Swimming against the tide: Flexibility and constraint in marine dispersal and life histories

Background/Question/Methods

For bottom-dwelling marine invertebrates, dispersal occurs as microscopic larvae with minimal swimming abilities are transported by ocean currents. Selection on dispersal in benthic invertebrates may thus target different traits than in fish, with large larvae able to swim against currents, or terrestrial taxa that disperse as vagile adults. Marine invertebrates have dichotomous life histories that evolve to maximize fecundity (planktotrophy) or larval survival (lecithotrophy), mediated through differences in offspring provisioning. Planktotrophs produce many tiny larvae that can disperse long distances during an obligate feeding period; lecithotrophs produce fewer, larger larvae with an abbreviated planktonic period and concomitantly reduced dispersal potential. Selection on dispersal is thus inherently correlated with adult reproductive traits. Life-history evolution is also unidirectional in most marine groups, with dispersive larvae repeatedly lost and rarely regained. Remarkably, we have little theory to explain the selective regimes that favor such transitions; further, no trait has yet been correlated with transitions in larval type, impeding our understanding of why dispersal is repeatedly lost from marine life histories. I will present empirical work on a group of marine gastropods aimed at identifying reproductive traits and environmental conditions that favor the loss of dispersal.  

Results/Conclusions

Using comparative methods, I document 27 losses of dispersive larvae in a clade of 200 species. Such transitions were positively correlated with increased per-offspring investment through production of extra-embryonic yolk, linking selection on maternal traits with dispersal ability. I then focus on two species (one temperate, one tropical) with intra-specific variation in larval type, to identify environmental drivers of life-history shifts. Field studies, population genetic surveys, and laboratory rearing experiments indicate non-dispersive larvae are favored in environments where circulation patterns render dispersal inherently unlikely. For instance, studies on the only species exhibiting seasonal polyphenism in larval type support the hypothesis that shifts to non-dispersive offspring are an adaptation to summer closure of Californian estuaries. I will also show evidence that dispersal may be favored if it facilitates colonization of ephemeral but high-quality habitat patches, especially at range margins. Finally, using species that are over- or under-dispersed relative to their larval lifespan, I will discuss how taxa may evolve to escape the constraints of ancestry on dispersal.  Traits ranging from oviposition to larval hatching, swimming or settlement behaviors may allow planktotrophs to achieve local retention, or facilitate dispersal by lecithotrophs, and thus overcome the ‘dispersal bias’ intrinsic to alternative life histories.