The adaptive significance of seasonal reproduction in marine invertebrates: the importance of distinguishing between models

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Abstract
Seasonal reproduction is a dominant characteristic of the reproduction of major groups of marine invertebrates. The seasonality may be extreme culminating in very synchronised mass spawning events. Hypotheses to explain strongly seasonal reproduction are appraised in relation to demographic theory of life history which supposes that a limited resource (e.g., energy) may be allocated to (a) maintenance and defense against the environment (respiration, excretion, ionic regulation, etc.); (b) growth; or (c) propagule production and present reproduction. Allocation to each contributes to fitness through demographic components that can be defined by the Euler-Lotka equation. Hypotheses to account for strongly seasonal reproduction must explain the mechanism that confers selective advantage to highly seasonal non-random deployment of limited resources to reproduction. The various hypotheses proposed can be classified as (1) non-functional and (II) functional. Functional hypotheses suppose that a selective advantage accrues as a consequence of energy storage prior to allocation to reproductive function. There are costs associated with this energy storage that must be compensated by the selective advantage accruing from delayed deployment of resources. A number of different functional hypotheses can be identified which may be allocated to two sets of related hypotheses: (A) Synchronisation maximises offspring survival—the optimum larval survival strategy; and (B) Synchronisation maximises fitness independently of offspring survival—the optimum fertilisation strategy. Several versions of each can be identified; their predictions are examined and discussed in relation to possible consequences of enhanced rates of environmental change (global warming).