Many organisms rely on chemical signals and cues to determine habitat suitability and safety. Chemical signals can mediate many interactions, including those between predators and their prey. Altering prey behaviour, these non-consumptive effects (NCEs) can influence population and community dynamics. Understanding how NCEs influence early life history stages, such as ‘decisions’ of benthic species with planktonic larvae about where to settle, can provide useful information on the ecological functioning of these systems as well as the management for commercial usage, although most studies have so far focused on intertidal systems which are already subject to a set of stressful conditions. With a shallow subtidal field experiment we investigated NCEs of the common starfish Asterias rubens on one of its main preys, the blue mussels Mytilus edulis. We tested the hypotheses that (1) the presence of starfish reduces mussels settlement and that (2) the mussels that settle will invest more energy towards induced defences than to growth, and will thus remain smaller than mussels settling in an area without starfish. Two independent trials revealed a significant reduction of mussel spat on the collectors in the presence of starfish after a two-week deployment period. There was however no effect of starfish on the size distribution of the mussel spat. The delayed observation of effects of starfish, absent after the first week but evident afterwards, suggests a time dependency of NCE's on spat settlement. Harnessing this ecologically important information has the potential to increase yield of mussel seeds available for fisheries by either removing starfish from the ground-based settling areas at the onset and for the duration of spatfall or by using floating substrates that are away from the bottom-bound starfish. Moreover, these results also underlines the potential of using predator cues in the application for sustainable natural antifouling compounds in situations with low recruitment pressures.

Spatial patterns formed through the process of self-organization are found in nature across a variety of ecosystems. Pattern formation may reduce the costs of competition while maximizing the benefits of group living, and thus promote ecosystem persistence. This leads to the prediction that self-organizing to obtain locally intermediate densities will be the optimal solution to balance costs and benefits. However, despite much evidence documenting pattern formation in natural ecosystems, there is limited empirical evidence of how these patterns both influence and are influenced by tradeoffs between costs and benefits. Using mussels as a model system, we coupled field observations in mussel-culture plots with manipulative laboratory experiments to address the following hypotheses: 1) labyrinthine spatial patterns, characteristically found at intermediate to high patch densities, are the most persistent over time; this is because labyrinthine patterns 2) result in adequately heavy patches that can maximize resistance to dislodgement while 3) increasing water turbulence with spacing, which will maximize food delivery processes. In the field, we observed that labyrinthine ‘stripes’ patterns are indeed the most persistent over time, confirming our first hypothesis. Furthermore, with laboratory experiments, we found the ‘stripes’ pattern to be highly resistant to dislodgement, confirming the second hypothesis. Finally, with regards to the third hypothesis, we found positive effects of this pattern on local turbulence. These results suggest that the mechanisms of intraspecific facilitation not only depend on initial organism densities, but may also be influenced by spatial patterning. We hence recommend taking into account spatial patterns to maximize productivity and persistence in shellfish-cultivation practices and to increase the restoration success of ecosystems with self-organizing properties.