Seasonal reproduction: proximate and ultimate factors
Munro (1990b) has proposed a classification of the proximate cues that determine the occurrence of different portions of the reproductive cycle. He recognizes four factors that control the development and synchrony of breeding cycles.
1 Predictive cues are general periodic environmental events that a fish can use to predict that the spawning season is approaching. Changing day length and temperature are predictive cues that are likely to trigger the onset of gametogenesis and secondary sex character development. Gametogenesis may have an endogenous circa-annual rhythm that is entrained by
some predictive environmental cue (e.g., heteropneustid catfish, Rainbow Trout, sticklebacks).
2 Synchronizing cues signal the arrival of spawning conditions. Typically, the presence of a suitably appearing and behaving mate, perhaps releasing pheromones, may serve as such a cue, causing final gamete maturation and release. The pheromone may even be produced by another species, as in the case of minnows that are nest associates of other species and
spawn only in the presence of the host species (Rakes et al. 1999; The gender of care-givers). The presence of vegetation or other spawning substrates plays a role in some species. Synchrony is important not just to insure contact between the sperm and eggs and to prevent hybridization. In many species, gametes decline in fertility rapidly after ovulation and spermiation. Hence, a small temporal window of spawning receptivity and opportunity exists.
3 Terminating cues signal the end of the spawning period. Because breeding conditions remain optimal for a short period, including the above-mentioned changes in gamete viability, breeding seasons are typically short. Gonad regression occurs after breeding in response to environmental cues (i.e., changes in predictive cues), exhaustion of gametes, or the departure or changes in behavior of conspecifics. Nest guarding species may respond to the presence of eggs in a nest, causing hormonal changes that inhibit spawning and encourage egg care and aggression.
4 The first three categories of cues can all be modified by secondary factors such as water quality, lunar cycle, adult nutrition, predator presence, and social interactions. These modifying factors are the causes of intraspecific variation in breeding at different latitudes or in different habitats.
Evolutionarily, why is seasonal breeding so prevalent in fishes? Gamete production, particularly in females, is energetically expensive. Gametes are usually released in batches; time and energy are required to replenish gametic products, even in males (Nakatsuru & Kramer 1982; Shapiro et al. 1994). Courtship and spawning, and parental care where it occurs, require time and energy and expose participants to predators. Few fishes can therefore afford to reproduce year round. Hence a decision in evolutionary terms must be made as to the optimal time to reproduce, optimality being defined in terms of the relative costs and benefits of current versus future reproduction (see Life histories and reproductive ecology). The conditions for egg dispersal, larval survival and growth, and larval recruitment vary through the year and are dependent on seasonally driven climatic variation. In most species, spawning appears to be synchronized with periods most favorable for the survival of young. In temperate marine fishes with pelagic larvae, food availability is one critical determinant. Spawning coincides with seasonal blooms of zooplankton, thus maximizing the chances that larvae will encounter prey during the critical period shortly after they use up the energy stores of their yolk supply (the Match–Mismatch Hypothesis of Cushing (1973); Larval feeding and survival). Individuals that spawn at times when the probability of egg, larval, and their own survival are higher will be more successful than individuals that spawn at less suitable times (Munro 1990a).
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