Despite considerable progress in the study of predator-prey interactions, our understanding of the role of non-lethal effects in this relationship remains limited. Except of consumption the presence of a predator often causes a behavioural shift in the prey species, called risk effects. They occur immediately by appearance of the predator and can effect the entire prey population. This is the first study dealing with changes in reproduction as a behavioural response to predation in the common periwinkle Littorina littorea. It is a factorial laboratory experiment to analyse the risk effect of the common shore crab Carcinus maenas on the mating success of L. littorea quantified at the number of eggs per female and unit of time. Exposure to water-borne chemical cues from feeding predatory crab caused the snail to decrease their number of eggs significantly about 40%, whereas another crab species (Cancer pagurus), damaged conspecific as well as starving crabs had no effect on the mating success. Furthermore, there was a tendency for decreased feeding activity of the snails when C. maenas were present. These results demonstrate that L. littorea is able to detect risk cues from the widespread predator C. maenas and response with a risk effect in terms of a reduced number of eggs. This shift may be either a direct behavioural answer to increase the chance of survival because the act of copulation enhance visual, tactile and chemical conspicuousness to predators, or a result of diversion of energy or time from reproductive tasks into anti-predator behaviour as escape or refuge seeking. However, it illustrates the potential for chemical cues as a non-lethal interaction between predator and prey, released by actively foraging C.maenas, to have profound effects on gastropod reproduction and by implication on the abundance of the next snail generation.