Life history optimization and physiological constraints: Effects of temperature and oxygen on zebrafish growth differ as a function of cell size, sex and maturity.

Many ectothermic animals follow the temperature-size rule, whereby animals grow faster, but mature earlier at a smaller size in the warm. Here we investigate how temperature and oxygen affect growth in diploid and triploid zebrafish, the latter having larger cells. Cell size may modulate physiological constraints via effects on oxygen supply and demand. Small cells benefit from enhanced capacity for oxygen uptake and transport owing to a more favourable membrane surface-area-to-volume ratio, while large cells benefit from decreased costs related to ion pumping for maintaining electrochemical gradients over cell membranes. Life-history effects may arise because triploids are sterile; triploids may therefore allocate more energy towards growth. To test these ideas, we reared fish at different temperatures and oxygen conditions. In the cold, zebrafish grew slower but also took longer to mature, exhibiting greater size at maturity. Warmer temperatures generally reduced post-maturity growth, contrasting with the increase in growth observed before maturity. After maturation, we found differences between ploidy levels with triploid males having a better growth performance than diploid males, but only in the cold. As a result, thermal responses in asymptotic size were most pronounced in triploids. Hypoxia reduced post-maturity growth, especially in diploid females. Our results broadly matched expectations based on cell size. Maturation schedules seemed to govern size at maturity, while for asymptotic size, our results support a stronger role for physiological constraints, which appear more prevalent in triploids and in warm water.