Possibility of direct utilization of seagrass and algae as main food resources by small gastropod, Lacuna decorata, in a subarctic lagoon, Hichirippu, eastern Hokkaido, Japan with stable isotope evidences of carbon and nitrogen

Abstract

Abstract:The small gastropod, Lacuna decorata Adams, living on macrophytobenthos or surface sediment, is one of the most dominant species of macrozoobenthos in Hichirippu lagoon covered with seagrass and macroalgae, eastern Hokkaido, Japan. We measured the standing stocks of primary producers and macrozoobenthos, and determined the stable carbon and nitrogen isotope ratios of the primary producers and L. decorata. With these results, we identify the main food items for L. decorata and discuss the feeding strategy of the small gastropod. This gastropod occupied about 64% in density and about 25% in biomass of the macrozoobenthos at all six sampling stations in the lagoon. It occurred densely on the surface of the sediment with dense patches of benthic microalgae (BMA), which contained extremely high levels of Chl.-a between 84 to 226 mg m Ϫ2 throughout the period of this study. Nevertheless, the stable isotope signatures of carbon and nitrogen of this gastropod clearly show the direct utilization of organic matter derived from seagrass, Zostera japonica, in the areas where the seagrass luxuriated. However, it shows also a flexible feeding strategy in food preference. It fed green algae such as Ulva pertusa and Urospora wormskioldii in the areas where the seagrass grew scarcely.Key words: food resource, gastropod, green algae, Lacuna decorata, lagoon, seagrass, stable isotope, Zostera japonica * Corresponding author: Rumiko Kajihara; E-mail, rmk-kjhr@ees.hokudai.ac.jpThe description of feeding activities of the dominant species in the macrobenthic communities is important to follow the material or energy transportation from the primary producers to the primary consumers (the secondary producers) in the benthic ecosystem. However, it is often not easy to observe the feeding activities of small macrobenthic animals such as L. decorata and to identify their main food items by direct observation of feeding behaviors. For the description of food chain in the marine ecosystem, the techniques of stable isotope analysis have become popular recently, since the accumulation of empirical results on the carbon and nitrogen stable isotope ratios (d 13 C and d 15 N) has revealed that each primary producer, such as phytoplankton, benthic microalgae, seagrass and macroalgae, has a unique range of the stable carbon isotope ratios (Fry & Sherr 1984, France 1995, animals have a slightly enriched carbon stable isotope ratio (0.8Ϯ1.1‰, d13 C) to those of their main food items, and the enrichment of nitrogen stable isotope ratio between two different trophic levels in the food chain is relatively constant (d 15 N: 3.4Ϯ1.1‰) (DeNiro & Epstein 1978, Minagawa & Wada 1984. The determination of carbon and nitrogen isotope ratios of the small gastropod, L. decorata, also seems to be helpful for identification of main food items, particularly because it occurs in the two different habitats on the sediment of the sea floor and on the leaves of seagrass or the thalli of macroalgae.We have studied the structure of lagoon ecosystems in eastern Ho...

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Food Sources for Ruditapes philippinarum in a Coastal Lagoon Determined by Mass Balance and Stable Isotope Approaches

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