Non-lethal imaging and modeling approaches for estimating dry mass in aquatic larvae

Daniela Granados Frias, Najva Akbari, Lauren A O'Connell, Bryan Hernandez Juarez 

Body mass is crucial for scaling and comparing physiological rates. For example, dry body mass is important in determining an organism’s metabolic rate since it excludes metabolically inactive water weight. Obtaining repeated measurements of body mass throughout an individual’s lifetime is trivial. In contrast, we are normally able to obtain only a single estimate of dry body mass per individual since classical methods require end-point euthanasia followed by drying. We present imaging and modeling techniques for estimating individual dry body mass in African clawed frog (Xenopus laevis) tadpoles, which allows repeated sampling of the same individuals. We applied allometric principles and tested whether external anatomy would yield reliable estimates of dry body mass. Specifically, we describe a procedure to embed tadpoles in agarose media for obtaining morphological data in 3-D and then we evaluate dry mass predictions among nine cross-validated maximum likelihood and machine learning models. The best performing and flexible model is an allometric model that uses estimates of body volume to predict dry body mass (validation r2 = 0.75). However, other models based only on wet body mass or meant to reduce the number of necessary input variables may also be logistically tractable. We discuss the pros, cons, and future directions of all nine models and give practical advice for users on data collection and analysis. This research develops a strong foundation for continued research on the biological importance of dry body mass, particularly in the context of growth and physiological ecology. Future development of similar approaches is crucial for understanding the importance of body mass indices for standardization and comparison of physiological rates in plants and animals.