To help elucidate the reproductive characteristics of the Atlantic snow crab Chionoecetes opilio, the functional anatomy of the male reproductive system and the female spermatheca was investigated using histological methods, transmission and scanning electron microscopic techniques, and microscopic observation of fresh material.

Several fundamental corrections and additions to earlier descriptions of spermatozoan structure in C. opilio were made: radial arms are present, while a chromatin ring is not; the acrosome protrudes only slightly and spermatozoa are not mushroom-shaped. The spermatozoa and the matrix of the anterior vas deferens are packed into spermatophores and surrounded by a pellicle which appears to be secreted by the cells lining the anterior vas deferens proximal to the testis. The highly folded configuration of this pellicle may act as a safeguard against dehiscence induced by contact with sea water during copulation. The posterior vas deferens contains two distinct secretions which are probably ejaculated along with the spermatophores and the matrix of the more anterior vas deferens.

Anatomical and in vitro observations suggest that fertilization is initiated by exposure of the spermatophores to a hypotonic medium. Such a medium may be generated within the spermathecae by dilution of the seminal fluids/spermatophore storage matrix with sea water prior to egg mass extrusion. Devagination of the liberated spermatozoa may be facilitated by the same mechanism.

Most of the females processed for histology had distended spermathecae devoid of spermatophores or spermatozoa. It is thus impossible to deduce successful copulation and spermatophore storage without direct observation of spermathecal contents.