Effect of Aeolosoma sp. (Aphanoneura: Aeolosomatidae) on the population dynamics of selected cladoceran species S. Nandini 1 & S. S. S. Sarma 2 1 UIICSE, Division of Research and Postgraduate Studies, National Autonomous University of Mexico, Campus Iztacala, AP 314, CP 54000 Los Reyes, Tlalnepantla, State of Mexico, Mexico (e-mail: nandini@servidor.unam.mx) 2 Laboratory of Aquatic Zoology, Division of Research and Postgraduate Studies, National Autonomous University of Mexico, Campus Iztacala, AP 314, CP 54000 Los Reyes, Tlalnepantla, State of Mexico, Mexico (E-mail: sarma@servidor.unam.mx) Key words: Aeolosoma sp., cladocerans, predation, competition, defences Abstract Although oligochaete worms naturally coexist with cladocerans in many shallow freshwater ponds and lakes, their influence on the latter is not well established. In this work we studied the effect of Aeolosoma sp. on the population growth of Alona rectangula, Ceriodaphnia dubia, Daphnia pulex, Macrothrix triserialis and Moina macrocopa. Population growth studies were conducted at one algal food density (1 · 10 6 cells ml )1 of Chlorella vulgaris). The experimental design was similar for all five cladoceran species, where we used 100 ml capacity transparent jars containing 50 ml of EPA medium with the desired algal density and three replicates for each treatment. The test medium was changed daily and fresh algal food was added. The initial density of each of the cladoceran species in the population growth studies was 0.4 ind ml )1 while that of the worms 1.0 ind ml )1 . Following inoculation, we estimated daily the number of cladocerans and the worms for duration of 21 days. Regardless of the presence of worms, Moina macrocopa and Macrothrix triserialis showed rapid population growth while A. rectangula took more than 2 weeks to reach peak abundances. With the exception of M. triserialis, all the other our cladoceran species declined in the presence of Aeolosoma sp. The lowest peak population density (about 1 ind ml )1 ) was observed for M. triserialis in controls. The remaining species had peak densities of about 3–5 ind ml )1 . The rates of pop- ulation increase per day varied from 0.03 to 0.19 depending on the cladoceran taxa and the treatment. In general we found that pelagic taxa were more adversely affected by the presence of the worms than were the littoral cladocerans. Introduction Among diverse aquatic organisms interacting with cladocerans are the oligochaete worms (Brinkhurst & Gelder, 2001). Since many taxa of cladocerans are mainly periphytic or benthic (Dodson & Frey, 2001), they are generally influ- enced by the presence of other organisms notably annelids, which often share the same habitat. While there exists a large database of information on the response of cladocerans to the presence of predators such as fishes, and competition with organisms such as ciliates, rotifers and copepods (Tollrian & Harvell, 1998), little is known about their interactions with oligochaete worms (Brinkhurst & Gelder, 2001). Some of these in- clude: (a) competition for limited food, (b) pre- dation on weak individuals of cladocerans or taxa that can be easily penetrated, and (c) pro- duction of some kind of allelochemicals that could reduce the survival or reproduction of cladocerans. These strategies are needed to persist in waterbodies since tube-dwelling oligochaetes could face intense competition pressure from the Hydrobiologia 526: 157–163, 2004. J. Pijanowska, P. Dawidowicz, A. Jachner & K. Szeroczyn ´ska (eds), Cladocera. Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands. 157