COULOMETRIC MEASUREMENT OF OXYGEN-CONSUMPTION DURING DEVELOPMENT OF MARINE INVERTEBRATE EMBRYOS AND LARVAE

Determining the metabolic rate of larval invertebrates from aquatic ha bitats is complicated by the problems of small size and the scarcity o f suitable measurement techniques, In this study, coulometric respirom etry (a new technique for the study of marine embryos and larvae) was used to explore several issues associated with the rate of energy use during embryonic and larval development of marine invertebrates from t hree phyla, Coulometric respirometry measures rates of oxygen consumpt ion under normoxic conditions by electrochemically replacing the oxyge n consumed by organisms during an experiment, This technique is based on the assumption that all electrons consumed by the anodic reactions result in the production of oxygen, We verify this assumption using di rect measurements of oxygen production and show that the technique is sensitive enough (1 nmol O-2 h(-1)) to quantify the oxygen consumption of a single individual swimming freely in a relatively large volume ( 2 ml), Continuous measurements can span days, and embryos in the coulo metric respiration chambers develop to the larval stage at normal rate s of differentiation. Measurements of metabolic rates were made with t he coulometric respirometer during the complete life-span of larvae of three species (asteroid, Asterina miniata; bivalve, Crassostrea gigas ; echinoid, Dendraster excentricus), For these species, metabolic powe r equations had mass exponents near unity (0.9-1.1), showing that meta bolic rate scales isometrically with mass during larval growth, Metabo lic rates were independent of the concentration of larvae used in the respirometer chambers for a range of larval concentrations from 4 to 4 00 larvae ml(-1) (coulometric respirometer) and from 241 to 809 larvae ml(-1) (polarographic oxygen sensor), Metabolic rates were measured u sing coulometric respirometry and two other commonly used techniques, polarographic oxygen sensors and Winkler's titration, Polarographic ox ygen sensors in small, sealed chambers (100 mu l) consistently gave th e lowest values (by as much as 80%) for the asteroid, echinoid and mol luscan larvae. By comparison, rates of oxygen consumption measured usi ng coulometric respirometry and Winkler's titration (to measure the ch ange in oxygen concentration over time) were similar and consistently higher, Although the polarographic oxygen sensor is the most widely us ed method for measuring the metabolism of small animals in sealed 100- 1000 mu l chambers, it appears that the metabolism of some larvae is a dversely affected by the conditions within these respirometers.