynn and Clark 1975). Also in Penaeid shrimp, the formation of a “hatching” envelope is induced by contact with seawater, requiring only external Mg2+ when the eggs are fertilized, and both Ca2+ and Mg2+ when they are not (Pillai and Clark 1987). Finally, a complex cortical reaction has been described in the eggs of crabs (Goudeau and Lachaise 1980 a,b; Goudeau and Becker 1982), and lobsters (Talbot and Goudeau 1988). The cortical reaction in crab eggs is a two-step phenomenon that consists of I) the exocytosis of a fine granular material, elicited by contact with seawater (Goudeau and Goudeau 1985), and 2) a slow and long-lasting exocytosis of ring-shaped elements, which is sperm-dependent and leads to the elaboration of a thick extracellular capsule (Goudeau and Lachaise 1980b; Goudeau and Becker 1982). In prawn eggs, the cortical reaction requires external Mg2+, and is independent of fertilization (unpublished results).