Physiological adaptations and the concepts of optimal reproductive strategy and physiological constraint in marine invertebrates.

The dominant 'demographic' theory of life history evolution supposes that different lifetime patterns of reproduction are the result of selection of alternative optimal solutions for the allocation of limited resources between somatic and reproductive functions. A number of trade-off possibilities have been recognized--those between current reproductive output and residual reproductive value and between fecundity and initial offspring size being considered especially important. Many theoretical studies assume that natural selection will favour the adoption of optimal solutions, but it has been pointed out that such solutions may not be obtainable due to design constraints and the development of physiological adaptations to specific reproductive traits which limit subsequent evolutionary potential. The validity of this idea is examined in this paper through a review of the major reproductive strategies available to marine invertebrates and the physiological adaptations associated with them. The ecologically important distinction between planktotrophic and lecithotrophic development is not necessarily associated with major physiological adaptations in the adults, but the distinction between strictly semelparous and iteroparous life histories is. This is demonstrated in a survey of the endocrinological and environmental control of reproductive processes in related organisms with contrasting modes or reproduction. Particular reference is made to the Polychaeta, in which the contrast between semelparous and iteroparous life histories is particularly marked. A similar contrast is found between cephalopoda and other mollusca, and the discussion of physiological adaptations is extended to include these groups and the Echinodermata.