Regeneration physiology of invertebrates

One of the great puzzles in biology is to understand the mechanisms underlying animal regeneration. Most recent efforts have used developmental and informatics approaches to understand how regenerated structures are formed, framing regeneration as a developmental outcome. However, regeneration is a complex process that also involves dynamic physiological mechanisms that support and fuel the rebuilding of lost structures. To develop a full understanding of regeneration, including how it relates to the ecology and evolution of organisms, it is essential to understand regeneration physiology. Despite the importance of physiological processes for regeneration, studies of regeneration focused on energetics, metabolism and environmental effects are scarce and have not been synthesized. This Review discusses the current understanding of regeneration physiology, focusing specifically on data from invertebrate animals where such information is especially dispersed and in need of synthesis. Considering data from diverse animal phyla, we review evidence for the consumption of different nutritional substrates during regeneration, summarize how aerobic and anaerobic metabolism appear to be broadly important to regeneration across animal phyla, and discuss how environmental and biotic factors can affect regeneration outcomes. We also introduce the concept of the 'physiological regeneration niche', describing the abiotic and biotic parameters where regeneration is possible, to expand consideration of regeneration in an ecological context. Significant gaps remain in understanding the physiological processes that underlie invertebrate regeneration, and we highlight some of these, including the need for broader taxonomic sampling, assessments of anaerobic metabolism during regeneration, investigations of multiple stressor effects on regeneration and comparisons between regenerators and non-regenerators.