Carbon and nitrogen balance of leaf-eating sesarmid crabs (Neoepisesarma versicolor) offered different food sources Nalinee Thongtham a, * , Erik Kristensen b a Phuket Marine Biological Center, P.O. Box 60, Phuket 83000, Thailand b Institute of Biology, University of Southern Denmark, DK-5230 Odense M, Denmark Received 11 October 2004; accepted 30 May 2005 Available online 2 August 2005 Abstract Carbon and nitrogen budgets for the leaf-eating crab, Neoepisesarma versicolor, were established for individuals living on pure leaf diets. Crabs were fed fresh (green), senescent (yellow) and partly degraded (brown) leaves of the mangrove tree Rhizophora apiculata. Ingestion, egestion and metabolic loss of carbon and nitrogen were determined from laboratory experiments. In addition, bacterial abundance in various compartments of the crabs’ digestive tract was enumerated after dissection of live individuals. Ingestion and egestion rates (in terms of dry weight) were highest, while the assimilation efficiency was poorest for crabs fed on brown leaves. The low assimilation efficiency was more than counteracted by the high ingestion rate providing more carbon for growth than for crabs fed green and yellow leaves. In any case, the results show that all types of leaves can provide adequate carbon while nitrogen was insufficient to support both maintenance (yellow leaves) and growth (green, yellow and brown leaves). Leaf- eating crabs must therefore obtain supplementary nitrogen by other means in order to meet their nitrogen requirement. Three hypotheses were evaluated: (1) crabs supplement their diet with bacteria and benthic microalgae by ingesting own faeces and/or selective grazing at the sediment surface; (2) assimilation of symbiotic nitrogen-fixing bacteria in the crabs’ own intestinal system; and (3) nitrogen storage following occasional feeding on animal tissues (e.g. meiofauna and carcasses). It appears that hypothesis 1 is of limited importance for N. versicolor since faeces and sediment can only supply a minor fraction of the missing nitrogen due to physical constraints on the amount of material the crabs can consume. Hypothesis 2 can be ruled out because tests showed no nitrogen fixation activity in the intestinal system of N. versicolor. It is therefore likely that leaf-eating crabs provide most of their nitrogen requirement from intracellular deposits following occasionally ingestion of animal tissue (hypothesis 3). Ó 2005 Elsevier Ltd. All rights reserved. Keywords: assimilation efficiency; carbon and nitrogen budgets; defaecation; ingestion; Neoepisesarma versicolor; sesarmid crab 1. Introduction Leaf-eating sesarmid crabs play a significant role in the detritus food-chain and energy flow in most mangrove environments (Robertson, 1986; Poovachir- anon and Tantichodok, 1991; Lee, 1998; Skov and Hartnoll, 2002). The crab Sesarma messa is known to process 30e70% of the total leaf production in Australian mangrove forests by eating both attached leaves and fallen leaf litter (Robertson et al., 1992). Similarly, populations of sesarmid crabs (e.g. Neo- episesarma versicolor) can handle 87% and potentially ingest 52% of the daily leaf litter fall in mangrove forests of Thailand (N. Thongtham, unpublished). By feeding on leaf litter and other debris that has fallen from trees, sesarmid crabs prevent tidal export of valuable nutrients from the mangrove environment (Lee, 1998). The crabs * Corresponding author. E-mail address: nalineeth@hotmail.com (N. Thongtham). 0272-7714/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2005.05.014