Carbon Dioxide Fixation in Marine Invertebrates: III. THE MAIN PATHWAY IN FLATWORMS

Abstract

primarily in succinic, fumaric, and malic acids, suggested that most animals share the main pathway demonstrated in the oyster (2), namely, the carboxylation of propionate to form succinate, which is then metabolized through the citric acid cycle. The flatworms, Stylochus zebra and Bdelloura candida, however, had relatively little Cl4 in succinic acid and a large amount in an acid not identifiable with those of the cycle (1). The purpose of the experiments described in this report was to determine the principal pathways of COz fixation in free-living flatworms. The common fresh-water planarian Dugesia tigrina, the two marine turbellarians of the previous study, and a trematode Entobdella bumpusi were found to exhibit very similar patterns of labeling of organic acids after incubation for various periods in water containing NaHC1403. These flatworms form a series of increasing closeness of association with larger animals, from Dugesia, which has no association, to Stylochus, which inhabits the same shell as hermit crabs, to Bdelloura, which attaches to the legs and gills of the horseshoe crab. The monogenetic trematode E. bump& is an ectoparasite of the sting ray. The general results indicated that, although propionate is utilized by flatworms, the principal carboxylation in the absence of added substrate is the conversion of pyruvate to malate.