Morphological responses of Daphnia pulex to Chaoborus americanus kairomone in the presence and absence of metals.

Daphnia pulex neonates develop neck teeth in the presence of predatory kairomone from Chaoborus americanus that are fed D. pulex. These neck teeth reduce the susceptibility of the neonates to predation. Evidence suggests that aqueous metals interfere with chemical communication in fish. The objective of our study was to determine if Cu or Ni at environmentally relevant concentrations affects predatory kairomone response in D. pulex. To test this possibility, D. pulex were placed in increasing waterborne concentrations of Cu or Ni in the presence or absence of predatory kairomone. Both Cu and Ni reduced neck tooth induction in D. pulex neonates in the presence of predatory kairomone. Copper had a significant nonlinear effect on neck tooth length consistent with a hormetic response, where neck tooth length was highest at 5 microg/L Cu, but not significantly different than 0 microg/L Cu at higher Cu concentrations. A Ni concentration of 200 microg/L caused D. pulex to become hypersensitive to Chaoborus regardless of Chaoborus' diet, leading to increased neck tooth number but decreased neck tooth length. Neither Ni nor Cu produced any significant effects on body length or brood size. These results suggest that metal inhibition of neck tooth induction probably occurs along the signal transduction pathway. Impairment of chemosensory response to predatory chemical cues may have widespread ecological consequences in aquatic systems contaminated by metals.