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Research Article
 

Leaf Choice of Herbivorous Mangrove Crabs



S. Ravichandran, A. Anthonisamy, T. Kannupandi and T. Balasubramanian
 
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ABSTRACT

Leaf consuming mangrove crabs play an important role in the initial processing of litter in low to mid intertidal riverine and fringing forests. In Pichavaram mangrove, the sesarmid crabs are dominant, leading mostly herbivorous mode of life and play a vital role in the process of leaf degradation and thus in biogeochemical cycles. The gut content analysis also showed they are mainly consumed vascular plant matter (58.33 to 72.54%). Sesarmid crabs strongly preferred 40 day decomposed Avicennia marina leaves when both fresh and 10, 20, 40, 60, 80, 90 and 100 days decomposed leaves of A. marina, Rhizophora mucronata and Acanthus ilicifolius were offered. Sesarmid crabs are important not only because of their burrowing activities, which can affect nutrient cycling and forest productivity but also their role as a link in the food web in the mangrove ecosystem.

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  How to cite this article:

S. Ravichandran, A. Anthonisamy, T. Kannupandi and T. Balasubramanian, 2007. Leaf Choice of Herbivorous Mangrove Crabs. Research Journal of Environmental Sciences, 1: 26-30.

DOI: 10.3923/rjes.2007.26.30

URL: https://scialert.net/abstract/?doi=rjes.2007.26.30

Introduction

Herbivores utilize plant material as a food source and although plants have a more predictable mass and availability than animal foods, they are nutritionally inferior. Degradation of mangrove leaf litter by sesarmid crabs play a key role as a major link between primary and secondary productions. Sesarmid crab species select particular litter types of the mangrove leaf species available, thus leading to variable rates of litter mineralization. Dahdouh- Guebas (1997), Ashton (2002), Buck et al. (2003), Thongtham and Kristensen (2003) and Schwamborn et al. (2006) analyzed the diets of sesarmid crabs and showed that their diet mainly consisted of mangrove leaves and in addition to bussle of animal matter. The sesarmid crabs may select particular litter types of the mangrove leaf species available, leading to variable rates of litter mineralization.

However, studies on herbivores crabs and their dietary and nutritional aspects in the mangrove environment are lacking for Indian mangroves. Hence the present study has been carried out to assess the leaf preference and to know the role of grapsid crabs in mangrove litter processing including the analysis of gut contents in different crabs. Further the food preference of five different mangrove crabs with six different mangrove diets are also reported here.

Materials and Methods

Leaf Preference Experiments
Individuals of the grapsid crabs such as S. brockii, S. plicatum, S. andersoni, Metopograpsus maculatus and M. messor were collected during the period of 2004-2005 from the Pichavaram mangrove environment, southeast coast of India (Lat.11°27’N; Long.79°47’E). Fresh green leaves and senescent ones were collected from the canopy of A. marina, R. mucronata and A. ilicifolius.

In order to recognize leaf identity after crab consumption, the leaves of the six types were cut into different geometric shapes of the same area (4 cm2). The leaf pieces were offered to the crabs and the consumption rates of three leaf types were compared after 24 h. The amount of leaf biomass offered for each leaf type was smaller than the expected consumption by the crabs in 24 h, for encouraging the crabs to feed on progressively less desirable leaf types after exhausting the more desirable leaf types.

Gut Content Analysis
Sesarma brockii
, S. andersoni, S. plicatum, Metopograpsus messor and M. maculatus crabs were collected randomly from all over the mangrove. All the contents from the stomach and rectum were removed and stirred with distilled water in a square petri-dish. The samples were then smeared on a microscopic slide and five random samples were observed at 100 and 400 X magnifications for large and small organisms. The contribution of each food item from the total diet is expressed in terms of percentage of the field occupied by the different categories recorded. Fecal matter in the hindgut region were also analyzed and categorized.

Results

Leaf Preference
There was a strong tendency for most of the crab species to consume decomposed leaves of A. marina. The same pattern was found in all the cases, with a very high preference for decomposed leaves of A. marina and very low preference for fresh leaves of A. ilicifolius. All the experimental animals preferred decomposed leaves of A. marina and R. mucronata, more than other leaf types. M. maculatus did not consume fresh green leaves of A. ilicifolius during the experimental period (Table 1 and 2).

Table 1:

Leaf preference of mangrove crabs fed with various stages of decomposed A. marina leaves

Image for - Leaf Choice of Herbivorous Mangrove Crabs

Table 2:

Leaf preference by mangrove crabs

Image for - Leaf Choice of Herbivorous Mangrove Crabs

A- Avicennia marina decomposed leaf, B- A. marina fresh leaf, C- Rhizophora mucronata decomposed leaf, D- R. mucronata fresh leaf, E- Acanthus ilicifolius decomposed leaf, F- A. ilicifolius fresh leaf, Sample size = 30


Image for - Leaf Choice of Herbivorous Mangrove Crabs
Fig. 1:

Comparison of plant materials in the gut of mangrove crabs

In general, the experimental crabs strongly preferred 40 day decomposed A. marina leaves followed by 60 day decomposed R. mucronata leaves. Among the decomposed mangrove leaves, S. brockii preferred A. marina leaves first and eagerly fed on 40 day submerged leaves. Secondly they preferred 60 day submerged R. mucronata leaves. Most of the crabs moved away from the fresh A. ilicifolius leaves and preferred leaves of 40 to 60 days of decomposition.

