Data from: Dispersal duration mediates selection on offspring size

Offspring size varies at all levels of organisation, among species, mothers and clutches. This variation is thought to be the result of a trade-off between offspring quality and quantity, where larger offspring perform better but are more costly to produce. Local environmental conditions alter the benefits of increased offspring size and thereby mediate selection on this trait. For sessile organisms, dispersal is a crucial part of the offspring phase, and in animals, bigger offspring tend to better endure longer dispersal distances than smaller offspring because they have more energy. Theory predicts that increasing distances between suitable habitats strengthens selection for larger offspring. We manipulated the dispersal duration of offspring of different sizes in the bryozoan Watersipora subtorquata and then examined the relationship between offspring size and post-metamorphic performance in the field. We found that selection on offspring size is altered by larval experience. Larger offspring had higher post-settlement performance if the larval period was short but, contrary to current theory, performed worse when the larval period was extended. The reversal of the relationship between offspring size and performance by extending the larval phase in Watersipora may be due to the way in which offspring size affects growth in this species. Regardless of the mechanism, it appears that experiences in one life-history stage alter selection on offspring size in another stage, even when they occupy identical habitats as adults.子代大小在多个组织层级均存在变异，涵盖物种间、母本间以及每窝幼体间。该类变异被认为源于子代质量与数量之间的权衡：子代个体越大，其生存表现越优，但繁育所需的资源成本也越高。局域环境条件会改变子代体型增大所带来的收益，进而介导该性状所受的选择压力。对于固着生物而言，扩散是子代阶段的关键环节；而在动物中，子代个体越大，其耐受长距离扩散的能力越强，原因在于其储备的能量更多。理论预测，适宜生境间的距离越远，对更大体型子代的选择压力就越强。我们以苔藓虫（bryozoan）Watersipora subtorquata为研究对象，操控不同大小子代的扩散时长，随后在野外环境中检测子代大小与变态后生存表现之间的关联。研究发现，幼虫经历会改变对子代体型的选择压力。当幼虫期较短时，体型更大的子代在固着后的生存表现更优；但与现有理论相悖的是，当幼虫期延长时，这类子代的生存表现反而更差。在Watersipora subtorquata中，延长幼虫期会逆转子代大小与生存表现间的关联，这一现象可能源于该物种中子代大小对生长的调控方式。无论具体调控机制为何，研究结果均表明：即便成体占据完全相同的生境，某一生活史阶段的经历也会改变另一阶段对子代体型的选择压力。