Small (5-10 .9 mm carapace length), medium (11-15 .9 mm), and large (16-20 .9 mm) juveniles of Metapenaeus anchistus, Metapenaeus sp ., Penaeus monodon and P. merguiensis were stocked individually in glass tanks provided with sand substrate, sea water, artificial bamboo shelter, aeration and food . The seven activity types (recorded for each shrimp hourly for 24 h) were classified as below (burrowing) or above substrate (swimming, walking, stationary, in shelter, feeding and cleaning) . Shrimp juveniles exhibited a strong diet periodicity -emergence and activity at night and burrowing in the day. The chi-square test showed that type of activity (above/below substrate) was associated with period (light/dark). Diurnal burrowing was greater among Metapenaeus than Penaeus ; inversely, above substrate activities were more frequent for Penaeus species compared to Metapenaeus. Feeding was the major above substrate and nocturnal activity for M. anchistus, Metapenaeus sp . and P monodon . Only P monodon used the shelter consistently . Frequency of the 7 activity types was dependent on juvenile size for Penaeus, e .g ., the preference for shelters shifted to burrowing with increase in size in P monodon . Results are discussed in relation to the importance of mangrove habitats in providing shelter to penaeids, in particular the mangrove-associated P monodon and P. merguiensis .
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