or R-S), these were superseded by other site-specific features. For example, one R-S site had large numbers of swimming crabs that emerged from the adjacent sand at dusk and fed intensely on small mussels, indicating the potential for very localized, site-specific dynamics. In contrast to most studies on predation in intertidal communities, whelks played very little role in predation on small mussels, and in New Zealand, sea stars seem to be important only in the very low shore levels on some west coast benches (see, [29]). Combining the data in this study and previous results on the effect of seascape on the presence and activity of subtidal predators [27,28] allows generalizations to be made across a much greater spatial context. These studies show how fast-moving fish and crabs can forage intertidally on early life stages of intertidal habitat- dominating mussels, thus affecting their low-shore distribution and abundance. However, why seascape affects recruitment is still a matter of speculation. The greatest recruitment rates were always measured in at least one R-S site in each region and sometimes in both. Because R-R and R-S seascapes were paired within the same small region, they most likely shared a common nearshore pool of competent mussel larvae [32; Rilov unpublished data] and therefore differential availability of nearshore larvae between seascape cannot explain differential recruitment rates onshore. One suggestion is that the difference in recruitment rates between seascapes is a consequence of the combination of (1) the vertical positioning of competent mussels in the water column [in most cases, in both New Zealand and Oregon, the larvae are found in mid- rather than surface waters very close to shore, 32] and (2) the fact that intertidal mussel species can recruit and in some places form beds on subtidal reefs (e.g., Perna canaliculus is primarily a subtidal species). If this is true, then where subtidal reefs are present, a portion of the mussel larval pool may settle there and Intertidal Community Regulation therefore be unavailable to the intertidal zone. Where subtidal reefs are absent, competent larvae have little choice but to settle intertidally. Some 23–26% and 31–32% of the recruitment to collectors on rocks at R-R and R-S sites, respectively, is made up of secondary settlers (.1 mm) compared to almost zero to collectors placed on moorings for similar durations (Rilov unpublished data). It is possible that the slightly higher percent of secondary settlers in R-S sites is also a result of scarcity of subtidal substrate in this seascape. The positive effect of limited substrate (e.g., due to sand cover) on intertidal settlement rates has previously been suggested for barnacles [35,36,see, 37]. Similarly, in estuaries, isolated appropriate habitat (oyster reefs in mudflats) was shown to increase juvenile fish recruitment compared to other (vegetated) areas [38]. Another plausible explanation for aug- mented recruitment on R-S sites could be that very local hydrodynamics are different between seascapes due to different bathymetry and R-S seascape which generates hydrodynamic conditions that facilitate settlement to the rocks. There are of course other possible explanations for the lower recruitment on R- R sites such as the ‘‘wall of mouths’’ idea [39,40] where incoming mussel larvae are eaten by planktivores (e.g., fish) on the nearshore reefs in R-R sites before they can make it to shore, and thus intertidal recruitment is reduced at such sites. Here, a conceptual model is proposed (Fig. 6) that describes different predator-presence/absence scenarios (driven either by the regional assemblage of predators or by seascape) that could explain the observed distribution of mussels on rocky shores worldwide in the context of high (for R-S sites), medium (for R-S or R-R sites) or low (for R-R sites) mussel recruitment rates. Of course, not all situations may exist in all biogeographic regions because both shoreline topography and the regional predator guild Figure 6. Conceptual model describing the influence of seascape, the presence of influential intertidal/subtidal predators (predators that have a strong influence on intertidal prey abundance) and recruitment on the distribution of mussel beds in the rocky shore in regions where mussel beds or patches are dominate features on the rocky shore (see text for explanation). The outcomes for the different scenarios are labeled alphabetically with bold capital letters. Examples of sites with the specific configuration are given in parentheses. Geographical regions: OR = Oregon (information from Menge publications and from Rilov unpublished data and personal observations), NZ = New Zealand. Site code names are from Fig. 1 and for Oregon sites as follows: SH = Strawberry Hill, YB = Yachats Beach, BB = Boiler Bay, FC = Fogarty Creek. doi:10.1371/journal.pone.0023958.g006 PLoS ONE | www.plosone.org 9 August 2011 | Volume 6 | Issue 8 | e23958