Proportions of Materials in the Gut
The crabs exhibited greater variability in the amount of food in foreguts. The foregut content consisted of vascular plant matter ranging from (40.27 to 72.54% of total, Fig. 1). S. brockii, S. andersoni and S. plicatum showed almost similar diets and they consumed high amount of vascular plant matter (58.33-72.54%). However, M. maculatus (52.94%) and M. messor (40.27%) consumed low amount of plant materials. The second most important category was sediments, i.e., sand, silt and clay particles, constituting the gut content ranging from 6.66 to 17.64%. Maximum amount of sediment (17.64%) was collected from the gut of M. maculatus. Other gut contents were found to be less quantity and they include unidentified debris (5.08-16.58%), filamentous algae (1.66-5.0%), leaf associated fungal hyphae (1.3-4.41%), micro algae (1.6-5.0) and macro-algae (2.84-8.33%).

Faecal pellets observed in the hindgut were mostly of degraded leaf containing fine particles. Further, leaf material ingested by S. brockii was completely digested into fine particles and released back into the environment. The difference in the food items of S. brockii in the foregut and hindgut is statistically insignificant (p>0.05, t-test).

Discussion

In Pichavaram mangrove, the sesarmid crabs are dominant, leading mostly herbivorous mode of life. Although plants have a more predictable mass and availability than animal foods, they are nutritionally inferior. Plants have low nitrogen content which can prove to be an important limiting nutrient for herbivores, since energy (in the form of carbon) is readily available in plants (Boyd and Goodyear, 1971; Mattson, 1980). From the present findings, crabs strongly preferred 40 day decomposed A. marina leaves. The preference was for A. marina leaves over R. mucronata and A. ilicifolius as A. marina have high protein and low tannin contents (Kathiresan, 2000; Ravichandran et al., 2000; Ravichandran and Kannupandi, 2004, 2005).

Preference of sesarmine crabs for decaying mangrove leaves is expected, since only the food materials which are having C: N ratios lower than 17 are considered nutritious to marine invertebrates (Russel-Hunder, 1970). The fact that senscent mangrove leaves generally have high initial C: N ratios (up to 100) which decreases for decaying leaves. The feeding activities of crabs speed the decomposition of leaf litter and may facilitate the release of nutrients to mangal system (Robertson and Daniel, 1989; Lee, 1997; Nordhaus et al., 2006). Giddins et al. (1986) discovered that assimilation efficiency of Neosarmatium smithi for carbon, nitrogen and organic matter increased rapidly with the age of the decaying leaf litter they consumed. Crabs prefer A. marina leaves because of low soluble tannin and rich nutrient contents as mentioned earlier.

The analysis of gut contents indicated that most of the sesarmid crabs are primarily herbivores. The proventricules and rectum contain very high percentage of mangrove leaf material. Small-sized detrital particles occur more in the hindgut than in the foregut region. The proventricule is a site of particle reduction, where much of the materials are reduced. Similar results were observed in the proventriculus of sesarmid crab, Chiromanthes oncychophorum (Malley, 1978).

In the Pichavaram mangrove forests, sesarmid crabs descend from their burrows above the high tide mark to feed on mangrove leaves which are then taken back into their burrows. Storage of mangrove litter within the crab burrows, however, does not seem to be the common feature of all sesarmid species found in the mangroves suggesting that this behavior might be influenced by ecological constrains rather than food quality. This species consumes significantly more aged (6-weeks old) litter, as compared to fresh litter, which indicates that leaves are stored in burrows to leach tannins before consumption (Nielson et al., 1986). In the present study also, S. brockii stores leaves in the artificial burrows soon after the leaves are offered. Approximately 50% of original leaf tannin content is lost by leaching process during the first week of decompositions and increasing palatability of litter (Robertson, 1988; Ravichandran and Kannupandi, 2004). Grapsid crabs are helpful for the production of plant detritus. They are linked most closely to detritus, which may be linked most closely to secondary production of crabs in the offshore mangroves where grapsids are abundant. The present study clearly shows that S. brockii plays a key role for major link between primary and secondary productions. Grapsid crabs are mainly responsible for leaf litter production and there fore the species link the detritus food chain in the mangrove environment.

Acknowledgments

The authors are grateful to the authorities of Annamalai University and staff members of ENVIS Centre for their keen interest and encouragement.

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