Pelagic propagule duration and developmental mode: reassessment of a fading link
As the main or sole motile stage of many aquatic taxa, propagules play a central role in their population dynamics, macroevolution and biogeography. The premises of studies concerned with dispersal, on issues as diverse as marine protected areas, sustainable fisheries and invasive species, commonly make simplified assumptions linking larval development mode (planktotrophic, lecithotrophic) and pelagic propagule duration (PPD) or pelagic larval duration. Because general empirical tests of these connections have yielded equivocal results, we critically examined the relationship between larval mode and PPD in light of regional and taxon-specific criteria.
We collated data from 190 species from the phylum Echinodermata and used two-way and one-way ANOVA to compare PPD among larval modes, taxonomic classes, geographic regions and climate zones. We also assessed the contribution of egg size, temperature and other continuous predictors using multiple regressions and ANCOVA, and examined whether geographic range size differed among larval modes.
Our results depart from the common assumption that planktotrophs have longer PPDs than lecithotrophs. Instead, we detected a synergistic influence of phylogeny and climate. No significant difference in PPD between planktotrophs and lecithotrophs occurred in two of the four classes and only four out of nine major ocean basins contained species with significantly different PPDs on the basis of larval mode. Species from colder climates displayed relatively constant PPDs regardless of larval mode, whereas temperate and warm-water species displayed a clearer dichotomy. PPD did not correlate with geographic range size nor did planktotrophs and lecithotrophs exhibit significantly different range sizes.
Our findings indicate that larval mode is not a reliable predictor of PPD across broad geographic and taxonomic scales. Instead developmental strategies may instigate shorter or longer PPDs in schemes that are influenced by evolutionary and environmental pressures.