Effects of grazing by deposit feeders on biogenic hydrocarbons in coral-reef surface sediments

The Fraction I hydrocarbons (containing alkanes, some alkenes, cyclo-alkanes or-alkenes) in the top 2 cm of sediment, in two species of deposit feeders (the holothurian Holothuria atra and the gastropod Strombus luhuanus), and in their feces were analyzed. Collections were made at Heron Island in December, 1982, and analyses were completed in January 1983. On the basis of their relative concentrations in whole samples, and in the lipid fractions of sediment and animal tissue, the Fraction I hydrocarbons were placed in three categories. Some compounds give evidence of passive movement from a high concentration in sediment lipid into an area of lower concentration in animal lipid fractions. Others may be selectively excreted and/or degraded by animals; while still others appear to be selectively accumulated and/or produced in animal tissue. The relatively short-chain n-alkanes, mono-olefins and C₂₅ cyclo-alkanes or alkenes in sediment and feces showed a net loss from sediment on its passage through the animals, whereas C₂₂ to C₃₂ n-alkanes, showed a net production in the guts of the animals and contributed to the pool of these compounds in sediments. About 40 to 55% of the group of short-chain n-alkanes, mono-olefins and C₂₅ cyclo-alkanes or alkenes in sediment are converted into long-chain n-alkanes as the sediment passes through the guts of H. atra and S. luhuanus. H. atra remove about 0.2% of the standing pool of short-chain n-alkanes, mono-olefins and C₂₅ cyclo-alkanes or alkenes in an average square-metre of sediment, of 2 cm depth, per day, and they contribute an amount of n-C₂₂ to n-C₃₂ alkanes equal to about 1.1% of the standing pool of these compounds.