Consumption of terrestrial organic matter by estuarine molluscs determined by analysis of their stable isotopes and cellulase activity Emily S. Antonio a, * , Akihide Kasai b , Masahiro Ueno a , Yoshiro Kurikawa b , Kanako Tsuchiya b , Haruhiko Toyohara b , Yuka Ishihi c , Hisashi Yokoyama c , Yoh Yamashita a a Maizuru Fisheries Research Station, Kyoto University, Nagahama, Maizuru, Kyoto, 625-0086 Japan b Graduate School of Agriculture, Kyoto University, Kyoto, 606-8502, Japan c National Research Institute of Aquaculture, Fisheries Research Agency, Minami-Ise, Mie 516-0193, Japan article info Article history: Received 31 January 2009 Accepted 16 May 2009 Available online 28 May 2009 Keywords: stable isotope feeding niche molluscs estuary abstract We examined the feeding niche of four species of molluscs by analysis of their stable isotope signatures and cellulase activities to determine if they could utilize terrestrial organic matter. The molluscs and potential food sources were collected from the upper, middle and lower estuary of the Yura River from spring 2007 to winter 2008. All species showed positive cellulase activity which highlighted their potential to digest terrestrial organic matter. Consumption and assimilation of terrestrial organic matter by estuarine molluscs however varied spatially and temporally, reflecting species-specific differences in feeding niche and in response to variations in food availability in the estuary. Thus, terrestrial primary production in the catchment area supports secondary production of molluscs in the Yura River estuary. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Estuarine molluscs constitute a diverse and abundant tax- onomical group occupying a wide habitat range along river– estuary–coastal systems. They occupy variable feeding niches and provide an important link between organic matter and higher consumers in aquatic food webs. Most gastropods are considered to be deposit feeders of sedimentary materials (Kesler, 1983) while bivalves are filter feeders of suspended materials (Yamamuro and Koike, 1993). Organic matter in estuaries comes from various origins but the relative importance of these sources to estuarine molluscs is poorly resolved. Many studies have shown that primary production of marine phytoplankton and benthic microalgae are the main energy sources of estuarine (Herman et al., 2000; Moens et al., 2002; Kasai et al., 2004) and coastal (Kang et al., 2006; Yokoyama et al., 2005; Yokoyama and Ishihi, 2007) benthic consumers, while other studies emphasized the importance of littoral plants such as reeds and macrophytes (Tenore, 1983; Currin et al., 1995; Kang et al., 2007). However, terrestrial organic matter may be an important food source for estuarine consumers as it accumulates rapidly on the bed and along flood plains of the river– estuary systems, supplying large amounts of allochthonous input. Few studies have shown the importance of terrestrial plants to aquatic consumers (Riera and Richard, 1996; Kasai and Nakata, 2005; Kanaya et al., 2008). This may be due to the fact that terrestrial detritus is typically composed of structural refractory materials such as cellulose and lignin that most macrobenthic detritus feeders are unable to digest and assimilate (Cividanes et al., 2002). Detritus from vascular plants is characterized by low caloric content, typically resistant to decay and becomes available to macroconsumers only after microbial breakdown (Tenore, 1983). However, animals possessing enzymes that have the ability to break glycosidic bonds of cellulose into oligosaccharides could take advantage of this energy source directly (Kesler, 1983). Gut extracts of freshwater molluscs and crustaceans were found to strongly hydrolyze cellulose (Monk, 2006). Endogenous cellulase genes have been cloned from clam Corbicula japonica (Sakamoto et al., 2007), abalone (Suzuki et al., 2003), snail (Wang et al., 2003), mussel (Xu et al., 2001) and crayfish (Byrne et al., 1999) proving direct digestion of cellulose by those aquatic invertebrates. Digestive enzymes could be used as a complementary tool for determining which dietary components are effectively metabolized (Johnston and Freeman, 2005) but it remains insufficient to determine to what extent these components contributed to the organism’s diet. * Corresponding author. E-mail address: emily@kais.kyoto-u.ac.jp (E.S. Antonio). Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2009.05.010 Estuarine, Coastal and Shelf Science 86 (2010) 401–407