Burrow-morphological characters of the fiddler crab, Uca annulipes (H. Milne!Edwards, 1837) and ecological correlates in a lagoonal beach on Pulau Hantu, Singapore

Nine burrow-morphological characters (burrow diameter, BD; burrow volume, BV; total burrow depth, TBD; burrow neck height, BNH; curved burrow length, CBL; horizontal length, HL; chamber diameter, CD; angle between descending neck of burrow and the substrate surface, beta; angle between the bend in the burrow and the perpendicular, theta) were defined from a study of 127 complete burrow casts of the fiddler crab, Uca annulipes. Crabs in 14 one-m(2) quadrats randomly sited in low, mid, and high shore levels on a lagoonal beach were sexed and categorized into small, medium, and large size-classes. Sediment type in each quadrat was assessed as anoxic or non-anoxic. Density was estimated at 12.8 crabs m(-2); mean sex ratio was 2.75 male:1 female. Five burrow-morphological characters (BV, TBD, BNH, CBL, and CD) differed significantly between sexes, being larger in male than in female crab burrows (p < 0.05). Differences in these burrow characters were indicative that males excavated larger burrows than females. Burrow mid-angle was significantly more obtuse in female crab burrows (p < 0.05), indicative of a more gradual turn in the descent of the burrows made by males. We attributed this to the poorer manoeuvrability of the males in the burrows due to their large chelipeds. Burrows of larger-sized crabs had greater BD, larger BV, and bigger CD than burrows of small- and medium-sized crabs. Significantly deeper burrows were dug at the high shore level than at both mid- and lower shore regions. We hypothesize that digging deeper burrows in the high intertidal is one way in which the fiddler crab can maintain lower burrow temperatures when the tide recedes. Burrows in anoxic sediments had significantly shorter TBD, BNH, and CBL than burrows in non-anoxic sediments. We postulate that shorter burrows in anoxic sediments may help improve the aeration of the burrows as opposed to longer burrows that accumulate a greater amount of stagnant air.