Reference: Biol. Bull 176: 317-326. (June. 1989) Amino Acid Uptake and Metabolism by Larvae of the Marine Worm Urechis caupo (Echiura), a New Species in Axenic Culture WILLIAM B. JAECKLE 1 AND DONAL T. MANAHAN 2 Department of Biological Sciences, University of Southern California, Los Angeles, California 90089-0371 Abstract. Axenic (bacteria-free) larval cultures of the marine echiuran worm, Urechis caupo, were reliably ob-tained by aseptically removing gametes directly from the gamete storage organs. Trochophore larvae only re-moved neutral amino acids from seawater as measured by high-performance liquid chromatography (HPLC). There was no detectable uptake, as measured by HPLC, of acidic or basic amino acids. Kinetic analysis showed that the transport system for alanine in 4-day-old larvae had a K, of 4-6 nM and a J max of 9-10 pmol larva' ' h '. Following a 50-min exposure, the majority of the radio-activity (95%) from ' 4 C-alanine was found in the trichlo-roacetic acid-soluble fraction. Very little label appeared as acid-insoluble material, and there was no detectable lipid biosynthesis from l4 C-alanine. Approximately 12% of the total alanine transported was released in the form of 14 CO 2 . Thin-layer chromatography of intracellular free amino acid pools demonstrated that aspartic acid and glutamic acid were radiolabeled from the alanine precursor. A comparison of the energy acquired from the transport of alanine, with the metabolic rate of 4-day-old larvae, revealed that 51% of the metabolic demand could be provided by the transport and complete catabolism of this single amino acid at a concentration of 595 nA/ in seawater. Received 21 September 1988; accepted 27 March 1989. ' Present address: Harbor Branch Oceanographic Institution, 5600 Old Dixie Highway. Fort Pierce. Florida 34949. : To whom reprint requests should be addressed. Unusual abbreviations: DAPI. 4'.6-diamidmo-2-phenylmdole; DOM, dissolved organic material; HPLC, high-performance liquid chromatography. Introduction Planktotrophic (feeding) larvae of marine inverte-brates must obtain food from the environment in order to supply energy for growth and metabolism (Thorson. 1 946; Mileikovsky, 1971). These larvae possess anatomi-cal adaptations to concentrate and clear particles from seawater (Strathmann, 1971; Strathmann et al, 1972). However, adaptations for energy and nutrient acquisi-tion need not exist only for the capture of particulate food. Larvae have a large surface area to volume ratio owing to their small size. Structural elaborations for lo-comotion and particle capture also increase the surface area (e.g., molluskan velum, echinoderm ciliated bands). The total surface area of the epithelium is further en-hanced by the presence of an apical brush border on cer-tain cells (e.g.. Waller, 1981; Amieva and Reed, 1987). In addition to the anatomical modifications used for the capture of particles, both larval and adult soft-bodied marine invertebrates can take up dissolved organic mate-rial (DOM) directly from seawater across their body-wall (see review by Stephens, 1988). Uptake of DOM by larvae has been primarily studied as the fluxes of free amino acids from seawater. To date, using a variety of analytical techniques, amino acid transport has been demonstrated for a number of plank-totrophic larvae. Larvae of the annelids Nereis virens and Neanthes arenaceodentata accumulate radioactivity when exposed to 14 C-labeled amino acids in seawater (Bass et al., 1969; Reish and Stephens, 1969). Amino acid influx and net flux has been reported for plutei of two species of echinoid echinoderms (Strongylocentrotus 317
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