Hermaphroditism as a reproductive strategy for metazoans; some correlated benefits

Abstract

The concept of hermaphroditism is examined, the terminology is reviewed and several new terms are proposed. Euhermaphroditism is defined as the occurrence together of eggs and sperm in the individual that produced them; temporary hermaphroditism is defined as the few instances where euhermaphroditism is a brief transitional phase between sexes in protandry and protogyny and opsiautogamy designates the condition where sperm saved from an early male phase are used to fertilize eggs produced in a later female phase. An animal that functions successively in the different sex roles is considered distinct from one that functions as both male and female simultaneously. The circumstances in which these different sexual modes are appropriate and the advantages that accrue from them are different. For this reason protandry, protogyny and consecutive sexuality are not considered to be part of hermaphroditism, but rather aspects of labile gonochorism. A major idea advanced in this paper is that hermaphroditism is of primary importance as a reproductive strategy of very small metazoans. In such animals, whose eggs are so disproportionately large that very few can be produced, hermaphroditism ensures that through having the maximal number of females the maximal number of eggs are produced and fertilized economically. For ver small hermaphrodites, the mate-finding difficulties associated with small size and limited motility are reduced by converting each meeting into a potential mating. In hermaphroditism the frequency of mating is maximized, the effects of matings last for a long time through sperm storage and genetically diverse progenies are produced through multiple reciprocal copulations. The effective size of the gene pool is maximized, and the risk of genetic drift in sex ratio in small populations is avoided. Very small animals do not have the energy resources to produce large numbers of large, yolky eggs, nor do they have the space in which to store them before release. Tiny hermaphrodites tend to produce a few eggs at a time, but over extended or continuous breeding seasons. The effects of their reduced fecundity may be further offset by precise methods of fertilization, brooding and (commonly) the suppression of pelegic phases, thus minimizing losses from larval predation and overdispersal. Hermaphroditism may have been advantageous because of features often associated with it. Copulation with protected sperm transfer may have been especially important, not only because of economies in gamete production and use, but also in the exploitation of new habitats. In freshwater hermaphrodites copulatory or spermatophore exchange habits protect sperm from osmotic death; in endoparasites copulation protects sperm from digestive enzymes and osmotic stress in the host''s gut, and from antibodies and phagocytosis in the bloodstream.