Food Threshold for Diapause in Daphnia under the Threat of Fish Predation

Abstract

The chances of prey to survive and leave viable offspring may be determined by many environmental constraints, of which food conditions may be of key importance. When food is not limited, the high reproductive effort of a well-fed prey individual dis- playing various defensive responses may assure successful reproduction, whereas low food supplies may prevent it by limiting energy gains and expenditures on defense and procre- ation. Under low chances for survival of active forms in a local environment, production of diapausing forms of enhanced resistance to unfavorable conditions or dispersion to more favorable places may assure persistence. The laboratory batch-cultured experiments detailed here sought to test whether food concentration may influence probability of producing resting eggs by Daphnia when under chemically simulated predation pressure. In fish-free conditions, no production of resting eggs was observed with either the tested low (0.4 mg C/L) or high (1.6 mg C/L) food concentrations, whereas in fish treatments, resting eggs were produced by various proportions of females according to food concentration. With the limited food (0.4 mg C/ L) there was production of ephippial (dormant) eggs in the early broods of almost all females in the fish treatment. Where food concentrations were higher, proportions of ephippial females were progressively lower, with females postponing the formation of resting eggs until later broods. At the highest food concentration (1.6 mg C/L), only about 17% of females produced ephippia when exposed to cues. These results are considered to indicate an adaptive response of prey individuals to a simulated danger of predation in various food conditions, determining relative gains of producing active vs. dormant off- spring. It seems that D. magna may assess their chances of survival and successful reproduction in accordance with the anticipated danger of predation and food conditions, and enter diapause when the expected gains from diapause exceed those of active life. A graphical model explaining observed production of active vs. dormant offspring by D. magna under predation pressure at various food concentration is proposed.