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.