Some aspects of the ecology of littoral invertebrates in a coastal lagoon of southern Oman Reginald Victor* & Jayanthi R. Victor† *Department of Biology, College of Science, Sultan Qaboos University, P.O. Box 36, Postal code 123, Sultanate of Oman †Sultan Qaboos University Campus, No. 2, Street 11, Road E, P.O. Box 50, Postal code 123, Sultanate of Oman ( Received 2 N ovember 1996, accepted 3 February 1997) Some aspects of the ecology of benthic, planktonic and weed-associated littoral invertebrates of a coastal lagoon in southern Oman are described. Invertebrates were studied at five sites during two pre-flood months and one post-flood month; 38 taxa belonging to 22 major groups were recorded. Their distribution and relative importance were variable at sites and in sampling months. Five categories adapted to the fluctuating salinity regime of the lagoon were recognized. The overall littoral diversity was low. Similarity, diversity and evenness indices reflect the spatial and temporal response of communities to environmental conditions influenced by floods. ©1997 Academic Press Limited Keywords: Arabia; coastal lagoon; ecology; invertebrates; littoral; Oman Introduction The coastal lagoons of the Arabian Peninsula are known as khawrs. Khawrs occur at the mouth of seasonal rivers called wadis and are separated from the sea by sand bars. These lagoons are characterized by wide variations in salinity. The khawr contains fresh water when flood water piles up and during a flood. If the sand bar is breached, incoming tides will increase the salinity. When the khawr is isolated, ground-water discharge from the landward end will reduce salinity by dilution, while salt water intrusion from the seaward end and waves overtopping the sand bar will cause high salinities. Salinity variations occur horizontally from the landward to the seaward end and also vertically from the surface to the bottom, at least during some months. Thus, salinity is primarily controlled by the season, local rainfall and the seepage of ground- water. The salinity regime of khwars is the main influence on the ecology of plant and animal communities. Permanent residents of khawrs need to be adapted to the wide fluctuations in salinity and other associated physical and chemical factors. Changes in community structure in response to the fluctuations in salinity may also be possible through various adaptive strategies, and salinity ranges can be used to characterize species groups (Hedgpeth, 1983). Although the origins, dynamics and productivity of Journal of Arid Environments (1997) 37: 33–44 0140–1963/97/010033 + 12 $25.00/0/ae970256 © 1997 Academic Press Limited