What fraction of the organic carbon in sacoglossans is obtained from photosynthesis by kleptoplastids? An investigation using the natural abundance of stable carbon isotopes

Any sacoglossans (opisthobranch gastropods) have the potential for carbon a cquisition from photosynthesis by plastids sequestered from their macroalga l food as well as by ingestion, digestion and assimilation of the organic c arbon derived from the alga. A new method for obtaining a minimum estimate of the fraction of sacoglossan carbon supplied from photosynthesis by klept oplastids is suggested, based on the mass balance of stable carbon isotopes at the natural abundance level. The method involves comparison of C-13/C-1 2 ratios in sacoglossans with those of the algae on which they are found. D ifferences in ratios between alga and sacoglossan are used to give a minimu m estimate of carbon acquisition by kleptoplasty, granted assumptions about the range of C-13/C-12 fractionation values which can occur for marine pho tolithotrophs. The new method is applied to several green (ulvophycean) alg a-sacoglossan associations from Rottnest Island, Western Australia, and the values compared with those obtained previously by other means. The method suggests values of up to 0.6 of the total carbon input to the sacoglossans from photosynthesis by their kleptoplastids. To improve the estimates of th e minimum role of kleptoplastidy in the carbon nutrition of sacoglossans, f urther information is needed: (1) on the fidelity of a given sacoglossan to a given algal individual (or species), (2) on the C-13/C-12 ratio of the p art of the alga ingested by the sacoglossan, and (3) on the allocation of d ietary organic carbon and of kleptoplastidic photosynthate to carbon lost i n respiration, mucopolysaccharide production and gametes (and hence not sam pled with the animal).