Ejaculate-storage patterns and the site of fertilization in female snow crabs (Chionoecetes opilio; Brachyura, Majidae)

A light-microscopy study of mated female snow crabs (Chionoecetes opilio) was conducted to investigate the site of fertilization and to resolve how multiple ejaculates are stored in the spermathecae. In its basic configuration, an ejaculate consisted of a layer or patch of spermatophores enclosing mature spermatids that was capped by a relatively large volume of amorphous matter, which in turn could include a patch of spermatophores containing immature spermatids. Up to 10-12 ejaculates were stored in the spermathecae with the largest loads. An ejaculate was initially deposited in the intermediate chamber and ventral part of the spermatheca, and was displaced toward the dorsal part of the spermatheca when a new ejaculate was inserted. Ejaculates were neatly stratified along the ventrodorsal axis of spermathecae with small to moderate loads, but they were disrupted and the storage pattern was disorderly in spermathecae with large loads. Ejaculate stratification favors last-male sperm precedence and single-male paternity. However, multiple-male paternity might occur in females with large spermathecal loads, in part because several ejaculates can co-occur close to the oviduct opening. Mixing of male and female gametes in preparation for oviposition, and probably also fertilization, occurs to some degree in the ovaries.