The size—and hence the age—at first maturity is a very important feature of fish and other water-breathing ectotherms (WBE), as it determines, jointly with their subsequent growth, longevity, and fecundity, the reproductive output that their life-history attempts to optimize. Here, based on the fact that gill surface area (S) is related to their weight (W) according to S ∝ W^d, with d always < 1, which causes relative oxygen supply to decline with size, and body fluids and tissues to gradually become hypercapnic and acidic, we posit that a pH “reproductive respiration threshold” (RRT) triggers the hormonal cascade leading to maturation. Previous studies showed that such a RRT exists and takes similar values in hundreds of species of bony fish and invertebrate WBE. Here, we show that the RRT is significantly lower in non-viviparous marine Chondrichthyes, as assessed based on 1622 pairs of length at first maturity and maximum length in 918 species of Chondrichthyes, i.e., 414 species of sharks, 470 of rays, and 34 of chimaera. The hypothesis that the high concentration of urea retained for the purpose of osmoregulation by marine Chondrichthyes is the reason for this difference was tested through comparison of marine, euryhaline, and freshwater rays, which retain differing urea concentrations. We found that in rays, decreasing urea retention correlates with increasing RRT, with freshwater rays approaching the values occurring in bony fish. Finally, we found that viviparity, which increases the oxygen demand of pregnant Chondrichthyes, induces higher RRT, thus again linking respiration and the attainment of first maturity. These results support the existence of critical maturation RRT, while also suggesting a role for urea that was previously undocumented.

首次成熟时的体型大小以及年龄是鱼类和其他水呼吸变温动物 (WBE) 的一个非常重要的特征，因为它与随后的生长、寿命和繁殖力一起决定了它们一生的生殖输出。历史尝试优化。这里，基于鳃表面积（S ）与其重量（ W ）相关的事实，根据S∝Wd ^， d总是<1，这导致相对氧气供应量随着尺寸的增加而下降，体液和组织也随之减少 。 逐渐变得高碳酸血症和酸性，我们认为 pH 值“生殖呼吸阈值”（RRT）会触发激素级联反应，从而导致成熟。先前的研究表明，这种 RRT 存在，并且在数百种硬骨鱼和无脊椎动物 WBE 中具有相似的值。在这里，我们根据 918 种软骨鱼（即 414 种鲨鱼、470 种鳐鱼和 34 种软骨鱼）的 1622 对首次成熟长度和最大长度进行评估，结果显示，非胎生海洋软骨鱼的 RRT 显着较低。奇美拉。通过比较保留不同尿素浓度的海洋、广盐和淡水鳐鱼，测试了海洋软骨鱼为了渗透调节而保留的高浓度尿素是造成这种差异的原因的假设。我们发现，在鳐鱼中，尿素保留的减少与 RRT 的增加相关，淡水鳐鱼接近硬骨鱼中的值。最后，我们发现胎生会增加怀孕软骨鱼的需氧量，从而诱导更高的 RRT，从而再次将呼吸与首次成熟的实现联系起来。这些结果支持了临界成熟 RRT 的存在，同时也表明了尿素的作用，这是以前未记录的。