Influence of different chemical signals on the habitat selection of three zooplankters of Andean Patagonian ponds

TROCHINE, C, MODENUTTI, B. & BALSEIRO, E.

Punta del Este, Uruguay.

Conferencia; Shallow lakes Conference 2008; 2008

Zooplankton prey may react differently to chemical signals produced by macrophytes in shallow systems. They may be attracted by macrophytes, as the plants may be used as a refuge against predators or the plants may have a repellent effect (e.g. when the plants are habitat for numerous invertebrate predators or fish). In fishless Patagonian ponds, the structural complexity provided by macrophytes modulates the rate of predation of the invertebrate predator Mesostoma ehrenbergii. We performed a field study to analyze the coexistence of M. ehrenbergii and three selected prey (two copepods, the calanoid Boeckella gracilis and the cyclopoid Acanthocyclops rubustus, and the cladoceran Ceriodaphnia dubia) in four ponds. In two of these ponds we carried out day and night samplings in order to evaluate the influence of macrophytes stands in the distribution of these zooplankters. In laboratory experiments we analyzed the response of these zooplankters to the chemical signals produced by macrophytes (the emergent Juncus pallescens and the submerged Myriophyllum quitense), the predator M. ehrenbergii and the “alarm signal” provided by a homogenate of conspecifics.Our field studies demonstrated the coexistence of M. ehrenbergii and the selected prey in different seasons and that A. robustus and C. dubia choose the vegetated area (mixed bed of J. pallescens and M. quitense) over the non-vegetated one in natural habitats. The habitat choice experiments indicated that the presence of M. ehrenbergii may directly affect the habitat selection of B. gracilis because this zooplankter swam away from the predator. Besides, Mesostoma may indirectly affect the habitat selection of the cyclopoid copepod A. robustus and the cladoceran C. dubia as both zooplankters exhibited a negative response to the alarm signal produced by crushed conspecifics. The presence of the submerged M. quitense did not affect the horizontal movements of any of the studied zooplankters. In contrast, the experiments with the emergent macrophyte J. pallescens showed a positive response, suggesting that this aquatic plant may act as refuge for B. gracilis. This current work suggests that a predator-avoidance behavior such as habitat selection may also be observed in fishless environments in response to a tactile invertebrate predator like Mesostoma. In addition, the refuge effect of emergent macrophytes, enhancing the survival of pelagic zooplankters, may act as a key factor stabilizing the predator-prey interactions in Patagonian fishless ponds, as seen in northern temperate lakes with fish.