Endosulfan-enhanced crest induction in Daphnia longicephala: Evidence for cholinergic innervation of kairomone receptors

Daphnia longicephala were reared from early embryogenesis in the presence or absence of crest-inducing kairomones released by Anisops gratus (Notonectidae) and from birth in one of five concentrations of the organochlorine pesticide, endosulfan (0, 0.1, 1.0, 10 or 100 μg l^-1). The morphology of the daphnids was measured 3 days after birth and on production of the first brood of eggs. The reproductive parameters, first brood size, age at first reproduction, mean egg volume and total egg volume, were also measured. Endosulfan significantly induced crest development in 3-day-old daphnids at 0.1, 1.0 and 10 μg l^-1 in the presence and absence of Anisops kairomone, although crest size in the absence of kairomone was only marginally greater than controls. A concentration of 10 μg l^-1 endosulfan significantly enhanced crest growth of kairomone-exposed daphnids at maturity. The Anisops kairomone alone induced a large crest in D.longicephala, reduced first brood size, mean egg volume and total egg volume, and increased age at maturity. The coefficient of variation of mean egg volume was significantly reduced by Anisops kairomone. It is hypothesized that endosulfan enhances crest development by inhibition of γ-aminobutyric acid (GABA), an inhibitory neurotransmitter at cholinergic nerve junctions.