Organic matter processing by the shrimp Palaemonetes sp.: Isotopic and elemental effects Jason P. Landrum ⁎, Joseph P. Montoya Georgia Institute of Technology, School of Biology, Environmental Science and Technology Building, Atlanta, Georgia 30332, United States abstract article info Article history: Received 8 February 2009 Received in revised form 4 August 2009 Accepted 31 August 2009 Keywords: Carbon Feces Isotope Nitrogen Shrimp Aquatic crustaceans often play a major role in organic matter (OM) transformation and recycling through their feeding and excretory activities. In this study, we measured the isotopic and elemental composition of organic matter fed to Palaemonetes sp. shrimp and the fecal pellets they produced. Nitrogen (N) content of the food (8.2 ± 0.2%, mean ± SD) was significantly higher than the fecal pellets (2.0 ± 0.9%), a pattern that also applied to the carbon (C) content of food (46.7 ± 1.0%) and fecal pellets (14.3 ± 6.8%). We also found a significant decrease in the N content of undigested, macerated food (6.1 ± 0.9 %) relative to food that had been soaked in artificial seawater (ASW) and artificial seawater that had previously contained shrimp (CASW) in the absence of feeding shrimp. We found no significant difference in N or C isotopic composition between the dry food, ASW- and CASW-soaked control food, and fecal pellets. We did, however, observe a significant increase in δ 15 N of the undigested, macerated food (δ 15 N =6.3 ± 0.6‰) relative to both the dry flake food (δ 15 N =5.6 ± 0.2‰) and controls incubated in the absence of shrimp in either ASW (δ 15 N=5.6± 0.3‰) or CASW (δ 15 N=5.8±0.1‰). Our results differ from previous findings of isotopic alteration of OM during processing by crustaceans (copepods), suggesting that isotopic changes related to feeding might be either taxon- or food-specific. This study also provides information on the influence of grazers/shredders on both the elemental and isotopic composition of POM, suggesting that larger aquatic shredders can influence the chemical composition of particles by either physical manipulation of the POM (release of DOM) or by facilitating microbial colonization of the POM. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Aquatic invertebrates play a central role in the transformation and movement of energy and nutrients through ecosystems (Banse, 1995; Graca, 2001; Frangoulis et al., 2005). Particulate organic matter (POM) consumed by invertebrates is partitioned into invertebrate biomass, dissolved excreta (e.g., NH 4 + , dissolved organic matter (DOM)), and fecal matter. Fecal matter plays an important role in POM flux and nutrient cycling in aquatic ecosystems (Wotton and Malmqvist, 2001). In pelagic environments, fecal material can dominate the vertical flux of POM, and is a critical component of the biological pump transporting C and N downward through the water column (Emerson and Roff, 1987; Dam et al., 1995; Sarnelle, 1999; Dagg et al., 2003; Frangoulis et al., 2005). Aquatic invertebrate grazers may also indirectly influence POM transformation, and DOM release via “sloppy feeding” (Moller, 2005; He and Wang, 2006; Moller, 2007), providing an important pathway for movement of energy and organic matter to other heterotrophs (e.g., microbes, coprophagous invertebrates). Palaemonetes sp. play an important role in estuarine food webs through their feeding and metabolic activities (e.g., consumption and excretion), and are an important resource for predators (Welsh, 1975; Morgan, 1980; Quinones-Rivera and Fleeger, 2005). Palaemonetes sp. are also good model crustaceans for studies of digestive processes given their size (e.g., large fecal pellets), voracious appetite, transparent gut, and long life spans (Johannes and Satomi, 1966; Welsh, 1975). In view of their role as grazers in estuarine systems, it is important to assess their impact on OM transformation through consumption and excretion, as well as through other indirect pathways (e.g., shredding). Stable isotopes of nitrogen and carbon have proved useful in tracing biologically-mediated input (Capone, 2001; Montoya et al., 2002), transformation (Wada and Hattori, 1976; Montoya et al., 1992; Nakatsuka et al., 1997; Breteler et al., 2002; Lehmann et al., 2002), and export (Altabet et al., 1991; Altabet and Francois, 1994; Voss et al., 1996; Smith et al., 2002; Lourey et al., 2003; Lourey et al., 2004) of these elements in marine ecosystems. Various studies have focused on the isotopic fractionation associated with the movement of carbon and nitrogen through food webs (Deniro and Epstein, 1981; Peterson and Fry, 1987; Wada et al., 1987; Kling et al., 1992; Montoya et al., 1992; Montoya et al., 2002), while other studies have addressed the potential for isotopic alteration of OM as a result of processing by Journal of Experimental Marine Biology and Ecology 380 (2009) 20–24 ⁎ Corresponding author. Present address: Georgia Institute of Technology, Sam Nunn School of International Affairs, Habersham Building, Atlanta, Georgia 30332, United States. Tel.: +1 404 385 8567; fax: +1 404 894 1900. E-mail address: jason.landrum@gatech.edu (J.P. Landrum). 0022-0981/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2009.08.020 Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe