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.