Differential effects of temperature on multiple components of fitness in a modular animal (Bugula neritina) reveal how temperature affects reproductive capacity

Thermal performance curves (TPCs) are important tools for predicting the sensitivity of populations to climate change. The goal of this study was to assess and compare the relationships between temperature and different life-history components in a modular animal to reveal the mechanisms underlying TPCs for fitness. We reared replicated clones of the marine bryozoan Bugula neritina across a thermal gradient (16 values) ranging from 23 to 32°C, which reflected the upper thermal range of seasonal variation in the field. TPCs were constructed for survival (measured as zooids states within a colony), growth rate, development to reproductive maturity and reproductive capacity, which were measured over much of the realized lifespan expected under field conditions (~30 days). The effect of temperature was more acute on zooid states rather than whole-colony survival, and increased temperature increased the frequency of polypide regression. Most colonies reached reproductive maturity up to ~30°C, but growth rate and reproduction decreased at temperatures beyond ~25°C. The decline in reproductive capacity over temperatures above ~25°C was then due to the decline in the production of zooids capable of brooding embryos and zooids transitioning to regressed states up until about 30°C and transitioning to dead state beyond that.热性能曲线（Thermal performance curves, TPCs）是预测种群对气候变化敏感性的重要工具。本研究旨在评估并比较模块化动物体内温度与不同生活史组分间的关联，以揭示适配度相关TPCs的内在作用机制。我们在覆盖野外季节波动高温上限的23℃至32℃温度梯度（共16个梯度水平）下，繁育了海洋苔藓虫膜孔苔虫（Bugula neritina）的重复克隆株。本研究针对存活情况（以群体内个员状态为衡量指标）、生长速率、发育至生殖成熟的进程以及生殖能力构建了TPCs，相关指标的测定覆盖了野外条件下预期实际寿命的大部分时长（约30天）。研究发现，温度对个员状态的影响显著强于群体整体存活，且温度升高会提升虫体退化的发生频率。多数群体可在约30℃前达到生殖成熟，但当温度超过约25℃时，生长速率与生殖能力均会下降。当温度高于约25℃时，生殖能力的下降源于两方面因素：一是能够育幼胚胎的个员生成量减少，二是直至约30℃时向退化状态转变的个员比例上升；当温度超过30℃后，个员则会直接转为死亡状态。