Single or multiple spawning? Comparison of breeding strategies of freshwater Unionidae mussels under stochastic environmental conditions

Abstract

The life cycle of unionids is characterized by a obligatory period of larval parasitism on a fish host, any disturbance of which might cause a large-scale decline in the mussel population. Because the probability of fish infestation is so important, we modelled what would happen to a population (in terms of population growth and probability of extinction), if the same number of glochidia were released in one or more separate spawning events, by a hypothetical mussel population living in conditions differing in the probability of fish infestation (a “neutral” scenario, a “good” one and three variants of a “bad” one). The single brood strategy was the best in the “good” scenario. However, when the frequency of unfavourable stochastic events increased (“bad” scenarios), all strategies led to population decline, the single brood strategy being the worst. In “good” and moderately “bad” conditions the double brood strategy performed better than the other multiple brood strategies, but as infestation conditions deteriorated, a greater number of spawning events ensured a slower population decline and longer persistence. Our model can facilitate a better understanding of this problem and set up a framework for further tests in other unionid species and their environmental conditions.