Hydrobiologia 387/388: 421–425, 1998. E. Wurdak, R. Wallace & H. Segers (eds), Rotifera VIII: A Comparative Approach. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 421 Colony size in Conochilus hippocrepis: defensive adaptation to predator size Maria Di´ eguez & Esteban Balseiro Dept. Ecology, Centro Regional Universitario Bariloche, Unidad Postal Universidad, 8400 Bariloche, Argentina Key words: Conochilus, coloniality, copepod predation, copepod development Abstract Conochilus hippocrepis colonies were analysed in relation to the presence and size of the predaceous calanoid copepod Parabroteas sarsi. Conochilus colonies increase in size throughout the season from May to August and then disappear from the lake. Simultaneously, Parabroteas developed from CI to CV and adults. We observed that when the predaceous copepod begins to prey on Conochilus, colony size increases in relation to maxilliped length of the predator. Our results show that the increasing size of the colony of Conochilus is an effective defense against Parabroteas predation. Introduction Planktonic rotifers are an important component of the diet of aquatic predators, especially of inverteb- rate predators, who prefer smaller prey individuals such as rotifers (Williamson, 1983). Since carnivorous planktonic invertebrates are size-dependent predators (Zaret, 1978; Gliwicz & Pijanowska, 1989), changes in the body shape and size of their prey may be effect- ive antipredator responses (Sih, 1987; Dodson, 1989). Among the different defenses of planktonic rotifers against predators, coloniality increases effective size and therefore may protect individuals in the colony from predation (Stemberger & Gilbert, 1987; Wallace, 1987). All species of the genus Conochilus form colonies of different sizes. This feature may provide protection from predation by copepods, Asplanchna and Lepto- dora (Gilbert, 1980; Williamson, 1983; Edmondson & Litt, 1987; Wallace, 1987). Conochilus colonies often persist in Northern Hemisphere lakes in the presence of copepod predators like Epischura and Diacyclops (Stemberger & Evans, 1984; Stemberger & Gilbert, 1987). Similarly, in a small fishless pond of the South- ern Andes (41 ◦ S) Conochilus hippocrepis (Schrank) coexists with the large predaceous calanoid copepod Parabroteas sarsi Daday. Early studies revealed that total prey length of Daphnia is a good predictor of susceptibility to P. sarsi (Balseiro & Vega, 1994). In this study we analysed by indirect evidence the effectiveness of the coloniality of Conochilus hippo- crepis in permitting the coexistence with the predator Parabroteas sarsi. Methods The study was carried out in Laguna Fantasma, a small, fishless, temporary pond near Nahuel Huapi lake (41 ◦ S, 72 ◦ W, Argentina). The hydroperiod of the pond extends from autumn (April) to early sum- mer (December). From mid summer to early autumn, the pond remains dry and often freezes during winter (July). In particular, in 1994 the ice remained only for one week. A maximum depth of 1.3 m was recor- ded during June and July. Based on our unpublished observations of samples taken in early autumn, we know that Conochilus hippocrepis population devel- ops from resting eggs that hatch just after the first autumn rainfalls fill the pond. The pond was sampled for C. hippocrepis approximately every 10 days (range = 6–20 days) over a three month interval (May 31, 1994 to August 31, 1994). We both took qualitative and quantitative samples of the zooplankton of Laguna Fantasma to monitor the population dynamics and an- imals sizes of C. hippocrepis and P. sarsi. Qualitative