Various factors can affect the rate of embryonic development of ectotherm organisms. For example embryos in relatively small eggs develop faster than embryos in relatively large eggs. Temperature also affects rate of development significantly, where embryos develop faster at warmer temperatures.

Changes in rate of development can be crucial in shaping life history and behaviour later in the life, and may be important for the evolution of diversity. This aspect is of special importance today when global temperatures are rising at rates previously unknown.

In the present study we examined the rate of development of Arctic charr (Salvelinus alpinus) embryos and juveniles. We exposed several sibling groups of variable egg sizes to two temperatures and measured various variables over development. All embryos and juveniles were followed individually before and after hatching. Our results show that although temperature is important in increasing developmental rate, absolute time of development was also important. Our findings question the use of meters such as degree-days classically used in developmental studies.