Data from: The evolutionary puzzle of egg size, oxygenation and parental care in aquatic environments

Offspring fitness generally improves with increasing egg size. Yet, eggs of most aquatic organisms are small. A common but largely untested assumption is that larger embryos require more oxygen than they can acquire through diffusion via the egg surface, constraining egg size evolution. However, we found no detrimental effects of large egg size on embryo growth and survival under hypoxic conditions. We tested this in the broad-nosed pipefish, Syngnathus typhle, whose males provide extensive care (nourishment, osmoregulation and oxygenation) to their young in a brood pouch on their bodies. We took advantage of this species' pronounced variation in egg size, correlating positively with female size, and tested the effect of hypoxia (40% dissolved oxygen) versus fully oxygenated (100%) water on embryo size and survival of large versus small eggs after 18 days of paternal brooding. Egg size did not affect embryo survival, regardless of O2 treatment. While hypoxia affected embryo size negatively, both large and small eggs showed similar reductions in growth. Males in hypoxia ventilated more and males with large eggs swam more, but neither treatment affected their position in the water column. Overall, our results call into question the most common explanation for constrained egg size evolution in aquatic environments.

后代适合度通常随卵尺寸的增大而提升。然而，绝大多数水生生物的卵却体型偏小。学界普遍提出但尚未得到充分验证的一项假设认为：更大的胚胎所需的氧气量，超出了其通过卵表面扩散所能获取的上限，这一约束限制了卵尺寸的演化方向。但本研究发现，在低氧（hypoxia）环境下，卵尺寸偏大并不会对胚胎的生长与存活产生不利影响。我们以宽吻海龙（Syngnathus typhle）为实验对象，该物种的雄性可通过体表的育幼袋（brood pouch）为幼崽提供全方位抚育，涵盖营养供给、渗透压调节与供氧。该物种的卵尺寸存在显著个体差异，且与雌性体型呈正相关，我们借此特性开展实验，对比了低氧（溶解氧占比40%）与完全充氧（溶解氧占比100%）水环境下，经雄性育幼18天后，大卵与小卵对应的胚胎尺寸与存活率差异。无论氧气处理条件如何，卵尺寸均未对胚胎存活率产生显著影响。尽管低氧环境会对胚胎尺寸产生负向影响，但大、小卵对应的胚胎生长降幅并无显著差异。处于低氧环境的雄性个体换气频次更高，携带大卵的雄性游动频次也更高，但两种氧气处理均未改变雄性在水层中的分布位置。综上，本研究结果对水生环境中卵尺寸演化受约束的主流解释提出了质疑。