Authors: Dustin J Marshall and Peter D Steinberg
Published in: The Journal of Experimental Biology, volume 217, issue 22 (November 2014)
The relationship between offspring size and performance determines the optimal trade-off between producing many small offspring or fewer large offspring and the existence of this relationship has become a central tenet of life-history theory.
For organisms with multiple life-history stages, the relationship between offspring size and performance is the product of the effects of offspring size in each life-history stage.
Marine invertebrates have long been a model system for examining the evolutionary ecology of offspring size, and whilst offspring size effects have been found in several life-history stages, the crucial stage of colonization has received less attention.
We examined the effect of offspring size on the settlement response of sea-urchin larvae (Heliocidaris erythrogramma) to preferred and less preferred hostplants, how these effects changed over the larval period and estimated the success of juveniles in the field on preferred and less preferred host plants.
We found that smaller larvae became competent to respond to preferred host plant cues sooner than larger larvae but larger larvae rejected less preferred host plants for longer than smaller larvae. Overall, smaller H. erythrogramma larvae are likely to have less dispersal potential and are more likely to settle in less preferred habitats whereas larger larvae appear to have an obligately longer dispersal period but settle in preferred habitats.
Our results suggest that marine invertebrates that produce non-feeding larvae may have the potential to affect the dispersal of their offspring in previously unanticipated ways and that offspring size is subject to a complex web of selection across life-history stages.
Marshall DJ, Steinberg PD (2014) Larval size and age affect colonization in a marine invertebrate, Journal of Experimental Biology, 217: 3981–3987 PDF 403 KB doi:10.1242/jeb.111781
Authors: Scott C Burgess, Michael Bode and Dustin J Marshall
Published in: Functional Ecology doi: 10.1111/1365-2435.12080
Much of the theory on offspring size focuses on the effects of habitat quality on the relationship between offspring size and fitness. Habitat spacing may be another important factor that affects selection on offspring size when offspring disperse prior to colonization and accrue deferred costs that are mediated by offspring size.
We developed a theoretical model, based on a well-known optimality model, of how selection on offspring size changes with dispersal distance. The model assumes that offspring fitness depends on both offspring size and dispersal duration and that dispersal time and distance are positively related. Such assumptions are based on thousands of marine invertebrate species with non-feeding larvae, but our model also applies more generally to any organism where offspring size modifies the energetic costs of dispersal, and there is a positive relationship between dispersal duration and distance.
Our model predicts that, even when habitat quality does not vary, more isolated habitats may favour the production of fewer, larger offspring if smaller offspring incur greater deferred costs of dispersal. We then empirically demonstrate that offspring size and dispersal duration have interactive effects on post-settlement survival in a marine invertebrate (Bugula neritina), and such size-dependent deferred costs of dispersal are of a magnitude sufficient enough to potentially favour larger offspring in isolated habitats.
Together, our results indicate that the spatial pattern of suitable habitat could impose very different selective regimes on offspring size compared with the effects of habitat quality. Furthermore, our predictions contrast to those predicted for seed size and dispersal in plants, where the production of smaller, more numerous seeds is often a more efficient way for mothers to access distant, suitable habitat.
Burgess SC, Bode M, Marshall DJ (2013) Costs of dispersal alter optimal offspring size in patchy habitats: combining theory and data for a marine invertebrate. Functional Ecology PDF 329 KB doi: 10.1111/1365-2435.12080