Early metamorphosis is costly and avoided by young, but physiologically competent, marine larvae

Dispersing organisms often cannot assess habitat quality directly, so they employ proxies (cues) to choose habitats that maximise fitness. Theory suggests organisms should settle as soon as they find appropriate cues in order to reduce physiological costs and mortality risk incurred whilst searching. We propose that for planktotrophic marine larvae, when resources are plentiful, development of adult structures during an extended larval phase provide post-metamorphosis benefits that offset the costs of remaining in the plankton. To test this, we measured fitness consequences of metamorphosis in response to habitat cues at a range of developmental maturities in 2 sea urchin larvae, Tripneustes gratilla and Centrostephanus rodgersii. We found larvae that were capable of responding to cues and settling accrued significant benefits if they extended their pelagic development. Compared to more developed larvae, larvae without adult structures took longer to metamorphose, and after metamorphosis were 11 to 25% smaller, 0.1 to 6 times more likely to lack defensive structures and 3 to 13 times more likely to have abnormal morphology. Most early settlers died within 1 wk compared to >40% survival for more developed larvae. We found larvae avoid the costs of early metamorphosis by only responding to low concentrations of cues in the water column once they have adult structures. Our results contrast with models of habitat selection that suggest organisms should settle in habitat quickly to minimise search costs. Incorporating the trade-off between the benefits of larval development and search costs into current models of habitat selection may provide new insights into how fitness consequences affect habitat selection.