107 Cey. J. Sci. (Bio. Sci.) 35 (2):107-114, 2006 MANGROVE LEAF LITTER PROCESSING BY SESARMID CRABS S. Ravichandran*, T. Kannupandi and K. Kathiresan Centre of Advanced Study in Marine Biology, Annamalai University, Parangipettai – 608 502, India. Accepted: 30 May 2006 ABSTRACT The sesarmid crabs belonging to the Family Grapsidae and Subfamily Sesarminae are mainly herbivorous as proven in the present study by their gut-contents. Gut contents of Sesarma brockii, S. andersoni, and S. plicatum are almost similar containing high amount of vascular plant matter ranging from 58.33 to 72.54% of total diet. However, Metapograpsus maculatus (52.94%) and M. messor (40.27%) consumed a low amount of plant material. Under laboratory conditions, S. brockii preferred to consume 40-day decomposed Avicennia marina leaves. This serves as a good-quality food, helping growth, survival and moulting frequency of the crabs. Thus, the leaf-eating crabs breakdown the mangrove leaf litter to detrital sized particles and they regulate the nutritive pathway in the mangrove environment. Key words: Sesarmid crabs, Mangrove leaf litter INTRODUCTION Mangrove crabs especially sesarmids play an were collected from the Pichavaram mangrove important role in the initial processing of litter in forest (11° 27’ N; 79° 47; E) located about 250 km intertidal, riverine and fringing forests (Dahdouh- south of Chennai on the south-east coast of India. Guebas et al., 1997; Ravichandran et al., 2000; Kathiresan and Bingham, 2001). The sesarmid crabs For gut content analysis, 30 individuals of each Chiromanthes bidnes and C. maipoensis together crab species were collected randomly from the entire consume more than 50% of the daily litter mangrove study area. They were brought production in infrequently inundated, landward immediately to the laboratory for analysis. After mangroves at the Mai Po marshes, in Northwestern identification of the crabs, sex, carapace width, Hong Kong (Lee, 1989). The diets of sesarmid crabs weight and the approximate fullness of mainly consist of mangrove leaves and animal proventricules of the crabs were recorded. All the debris (Kwok and Lee, 1995; Dahdouh-Guebas et contents from the stomach and rectum were al., 1997). The sesarmid crabs may select particular removed and stirred with distilled water at 1:2 litter types of the mangrove leaf species available, volumes in a square petri-dish. The samples were leading to variable rates of litter mineralization. then smeared on a microscopic slide and five Studies on processing of mangrove leaf litter by random samples were observed at 100 X and 400 X sesarmid crabs are lacking for the Indian mangroves. magnifications for large and small organisms Hence, the present study has been carried out for the (Poovachiranon and Tantichodok, 1991). The first time to assess the role of Sesarmid crabs in contribution of each food item from the total diet is mangrove litter processing based on gut contents in expressed in terms of percentage of the visual field the crabs. Further, food preference of five different occupied by the different categories recorded. The mangrove crabs with six different mangrove diets categories used in classifying the stomach and rectal with reference to relative growth rate, assimilation contents were plant material, sand, silt, microalgae, efficiency and food conversion efficiency in crabs fungal material, macroalgae, crustacean debris, are also reported here. unidentified debris and filamentous algae. Faecal material in the hindgut region were also analysed and categorized. MATERIALS AND METHODS Leaf preference experiments Gut content analysis Individuals of the grapsid crabs namely Sesarma brockii, S. andersoni, S. plicatum, S. brockii, S. plicatum, S. andersoni, Metopograpsus Metopograpsus messor and M. maculatus crabs maculates and M. messor were collected from the *Corresponding author’s e-mail: [email protected] S. Ravichandran, T. Kannupandi and K. Kathiresan 108 Pichavaram mangroves. Fresh green leaves and Energy budget of S. brockii senescent ones were collected from individual S. brockii crabs collected from the Pichavaram healthy trees of Avicennia marina, Rhizophora mangroves were acclimatized for two days to mucronata and Acanthus ilicifolius. One set of captive feeding of fresh leaves and various stages of leaves was separately placed in nylon mesh bags and decaying leaves. Before the start of experiment, wet submerged for decomposition in water channels of weight, carapace width and sex were recorded for the Pichavaram mangrove forest. The leaves each crab. Ten crabs were kept in each tank. Seventy decomposed for 20,40,60,80 