Ontogenetic shifts in swimming capacity of echinoderm propagules: a comparison of species with planktotrophic and lecithotrophic larvae

Abstract

While developmental strategies can modulate the dispersal and recruitment of marine benthic species, the significance and drivers of propagule motility throughout ontogeny remain incompletely understood. Species with lecithotrophic (non-feeding) development are rarely studied, despite their predominance in some taxa, including echinoderms. Quantification of the swimming capacity (i.e. speed and trajectory) of early life history stages and its variability with environmental factors is required to improve the ability to predict population connectivity and assess trade-offs associated with complex life histories. In general, lecithotrophic larvae of echinoderms are ascribed weak swimming abilities relative to planktotrophic larvae, although explicit measures are scarce. Here, we explored selected metrics of swimming capacity in four co-occurring species of North Atlantic echinoderms displaying different types of pelagic development: planktotrophs represented by the sea star Asterias rubens and the sea urchin Strongylocentrotus droebachiensis, and lecithotrophs represented by the sea star Crossaster papposus and the sea cucumber Cucumaria frondosa. Swimming was characterized in still water based on the horizontal speed and path straightness of early life-history stages, from late blastula (hatched embryo) to late-stage larva. We tested the hypotheses that swimming capacity of propagules increases with progression through developmental stages and with increasing seawater temperature. Swimming speed increased with ontogeny in two of the four species (A. rubens and C. papposus) and with temperature in all species, although the effects of temperature were not uniform across life stages. The fastest swimming speeds across all species and temperatures were recorded in lecithotrophic propagules (i.e. max speed 1.2 mm s− 1 in the brachiolaria of C. papposus), whereas propagules of species with planktotrophic development displayed faster relative speeds (body lengths s− 1). Relative speeds increased with temperature in all tested species except C. papposus. Swimming paths typically increased in straightness from early to later developmental stages, and also became straighter with increased temperature in most species, except in C. papposus where they became more circular and complex. In general, planktotrophic and lecithotrophic propagules had similar swimming capacities when tested at the same level of increased temperature, though several stage-specific differences were detected; propagules of species with planktotrophic development had greater relative speeds at the gastrula stage and greater path-corrected speeds at the larval stage. Swimming paths and swimming speeds were similar between propagules of species with planktotrophic development and lecithotrophic development, suggesting that phylogenetically conserved, ontogenetic patterns of swimming capacity (seen here between two sea stars) may supersede the contribution of larval nutritional mode.