Sperm transfer mechanisms: some correlates and consequences

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Abstract
The methods by which the Metazoa bring their gametes into juxtaposition as a preliminary to fertilisation are reviewed. The method of sperm transfer employed by a particular organism or group of organisms is commonly more directly related to habitat than to phylogeny. This appears in part to be a consequence of certain methods of sperm transfer facilitating the exploitation of particular habitats. Non-exposure of sperm to hostile influences such as fresh water, digestive enzymes, antibodies, or the desiccating effects of air may have been important in the colonisation of fresh waters by animals with body fluids of high osmolarity, in the establishment of the endoparasitic habit, and in the colonisation of dry land. Mutual exchange of sperm with equal sharing of the reproductive burden by means of hermaphroditism, coupled with internal fertilisation, is seen as an important adaptation to very small body size, especially as associated with life in interstitial habitats. Efficient transfer of sperm, usually in conjunction with internal fertilisation, is seen as a prerequisite for the development of yolky eggs with fairly direct development of the young. It is also a prerequisite for establishing maternal nourishment of the young in viviparity and a consequent reduction or suppression of free larval stages. Yolky eggs confer freedom from the vicissitudes of fluctuation in larval food supplies, and eggs which contain all or most of the mineral salts required for development seem necessary for colonisation of fresh waters. Amongst freshwater fishes and anurans, close pairing of male and female minimises the effects of rapid destruction of sperm in species which do not copulate or use spermatophores.