Measurement of feeding rates, respiration, and pH regulatory processes in the light of ocean acidification research.

The physiology of marine larvae has received considerable attention in the context of anthropogenic ocean acidification (OA). Many marine larvae including those of echinoderms, hemichordates, and mollusks are characterized by a developmental delay when exposed to reductions in seawater pH with the underlying mechanisms being largely unexplored. A key task in the frame of OA research lies in the identification of unifying physiological principles that may explain reductions in growth and development. The sea urchin larva has been identified as a good model organism, and energy allocations toward compensatory processes were found to be key factors affecting development. However, physiological approaches to assess the animal's energy budget, as well as methods to characterize energy consuming processes (e.g., gut pH homeostasis and biomineralization) were scarce. During the last decade, a suite of physiological techniques was developed, to accurately determine the larval energy budget including feeding and metabolic rate measurements. To identify and characterize energy consuming processes, gastroscopic pH measurements in the larval gut and intracellular pH measurements of primary mesenchyme cells were developed. These techniques helped to understand fundamental processes of gut homeostasis and biomineralization in the developing sea urchin larva and their interaction with the environment. Using the sea urchin larva as a model these methods were successfully transferred to other echinoderm and hemichordate early developmental stages. This chapter explains and provides the methodological basis for the determination of feeding and metabolic rates as well as intracellular and extracellular pH measurements using the sea urchin larva as an example.