'Desperate larvae' revisited:: age, energy and experience affect sensitivity to settlement cues in larvae of the gastropod Alderia sp.

Formulated in the 1950s, the 'desperate larva' hypothesis postulated that lecithotrophic larvae become less discriminating in their settlement requirements over time, due to depletion of energy reserves. Its predictions have been confirmed for some taxa in which larvae settle gregariously or in response to biofilms, but the hypothesis has not been tested with larvae that exhibit a stringent requirement for prey- or host-derived settlement cues. Using the opisthobranch Alderia sp., we tested how age, nutritional status and prior experience affected larval response to settlement cues from the host alga Vaucheria longicaulis. Although able to metamorphose without feeding, larvae of Alderia sp. were facultatively planktotrophic if phytoplankton were available, allowing us to test the effects of energy limitation on dose-response. Unfed larvae became increasingly responsive to dissolved settlement cues with age, whereas the sensitivity of fed larvae did not change over time; larvae thus responded more strongly to habitat cues as energy reserves diminished. Rate of metamorphosis also accelerated with age; following initial settlement, 4 d old larvae completed metamorphosis 12 to 24 h faster than 1 d old larvae. Larvae may settle and then continue to evaluate their environment via perception of chemical cues, with older larvae initiating metamorphosis more rapidly. Prior exposure to a subthreshold dose of the natural cue triggered a higher response to a subsequent exposure in initial non-responders. Some families exhibited a lower threshold dose than others, suggesting that cue sensitivity may have a heritable component. Habitat choice thus depends on interactions between the genetic background, prior experience and energy content of a given larva. Our results support a modified version of the original desperate-larva hypothesis applicable to nongregarious species, in which energy-depleted larvae accept weaker cues and respond more quickly during habitat selection.