One possible interaction between organisms is direct predation, where a predator consumes prey and thereby decreases its density which may influence more organisms in that trophic food chain (e.g. the prey's food source). Another interaction is based on most prey's capability to detect predators and adapt to that threat in behaviourial, morphological or life history traits. These adaptions can be quite costly and it is expected that there is a trade off between predator avoidance and the costs to do so. In aquatic environments this way of interaction is often communicated via chemical “risk cues“, which can either originate from the predator alone (kairomones) or from startled (disturbance cues), injured (alarm cues) or consumed and digested prey (dietary cues). These cues are used by potential prey to assess the risk of predation and adapt accordingly. This study focuses on the effects of alarm and dietary cues on the egg-production and the mating and feeding behaviour of the common periwinkle (Littorina littorea) by using either crushed (injured) snails – the stomach juice from the shore crab (Carcinus maenas) with and without the addition of a snails body – or the faeces of shore crabs feeding on periwinkles. The research took place on Helgoland, from March to May (2011) as a manipulative lab experiment with factorial design. It was found that injured conspecifics reduced the mating behaviour of L. littorea probably because of the higher mortality risk associated with forming mating pairs. The faeces of crabs had no effect on periwinkles but that may be due to the point of time the investigation took place: egg production and mating frequency were decreasing. Stomach juice with in vitro digested snails showed differing results, reducing the egg numbers in one experiment but not in the other. This may be because of methodical reasons: in the experiment showing an effect the devices applying the stomach juice were changed more often and the room temperature was higher which could mean that the enzymes in the stomach juice were working more efficiently on the added snail tissue. Therefore a possible effect of stomach juice with digested snail is favored. There was no evidence found that pure stomach juice influences the snails while crabs feedings on conspecifics clearly had an effect on the numbers of eggs laid and the mating behaviour, as former studies hypothesised. There was no evidence in any experiment where the amount of food consumption differed from a control treatment, probably because L. littorea does not prefer Fucus. The results suggest that the trait-mediated indirect interaction between the predator C. maenas and its prey L. littorea is not communicated via predator's kairomones but more likely via risk cues originating from the periwinkle (alarm or dietary cues).