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 tradeoff 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.

子代大小在所有组织层级、不同物种、母本以及产卵群间均存在变异。该变异被认为源于子代质量与数量间的权衡（tradeoff）：子代个体越大，生存表现越佳，但繁育成本也越高。局域环境条件会改变增大子代大小所带来的收益，进而介导该性状所受的自然选择。对于固着生物（sessile organisms）而言，扩散是子代阶段的关键环节；而在动物中，子代个体更大者往往能比小型子代更好地耐受更长距离的扩散，因其储备的能量更多。理论预测，适宜生境间的距离增加会强化对大型子代的自然选择。本研究以苔藓虫（bryozoan）*Watersipora subtorquata*为对象，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表现间的关联。研究发现，幼虫经历会改变对子代大小的自然选择：当幼虫期较短时，大型子代的定居后表现更优；但与现有理论相悖的是，当幼虫期延长时，大型子代的表现反而更差。在*Watersipora subtorquata*中，延长幼虫阶段会逆转子代大小与表现间的关联，这一现象可能源于该物种中子代大小对生长的影响方式。无论具体机制为何，似乎某一生活史阶段的经历会改变另一阶段对子代大小的自然选择，即便成体占据的生境完全一致。