Data from: Oxygen limitation at the larval stage and the evolution of maternal investment per offspring in aquatic environments

Many selective agents have been implicated in the evolution of maternal investment per offspring. Although oxygen limitation and surface-area-to-volume relationships were historically thought to constrain investment in individual eggs in aquatic environments, a weight of evidence now indicates that egg size per se does not influence oxygen availability to embryos. Yet, larval size is related to egg size, and hence investment per offspring, thus the scaling of oxygen assimilation efficiency with larval size may still constrain investment per offspring in aquatic environments. We employ comparative methods in the Amphibia to investigate this problem. We demonstrate that the slope of species-mean egg diameter over habitat temperature is negative for species with aquatic larvae, and the decline in egg size is similar whether eggs are laid terrestrially or aquatically. Further, the slope of egg diameter over temperature is more negative for species with aquatic larvae compared to species that feature terrestrial eggs and no larvae. Finally, we show that egg size declines with temperature more strongly for species whose larvae cannot breathe aerially prior to metamorphosis, compared to those that can. Our results suggest that oxygen transport in larvae, not eggs, constrains investment per offspring in aquatic systems. This study also helps reconcile conflicting reports that undermined the generality of temperature-dependent oxygen limitation as a mechanism driving the temperature-size rule in aquatic systems.

诸多选择因子（selective agents）均被证实与每子代母体投入的演化过程相关。尽管长期以来学界认为氧限制（oxygen limitation）与表面积体积比（surface-area-to-volume）关系会约束水生环境中单枚卵的投入量，但如今大量研究证据表明，卵大小本身（egg size per se）并不会影响胚胎的氧气可获得性。然而幼虫体型与卵大小相关，进而与每子代母体投入相关，因此氧同化效率随幼虫体型的标度关系仍可能约束水生环境中的每子代母体投入。本研究采用比较研究法（comparative methods），以两栖纲（Amphibia）类群为研究对象探讨该问题。结果显示，具有水生幼虫的物种，其平均卵直径随栖息温度的变化斜率为负值；且无论卵产于陆地还是水生环境中，卵大小的下降幅度均相近。此外，与仅产陆地卵且无水生幼虫的物种相比，具有水生幼虫的物种，其卵直径随温度的变化斜率更负（即下降幅度更快）。最后，相较于幼虫在变态发育前可进行空气呼吸的类群，幼虫在变态前无法进行空气呼吸的物种，其卵大小随温度下降的幅度更为显著。本研究结果表明，水生系统中约束每子代母体投入的是幼虫的氧气运输能力，而非卵本身。本研究还有助于调和此前相互矛盾的研究结论——这些结论曾质疑"依赖温度的氧限制是驱动水生系统温度体型法则（temperature-size rule）的机制"这一观点的普适性。