New study from the Centre for Ocean Life show how the huge differences in offspring size between sharks and bony fish can be explained by differences in early life density dependence
How does the life-history strategy differ between elasmobranchs (sharks, rays etc) and teleosts (bony fish)? With respect to the size of their offspring, one difference is striking: elasmobranchs make offspring that is roughly a factor 100 (in weight) smaller than the adults, while most teleosts make offspring that is around 1 mg, independent of the adult size (see figure). We show that the difference in the offspring size strategy is unrelated to whether offspring is live or eggs, and there are no significant differences between growth, mortality etc. of elasmobranchs and teleosts.
How, then, can evolution maintain these two very different offspring size strategies among very similar groups of organisms? In a recent paper in Journal of Theoretical Biology we develop an evolutionary model to show how the two offspring size strategies can co-exists evolutionary — in technical terms, we can say that there exists two local evolutionary stable states. The difference between the two strategies boils down to a difference in density-dependent control in the populations: if newly hatched offspring compete with one another, the evolutionary stable strategy is one where offspring size proportional to the adult size (the elasmobranch strategy), while if early life is devoid of density-dependent competition, the strategy with small offspring independent of adult size emerges