“Collective dispersal leads to variance in fitness and maintains offspring size variation within marine populations”
Scott C. Burgess, Robin E. Snyder, and Barry Rountree (Mar 2018)
Collective dispersal selects for risk avoidance life histories, which consequently maintains variation in offspring size
Larval dispersal in coastal environments is influenced by turbulent eddies that are several kilometers wide and last several weeks. These eddies can collect the larvae of benthic marine species (such as lobsters, sea urchins, reef fish) into dense groups that travel as coherent ‘packets’. Such collective dispersal means that sibling larvae released within a few days of each other would succeed (return to the coast and settle) or fail (are lost offshore) in groups. Therefore, turbulent dispersal generates unpredictable variation in fitness because some groups of offspring are lucky and others are not. How do marine life histories evolve in this turbulent and unpredictable setting to avoid occasional recruitment failures? Is it better to increase mean fitness, which also increases variability in fitness, or is it better to reduce variability even at the cost of a lower mean? By analogy with financial investment strategies, reducing variation at the expense of the mean is known as “bet hedging.”
Scott Burgess, Robin Snyder, and Barry Rountree develop a mathematical model that predicts how turbulent dispersal influences the evolution of offspring size and spawning duration. They find that evolution favors offspring sizes that maximize fitness, even though this also increases the unpredictability of recruitment—there is no bet hedging. However, it can take a very long time for offspring sizes to evolve to the optimum, which means that types that differ in the size of offspring they produce can coexist for long periods. In nature, variation in offspring size within the same population is quite common. In the past, it’s been thought that this is the result of good and bad years or locations favoring different sizes. This paper shows that this kind of environmental variation is unnecessary to explain variation in size seen in nature: multiple offspring sizes can, in theory, coexist even in a uniform environment if larval dispersal is risky and the fates of larvae are correlated.
Variance in fitness is well known to influence the outcome of evolution but is rarely considered in the theory of marine reproductive strategies. In coastal environments, turbulent mesoscale eddies can collect larvae into ‘packets’ resulting in collective dispersal. Larvae in packets return to the coast or are lost offshore in groups, producing variance in fitness. Using a Markov process to calculate fixation probabilities for competing phenotypes, we examine the evolution of offspring size and spawning duration in species with benthic adults and pelagic offspring. The offspring size that provides mothers with the highest mean fitness also generates the greatest variance in fitness, but pairwise invasion plots show that bet-hedging strategies are not evolutionarily stable: maximizing expected fitness correctly predicts the unique evolutionarily stable strategy. Nonetheless, fixation can take a long time. We find that selection to increase spawning duration as a risk-avoidance strategy to reduce the negative impacts of stochastic recruitment success can allow multiple offspring sizes to coexist in a population for extended periods. This has two important consequences for offspring size: 1) coexistence occurs over a broader range of sizes and is longer when spawning duration is longer, because longer spawning durations reduce variation in fitness and increase the time to fixation, and 2) longer spawning durations can compensate for having a non-optimal size and even allow less optimal sizes to reach fixation. Collective dispersal and longer spawning durations could effectively maintain offspring size variation even in the absence of good and bad years or locations. Empirical comparisons of offspring size would, therefore, not always reflect environment-specific differences in the optimal size.