and 100 days along per cent of the tank was filled with muddy sand with freshly collected ones were used for leaf (collected from the mangrove) forming a slope. preference experiments for crabs. After 24 hours, residual leaf material was collected and weighed to calculate the quantity of leaf Sex and carapace width of the crabs were consumed. Feeding was terminated and the crabs recorded before they were placed in experimental purged until faecal production ceased. Water in the tanks. Each tank was filled with 50% sea water up to tank was changed daily. Weight gain, food 1 cm depth to avoid desiccation of the crabs. In consumed, feed conversion ratio and survival of order to recognize leaf identity after crab crabs fed with different mangrove leaves were consumption, the leaves of the six types were cut calculated after 60 days of feeding trial. Differences into different geometric shapes of the same area (4 in treatment means were determined by Duncan’s cm2). Multiple Range test (P<0.05) using SPSS/PC package. The leaf pieces were offered to the crabs and consumption rates of three leaf types were compared after 24 hours. The amount of leaf biomass offered RESULTS for each leaf type was smaller than the expected consumption by the crabs in 24 hours, for The carapace widths of the studied crabs ranged encouraging the crabs to feed on progressively less from 1.3 to 2.6 cm for S. brockii, 2.6 to 3.2 cm for desirable leaf types after exhausting the more S. andersoni, 1.2 to 2.4 cm for S. plicatum, 2.8 -3.4 desirable leaf types. Since the objective of this cm for Metopograpsus maculatus, and from 2.6 to experiment was to define leaf preference, 4.2 for M. messor. The fresh weight varied between consumption by the five crab species of the six leaf 2.02 and 6.20 g for S. brockii, 5.24 and 9.87 g for types was ranked for each crab-trial. Actual S. andersoni, 2.01 and 5.68 g for S. plicatum, 5.63 consumption was measured only for those with very and 23.47 g for M. messor and between 3.01 and similar consumption for different leaf species. 10.56 g for M. maculatus. The mean percentage of food items in relation to total volume found in the proventriculus of five sesarmid crabs is given in Table 1 & Fig. 1. Table 1. Mean percentage composition of food items found in the proventriculus of grapsid crabs. Crab species Carapace Weight Food items width (g) range (cm) Plant Sand, Animal Unidenti Filamentous Macro Micro Fungal materials silt & debris -fied algae algae algae material clay debris Sesarma brockii 1.3-2.5 2.02-6.2 72.54 9.8 1.96 5.08 1.96 3.92 1.96 1.96 S. plicatum 1.2-2.4 2.01-5.68 63.33 6.66 5 16.58 1.66 - 5 1.66 S. andersoni 2.6-3.2 5.24-9.87 58.33 6.66 5 13.33 5 3.33 1.6 3.33 Metapograpsus 2.8-3.4 3.01-10.56 52.94 17.64 4.41 2.94 2.84 1.47 4.41 maculatus 13.23 M. messor 2.6-4.2 5.63-23.47 40.27 6.94 31.94 5.55 2.7 8.33 2.7 1.3 Mangrove leaf litter processing by sesarmid crabs 109 Proportion of material in the gut The crabs exhibited greater variability in the Leaf tissue was found to have originated from the amount of food in foregut. The foregut content mangrove species present in the vicinity of the crabs consisted of vascular plant matter ranging from collected.The leaf fragments were mostly of two 40.27 to 72.54% of total (Fig. 1). Sesarma brockii, types and they could be identified as Avicennia and S. andersoni and S. plicatum showed almost Rhizophora species on the basis of stomatal features. similar diets and they consumed high amount of The inorganic particles in the gut contents were vascular plant matter (58.33–72.54%). However, p r edo m inantly clay. Animal debris and algae M. maculatus (52.94%) and M. messor (40.27%) contributed little to the gut contents and were consumed a low amount of plant materials. The arbitrarily lumped together. Unidentified material second most important category was sediments, included bacterial colonies and some fruiting bodies. sand, silt and clay particles, constituting the gut content that ranged from 6.66 to 17.64%. Maximum Comparison of fore and hindgut contents of amount of sediment (17.64%) was collected from S. brockii the gut of M. maculatus. Other gut contents were The principal difference between the foregut detected in lesser quantity and they included and hindgut contents is given in Table 2. The unidentified debris (5.08–16.58%), filamentous degraded leaf particles occupied 41.8% of foregut algae (1.66–5.0%), leaf associated fungal hyphae content and 71.2% of hindgut (rectal region). The (1.3–4.41%), micro algae (1.6–5.0) and macro-algae fresh leaf materials decreased from 30.08% in the (2.84 – 8.33%). foregut to 13.9% in the hindgut.