Feeding Behaviour of an Obligate Arboreal Crab Labuanium politum at Selected Nipa Forests in Sarawak, Malaysia

Nur Amirah, Mohamad Alwie (2020) Feeding Behaviour of an Obligate Arboreal Crab Labuanium politum at Selected Nipa Forests in Sarawak, Malaysia. Masters thesis, Universiti Malaysia Sarawak (UNIMAS).

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Abstract

Labuanium politum is an arboreal sesarmid crab and locally known as ‘geramak apong’ in Sarawak that inhabits the nipa palm (Nypa fruticans) forests. Its brief ecology reported by previous researchers does not cover on the feeding behaviour. Thus this pioneer study attempts to document the natural diet and feeding behaviour of L. politum in the western part of Sarawak, through stomach content analysis and feeding experiments. This study aimed to (i) determine the diet composition of L. politum according to different localities, seasons and nipa management level, (ii) investigate the feeding preferences of L. politum in captivity and (iii) determine the effect of different nipa leaf conditions on the survival and growth of L. politum in captivity. A total of 720 crabs (carapace width range: 8.03-33.02 mm) were collected from Tambirat and Tambak for stomach content analysis. of the Frequency of occurrence results showed that this crab consumed two main components, namely plant (97.92%) and sediment materials (98.06%), followed by diatom (21.11%), animal (12.5%) and unidentified materials (29.17%). In addition, the Weighted Resultant Index (Rw) gave similar findings whereby the two main components were significantly different from other food materials (p<0.001). Based on the degree of stomach fullness, crabs with 100% full stomach were higher (11.53%) than those with empty stomach (2.36%) whereby more male crabs have full stomach than the females, but vice versa for empty stomach. Results of the corresponding analysis (CA) demonstrated that food preferences did not vary between (i) different sizes of crabs but (ii) small crabs feed more than larger crabs. Furthermore, no variation was observed in feeding preferences of L. politum at different localities, seasons and nipa management level. Based on overall findings, the natural feeding mode of L. politum is herbivorous. For the first feeding experiments, L. politum preferences on three nipa leaf conditions was evaluated within three day period by placing one crab per glass aquarium together with green, yellow and brown leaves. This experiment involved eight crabs of both sexes. It was observed this crab significantly preferred green leaf over yellow and brown leaf (p<0.001). Similar results were obtained for male and female crabs (p<0.001). Next, the same setup and crabs were used for the multiple-choice feeding experiment but the foods given were mangrove slug, cricket, bee and green nipa leaf. Surprising outcome was obtained whereby L. politum strongly prefer the animal materials over green leaf, indicating a shift of feeding behaviour towards opportunistic omnivores although they are herbivores in their natural habitat. The last experiment measured the survival and growth performance of L. politum in captivity (60 days) under different nipa leaf conditions. Each leaf condition had triplicate of both crab sexes in which one crab was placed in each aquarium and the leaf was exchanged daily. The initial and final weights of the crabs were recorded on Day 1 and Day 60, while the leaves were weighed daily. It was found that green leaf gave the highest survival than the yellow and brown leaf. Even though the highest SGR was obtained in crabs that fed on green leaf, the difference in mean weight gain of the crabs was not significant. The best FCR was recorded by the green leaf (73.98%), but it is not an efficient feed for L. politum. Thus, green nipa leaf is substantial for L. politum survival but inadequate for their growth. Overall findings of this research had provided useful information on L. politum feeding behaviour that can be applied towards its future conservation and culture works.

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