1. 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 use specific microhabitats.
2. 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 use more performance-demanding microhabitats.
3. 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 use of microhabitats in the river that are characterised by a higher-flow velocity.
4. 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.
5. Taken together, our results suggest that placentation and superfetation may be hitherto unrecognised reproductive features that help to explain differences in ontogenetic and diurnal microhabitat preferences between sympatric live-bearing fish species living in environments characterised by large flow variation.

中文翻译：

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不同繁殖方式的同域活鱼在个体发育和昼夜微生境选择上的差异

1. 怀孕给女性带来了沉重的生殖负担。一些活的物种已经进化出繁殖适应性以减轻这种负担，这可能会影响它们使用特定微生境的能力。
2. 我们调查了两种这样的生殖适应性，胎盘（通过胎盘提供胚胎）和超胎（在不同发育阶段携带多个育雏的能力）是否影响五种同域哥斯达黎加活鱼物种（Poeciliidae）的微生境选择。理论预测，胎盘和超胎应该都可以减轻女性在怀孕期间的生殖负担，改善她们的身体流线型和游泳能力，从而使她们能够使用对性能要求更高的微栖息地。
3. 在这里，我们应用水下视觉鱼类调查来测试这一假设的关键预测，即这两种繁殖适应性的存在与河流中以较高流速为特征的微生境的使用相关。
4. 与我们的预测一致，我们观察到白天微生境使用的显着种间差异：在河流较深和流速较快的部分发现了具有胎盘和超胎的物种，缺乏这两种适应性的物种仅限于浅层缓慢流动的区域，以及具有一种适应性（即只有超繁殖）的物种栖息在中间区域。这种种间白天微生境的使用在生殖成虫中最强，在未成熟的中等，在幼鱼中不存在（后者都在浅层低速区发现），表明个体发育影响物种特异性微生境的使用。最后，到了晚上，所有的鱼，不分种类或年龄等级，都聚集在水流缓慢的浅水区，在河底休息（睡觉）。
5. 综上所述，我们的结果表明胎盘和超胎可能是迄今为止未被认识的生殖特征，这有助于解释生活在以大流量变化为特征的环境中的同域活鱼物种之间个体发育和昼夜微栖息地偏好的差异。