Osmoregulation, growth and moulting cycles of the giant freshwater prawn (Macrobrachium rosenbergii) at different salinities Do Thi Thanh Huong 1 ,Tobias Wang 2 , Mark Bayley 2 & Nguyen Thanh Phuong 1 1 College of Aquaculture and Fisheries, Can Tho University, CanTho,Viet Nam 2 Department of Biological Sciences, Aarhus University, Aarhus, Denmark Correspondence: D T T Huong, Department of Aquatic Nutrition and Products Processing, College of Aquaculture and Fisheries, Can Tho University, Campus 2,3/2 street, Ninh Kieu District, Can Tho City,Viet Nam. E-mail: dtthuong@ctu.edu.vn Abstract The giant freshwater prawn, Macrobrachium rosen- bergii , is a species with a high commercial value in aquaculture. Two experiments were performed to de- termine the e¡ects of salinities on the osmoregula- tion, growth and molting cycles of M. rosenbergii during growout. The ¢rst experiment was designed to determine whether these animals are capable of adapting to the changes in salinity seen in salinity intrusions in tropical deltas, with an incremental in- crease in salinity of 3% per day from 0% to 30% Haemolymph osmolality was rapidly regulated up to salinities of 15%, whereas animals conformed at higher salinities. The second experiment determined the growth, moulting cycle, osmolality, muscle water content and mortality during a 4-month experiment at 0%, 15% or 25% salinity. The weight gains in 0% and 15% were not signi¢cantly di¡erent and were comparable to the growth rates achieved in produc- tion farms with body mass increases of 2.6 and 2.3-fold their initial body mass, respectively, after 4 months. The 25% group su¡ered from low growth, high mortality and a signi¢cantly lower moulting frequency. These data show that this species can be reared in brackish water up to 15% , allowing for farming in the large areas impacted by salt water in- trusions in tropical deltas. Keywords: Macrobrachium rosenbergii , giant fresh- water prawn, salinity, growth, osmolality Introduction The giant freshwater prawn Macrobrachium rosenber- gii (de Man) is one of the world’s largest freshwater crustaceans and an important freshwater aquacul- ture species in many tropical countries, particularly in Asia. The production of M. rosenbergii is increasing rapidly in many South East Asian countries and land-based culture of Macrobrachium has expanded from 19035 tonnes in 1995 to 201467 tonnes in 2006 (FAO 2007). Thus, to sustain this increased pro- duction without imposing exorbitant demands on area use, it is vitally important to improve the rearing techniques and holding conditions for this species. Some provinces in the Vietnamese Mekong delta (Soc Trang,TraVinh, Kien Giang, Ca Mau, Ben Tre) are ex- posed to seasonal changes in salinity, rendering rice ¢elds unproductive for 6 months of the year. During these periods with saline intrusion, there are exten- sive areas amounting to 4600 000 ha where the salinity gradually increases during the dry season to a peak of 15ppt before declining again as the rainy season starts (Sam 2001). These areas currently sup- port low-quality rice crops for some of this time, but could be used more e⁄ciently for shallow water shrimp farming. It is relevant, therefore, to assess whether these areas could be used for Macrobra- chium culture during the dry season, resulting in a more pro¢table use of the area. Macrobrachium rosenbergii migrates from fresh- water to estuaries for spawning, and the larvae devel- op in brackish water for a month until the postlarvae return to freshwater to mature (New 2002). In line with this habitat choice, it is well established that Macrobrachium is an osmoregulator in freshwater up to salinities at the iso-osmotic point of  15%, whereas it is an osmoconformer at higher salinities (e.g. Castille & Lawrence 1981; Stern, Borut & Cohen 1987; Funge-Smith, Taylor, Whitley & Brown 1995; Wilder, Ikuta, Atmomarsono, Hatta & Komuro 1998; Aquaculture Research, 2010, 41 , e135^e143 doi: 10.1111/j.1365-2109.2010.02486.x r 2010 The Authors Journal Compilation r 2010 Blackwell Publishing Ltd e135