Incubation temperature affects development order of morphological features and staging criteria in turtle embryos

Embryonic development is strongly affected by incubation temperature in most animals, and the effects of incubation temperature continue after partition. However, relatively little is known about the effects of incubation temperature on the development of morphological features in reptiles. As a result, whether developmental rates of different traits change uniquely in response to incubation temperature remains unknown. In this study, we compared embryonic development of an Australian turtle incubated at different temperatures. We examined 43 internal and external morphological features of embryos at each week of development and compared presence and absence of each feature within and across weeks and temperature treatments, using a multivariate statistical approach. We identified morphological features that develop at different rates in turtle embryos incubated at different temperatures. Embryos incubated at 30°C hatched 3 weeks earlier, grew to hatchling size faster, and developed advanced morphological features earlier than those incubated at 26°C. Furthermore, embryonic features that appear late in development appeared progressively earlier in embryos incubated at 30°C. Embryos incubated at 26°C also retained some early embryonic features for longer periods of time during development, especially from weeks 2–4. Despite temperature effects, some traits appeared at the same week of development in embryos from both treatments, including a clearly distinguishable carapace and distinct gut villi at week 3. Variation in appearance and retention of features means that not all external features consistently appear in the same order, depending on incubation temperature. From a practical perspective, cool- and warm-incubated embryos may exhibit different sets of embryological features, which may not be captured by previously published embryological staging guides.