Predator specificity of kairomones in diel vertical migration of Daphnia: a chemical approach

Daphnia responds to chemical cues released by fish with diel vertical migration (DVM) as a behavioural predator avoidance. We used a bioassay to characterize the chemical nature of the kairomone. Cues released from stickleback (Gasterosteus aculeatus, Gasterosteidae) and a piscivorous pike (Esox lucius, Esocidae) were enriched from incubation water by reversed-phase sorbent extraction and were reversibly inactivated by acetylation. HPLC yielded only one active fraction with identical retention times for the kairomones of both species. Chemical features did not differ from those previously reported for Cyprinidae, indicating that the active compounds are very similar if not identical. From further investigation of the kairomone released by crucian carp (Carassius carassius, Cyprinidae), glucuronic acids and carboxy-, sulphate- and phosphate-groups can be excluded as essential for biological activity. The response of Daphnia increased with increasing concentrations of extracted kairomone. The kairomone was not released from mucus by digestion with hyaluronidase. Adsorption of the kairomone to food particles seems to be of minor importance suggesting that the cue is perceived as a freely dissolved molecule.