Predation during early post-settlement varies in importance for shaping marine sessile communities

Predation on newly settled sessile invertebrates is an important process shaping the structure of benthic marine communities in localised areas on the northeastern coast of North America. There are no studies that have tested whether predation acts similarly in other locations, so it is not clear whether generalisations can be made about the effects of predation on different sessile communities. In this study we determined whether predation on newly settled recruits altered the structure of 2 different sessile communities in Port Phillip Bay, Australia. We conducted a series of predator exclusion experiments using full cage, partial cage and no cage treatments at 2 study sites, Williamstown and Queenscliff. Full cage and partial cage treatments of either 1 cm or 2 mm mesh sizes were used to separate the effects of different size classes of predators. At both sites, a variety of colonial and solitary ascidians, bryozoans, sponges and polychaetes settled onto experimental surfaces. Predation had little impact on the recruitment success of taxa present at Williamstown and did not alter community structure. At Queenscliff, didemnid ascidians had higher abundances on completely caged plates (2 mm mesh) after 40 d, suggesting that they may have been preyed upon in treatments exposed to carnivores. However, predation had no effect on the densities of other taxa found on experimental surfaces, and there were no differences in overall community structure between treatments. Recruitment rates were low and predators were never observed on experimental surfaces at Queenscliff, so predation on newly settled recruits may be an uncommon occurrence for most taxa. In contrast, predators were commonly found on experimental surfaces at Williamstown, but recruitment rates were high and predators had little effect on the abundance of newly settled prey. The results of this study show that predators can have weak effects on recent recruits and that predation during early post-settlement is not a major process shaping the structure of all marine sessile communities.