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Burrow usurpation and duration of surface activity were studied for the ocypodid crab Scopimera globosa in the Fukuro River Estuary, southern Honshu, Japan. Burrow owners were displaced frequently during an exposure period, where burrow usurpation accounted for almost half of the causes of burrow abandoning. In most instances of burrow usurpation, the usurper was larger than its opponent, irrespective of the sex. The duration of surface activity during an exposure period was not correlated with crab size. Based on these facts, the reason for size variations in spatial distribution of S. globosa is discussed. For some ocypodid crabs inhabiting intertidal sand or mud fiat, it has been reported that larger crabs tend to occur at higher level than smaller conspecific ones: for Scopimera in.flata by Fielder (1971), Dotilla fenestrata by Hartnoll (1973), Ocypode cursor by Shuchman & Warburg (1978), Ocypode quadrata by Fisher & Tevesz (1979), Uca lactea by Frith & Brunenmeister (1980), and Scopimera globosa & lf:yoplax pusillus by Wada (1983). But mechanisms involved in such intraspecific distributional difference have not so far been fully analyzed. With the purpose of discussing this problem for Scopimera globosa De Haan, 1835, I have made experimental observations in the field, and present, in this paper, data on displacement of burrow owners and the relation between the duration of surface activity and crab size. In this connection, few works have been done on burrow abandoning or burrow obtaining in ocypodids in the field; Hyatt & Salmon (1978) reported the size relationships between wandering crabs and residents evicted by the former for Uca pugilator and U. pugnax, and Bertness & Miller (1984) examined crab recruitment to artificial burrows for U. pugnax. Furthermore, the duration of surface activity has not so far been studied in relation to individual size in ocypodids. Field study was conducted on an intertidal sand flat of the Fukuro River Estuary, Japan (33°38'N 135°24'E). Four fixed areas were established along the middle level (ca. 42 em above M.T.L.) of S. globosa distributional range which measured about 30m across the shore. On each day of spring and neap tides ofsummer (June 5 & 14, 1984) and of autumn (October 2 & 12, 1984), surface activities of crabs in the fixed areas were continuously recorded with four video cameras for 4.5 to 7 hours within daytime exposure periods. It was only on 12 October that recording almost fully extended over an entire exposure period. Data on observation time & area, number of burrows observed, and Publ. Seto Mar. Biol. Lab., 31 (3/6), 327-332, 1986. (Article 11)

Burrow Usurpation and Duration of Surface Activity in Scopimera globosa (Crustacea : Brachyura : Ocypodidae)