Although there have been comprehensive ecological surveys of impacted mangal and salt marshes in the Gulf states, especially Saudi Arabia, no data exists regarding the mangal or salt marshes fauna of Qatar, where recent replanting has expanded the area of mangal . The first aim of the present investigation was, to study the biodiversity of the Brachyura and fish living within these habitats quantitatively together with relevant features of the abiotic and biotic environments of natural, replanted mangal and salt marsh so that the progress of recolonisation of the new habitat could be evaluated. Measurements of sediment organic matter, grain size and moisture content indicate that natural mangrove areas have the finest grain size and highest organic and moisture contents while planted mangrove areas have a higher mean grain size, but lower organic and moisture content. Mean soil water pH within the natural mangrove areas was 7.21, in planted mangrove areas slightly higher with a value of 7.55, and 7.53 at the salt marsh, while sea water pH was 7.91 - 8.30. Differences in brachyuran species in planted and natural mangrove areas were found, but biodiversity was similar in salt marsh and natural mangrove areas. Nasima dotilliformis was the only species not to occur at any planted mangrove site, while Serenella leachii was missing from natural mangrove. Four crabs Nasima dotilliformis, Metopograpsus messor, Eurycarcinus orientalis, and Macrophthalmus depressus dominated natural and planted mangroves and salt-marsh, extending through the upper intertidal. Ilyoplax frater, Manningis arabicum, Macrophthalmus depressus occur in the mid intertidal zone. On the lower intertidal zone the two dominant species in all areas were M. depressus and Metaplax indica. In planted mangrove areas where sandy sediment dominates Scopimera crabricauda occurs between the upper intertidal to mid intertidal zone. Fish surveys indicate that Ablennes hians, Gerres oyena, Hemiramphus marginatus and Liza macrolepis, enter mangroves using these as nursery areas and significant differences occurred between sites demonstrating that mangrove areas, especially pneumatophores, form a special habitat for these small fish. The first zoeal larval stage for 6 common intertidal crabs is described, and new generic diagnoses are erected for Paracleistostoma arabicum and Cleistostoma kuwaitense, crabs belonging to the Camptandriinae. A modified key based on Manning and Holthuis (1981) is constructed to separate these from other members of the subfamily. The biological characteristics of 5 species of crab were monitored during a monthly sampling programme over the period June 1993-1994 including carapace widthweight relationship, size frequency, sex-ratio and breeding biology. Male: female ratios differed, indicating spatial and temporal variations by size-classes and season. The ovigerous females of N. dotilliformis and S. leachii were encountered over 7 months while those of M. depressus were seen almost throughout the year. Metopograpsus messor were ovigerous over a5 month period and E. orientalis over 6 months. From size frequency modes and data on recruitment and ovigerous females it appear that late spring and summer is the ecologically-active season. The mouthparts of 6 species of the family, Ocypodidae, 2 species of the family Grapsidae and 1 species of Xanthidae are described. These crabs were observed and collected from mangrove sites mud and saltflats between the midlittoral intertidal zone and supralittoral fringe. Detail of the mouthpart structure reveal differences between deposit feeders with spoon-tipped setae in sandy habitat dwellers and plumose setae in mud feeders, while spinose setae occur in omnivorous and carnivorous species. Scanning Electronic Microscope studies of the structure of proventriculus of these crab species again revealed different structures related to the type of feeding and particular type of sediment in which deposit feeding crabs live. In conclusion this study has demonstrated that mangrove in Qatar whether natural or planted acts to conserve species and enhance diversity and abundance. As yet recently planted mangroves (10 y) have not reached the full brachyuran diversity seen in natural mangroves, and present work demonstrates that this is only likely to occur when full physical habitat comparability with natural mangroves is attained.