Differences in ontogenetic and diurnal microhabitat selection by sympatric live-bearing fish species with different reproductive modes

A pregnancy imposes a heavy reproductive burden on females. Some live-bearing species have evolved reproductive adaptations to reduce this burden, which may influence their ability to utilize specific microhabitats. We investigate whether two such reproductive adaptations, placentation (embryo provisioning via a placenta) and superfetation (the ability to carry multiple broods at various developmental stages), influence microhabitat selection by five sympatric Costa Rican live-bearing fish species (family Poeciliidae). Theory predicts that placentation and superfetation should both reduce the reproductive burden of females during pregnancy, improve their body streamlining, and swimming performance, and consequently allow them to utilize 'more performance-demanding' microhabitats. Here we apply underwater visual fish surveys to test a key prediction of this hypothesis, which is that the presence of these two reproductive adaptations is correlated with the utilization of microhabitats in the river that are characterized by a higher-flow velocity. Consistent with our predictions, we observed significant interspecific differences in daytime microhabitat use: species that had both placentation and superfetation were found in deeper and faster-flowing parts of the river, species that lacked both adaptations were confined to shallow slow-flowing areas, and species with one adaptation (i.e. only superfetation) inhabited intermediate areas. This interspecific daytime microhabitat use was strongest in reproductive adults, intermediate in immatures, and absent in juveniles (the latter of which were all found in shallow low-velocity zones), suggesting that ontogeny influences species-specific microhabitat use. Finally, at night, all fishes, regardless of the species or age-class, congregated in shallow slow-flowing waters to rest (sleep) on the river bottom. Taken together, our results suggest that placentation and superfetation may be hitherto unrecognized reproductive features that help to understand differences in ontogenetic and diurnal microhabitat preferences between sympatric live-bearing fish species living in environments characterized by large flow variation.

妊娠会对雌性个体造成沉重的生殖负担。部分胎生物种已演化出生殖适应性特征以减轻此类负担，这或会影响其对特定微生境的利用能力。本研究旨在探究两种此类生殖适应性特征——胎盘生殖（placentation，即通过胎盘为胚胎提供营养）与超胎生（superfetation，即携带处于不同发育阶段的多窝幼体的能力）——是否会影响五种同域分布的哥斯达黎加胎生鱼类（花鳉科，Poeciliidae）的微生境选择。理论预测，胎盘生殖与超胎生均可减轻雌性妊娠期间的生殖负担，改善其躯体流线型形态与游泳性能，进而使其能够利用对运动性能要求更高的微生境。本研究采用水下视觉鱼类调查法，对该假说的核心预测进行检验：即这两种生殖适应性特征的存在，与河流中流速更高的微生境利用情况呈正相关。与研究预测一致，我们观察到日间微生境利用存在显著的种间差异：同时具备胎盘生殖与超胎生两种适应性特征的物种，栖息于河流较深且流速更快的区域；同时缺乏这两种适应性特征的物种，仅分布于浅水区缓流区域；而仅具备其中一种适应性特征（即仅超胎生）的物种，则栖息于中间过渡区域。这种日间微生境利用的种间差异，在繁殖成体中最为显著，亚成体次之，幼体则无此类差异（所有幼体均栖息于浅水区缓流区域），这表明个体发育进程会影响物种特异性的微生境选择偏好。最后，在夜间，所有鱼类无论物种或年龄组，均聚集在浅水区缓流区域，栖息于河底休憩。综上，本研究结果表明，胎盘生殖与超胎生可能是此前未被认知的生殖特征，有助于解释在流速变化显著的环境中栖息的同域分布胎生鱼类物种之间，个体发育及日间微生境偏好差异。