Hatching

Abstract Hatching in crustaceans is an active mechanism in which free, mobile individuals are released from the egg envelopes. For the majority of species, this marks the transition from the embryonic phase of the life cycle, which is spatially constrained by the egg, and the free-living phase. The hatching process of crustaceans has so far not been subject to a detailed comparative treatment across taxa and thus we know little of the diversity of mechanisms, timing in relation to other developmental processes, or evolutionary history. Here we attempt to provide an overview of this diversity throughout the Crustacea. To this end, we treat a particular set of subjects that we consider relevant to the hatching process: the morphology of the involved structures (egg membranes, specialized hatching structures of the hatchling, morphology of the hatchling itself), mechanics of hatchling release, biochemical processes involved in egg shell degradation, maternal and embryonic control and initiation of hatching, as well as the temporal pattern of hatching-related events. A common feature of the hatching mechanism in the majority of crustacean species is an osmotic swelling of the embryo caused by active water uptake prior to hatching, which builds up pressure against the inside of the envelopes. The remaining features vary according to developmental mode and ecological parameters, but the causality behind many hatching-related features remains unclear. However, we conclude that the particular life history strategy can have a strong impact on the relative timing of hatching events.