Role of the sea anemone Metridium senile in structuring a developing subtidal fouling community

In a variety of ecological settings, individual species have been shown to possess the ability to influence the structure and development of the community at large. However, within fouling communities, the influences of many sessile invertebrate species and how they drive patterns of succession remain relatively unexplored. The sea anemone Metridium senile is ubiquitous in cold temperate fouling communities, but its role within these communities has largely been overlooked. Natural fouling communities that had formed on plastic experimental panels were ‘gardened’ to form treatments containing only M. senile, compound ascidians, or erect bryozoans. These panels were deployed under floating docks and were surveyed photographically over 4 mo. M. senile maintained more free space through time within its experimental treatment community than did organisms contained within the other 2 (ascidian or bryozoan) treatments. Subsequent short-term field experiments on recruitment concluded that the presence of M. senile in fact enhances recruitment rates among fouling organisms (relative to controls), ruling out larval predation and larval avoidance as a cause for observed long-term patterns. However, when M. senile were placed upon panels containing natural recruitment in laboratory tanks, it was shown that the anemone ‘smothers’ the majority of these new recruits by sliding over them with its pedal disk, killing all other potential space occupiers and thereby generating increased free space. This study illustrates how a single species can have multiple, opposing influences on patterns of succession at different temporal scales.