Sexual Differentiation in Mollusks. II. Gastropods, Amphineurans, Scaphopods, and Cephalopods

Abstract

The author classifies the sexual manifestations of the mollusks into 2 main groups ambi-sexuality (monoecism, hermaphroditism) and unisexuality (dioecism, gonochorism), with 4 more or less intergrading categories of ambisexuality, namely: functional ambisexu-ality, consecutive sexuality, rhythmical consecutive sexuality and alternative sexuality. Two of the 3 orders of gastropods (Opisthobranchiata and Pulmonata) are almost exclusively ambisexual; the 3rd order (Prosobranchiata) has representatives with all 5 aspects of sexuality. Most of the amphineurans (with the exception of the wormlike spp. of Aplacophora) and all of the scaphopods and cephalopods, so far as known, are unisexual. In some cases of consecutive sexuality the individual has a highly specialized copulatory organ and masculine instincts when young; but after functioning as a [male] for some weeks or months the [male] organs are absorbed and are later replaced by specialized [female] organs, accompanied by appropriate feminine instincts. The primary, [male] phase may be prolonged and accentuated by favorable environmental conditions, particularly by opportunity for copulation with a [female]-phase individual but such association is not, as was formerly believed, a prerequisite for the development of the functional [male] phase. Other spp. show intermediate conditions relative to the differentiation of [male] and [female] phases. Many functional hermaphrodites are provided with the copulatory organs and instinctive behavior of both sexes and sperm are transferred either simultaneously or consecutively to each member of a mating pair. Other hermaphrodites experience self-fertilization and in one species the line has been continued by experimentally isolated individuals for 93 generations. All these manifes-tions of sexuality may be referred to the homozygosity or heterozygosity of the primary sex factors, with their associated modifying factors for the activation or suppression of either the [male]- or [female]-determining components at certain stages of development or in harmony with different degrees of physiological maturity. Environmental conditions influence the expression of sexuality in ambisexual spp. and the sex ratios in unisexual spp. but no exptl. proof of pheno-typic sex determination has been obtained.