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Gender and Species Differences in Sentinel Crabs (Macrophthalmus) Feeding on an Indonesian Mudflat

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AbstractThis study investigates the influence of gender on feeding and habitat niches in the ocypodid sentinel crabs Macrophthalmus convexus and M. definitus feeding on a mudflat in the Wakatobi Marine National Park, Indonesia. Male M. convexus possess sub-equal chelae that are considerably larger than those of females and of those of both genders of M. definitus. Habitat location, food (Chl a) biomass index in microalgae and seagrass mats on muddy sediments, faecal material, and feeding rates were analysed in females and males of both species throughout the lunar cycle in July and August 2002. Food biomass content was significantly higher in the upper than in the lower mudflat (P = 0.008). The particle size of faecal material did not vary between species or between genders of M. definitus, but the faeces contained larger particles in males of M. convexus compared with females and both gender of M. definitus. Macrophthalmus convexus predominantly inhabited pools on the upper and mid mudflats, whereas M. definitus was found in muddy sea-grass substrates on the mid and lower mudflats. Crabs were observed to emerge for feeding only on ebb tide during daylight. Macrophthalmus convexus showed significantly higher scoop rates than M. definitus (P < 0.05) during full and new moon. Feeding rates in male M. convexus were also significantly lower than of females (P < 0.001) throughout the lunar cycle. In contrast, feeding motions in M. definitus were generally variable and not gender-specific. However, males showed significantly reduced scoop rates in this species on the lower mudflat during the first quarter moon (P < 0.001). Scoop rate is not the only process affecting potential ingestion rate, however, in that several ‘pinches’ of sediment may be made by the chela before it is lifted towards the mouthparts.

Affiliations: 1: a(PMMS, correspondence) Department of Marine Sciences, Britannia Royal Naval College, Dartmouth, TQ6 0HJ, UK (; ; 2: b(RSKB) Department of Zoology, University of Cambridge, Cambridge, CB2 3EJ, UK, and Operation Wallacea Marine Research Centre, Hoga, Wakatobi Marine National Park, Sulawesi Tenggara, Indonesia (; ; 3: c(GJCU) Department of Biological Sciences, University of Essex, Colchester, CO4 3SQ, UK (; ; 4: d(PWJ) Department of Mathematics, Keele University, Keele, ST5 5BG, UK (


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