Desperate planktotrophs: decreased settlement selectivity with age in competent eastern oyster Crassostrea virginica larvae

For larvae of benthic marine invertebrate species, settlement from planktonic to benthic life is a critical transition. The 'desperate larva' concept describes the tendency of larvae to accept suboptimal settlement habitats as they age. We quantified swimming behavior in planktotrophic larvae of the eastern oyster Crassostrea virginica, to determine whether settlement behaviors such as swimming downward and remaining on the bottom increased with age, and whether these ontogenetic changes were more apparent in larvae exposed to suboptimal conditions than to preferred conditions (settlement cue absent or present, respectively). In 2 experiments, the proportion of competent larvae remaining near the bottom of experimental flasks (indicating settlement) increased with larval age, but only in larvae that were not exposed to the settlement cue. This result is consistent with the hypothesis that larvae encountering suboptimal habitat become 'desperate' (i.e. more likely to settle) as they age. Exploratory behaviors, such as upward swimming, meandering, or helices, were expected to decrease with age, especially in the absence of the settlement cue, but this pattern was detected in only 1 of the 5 swimming metrics tested (helices in downward swimming larvae). Surprisingly, pre-competent larvae exhibited settlement behavior when exposed to the cue, raising the question of whether a response at this stage would have positive or negative consequences. Acceptance of suboptimal settlement habitats by aging larvae may increase the resilience of a species by allowing populations to persist in variable environmental conditions.