Marine invasive species are much less common in open coast habitats than in bays, estuaries and marinas. However, the colonial marine bryozoan Watersipora is a successful invader of both estuarine and outer coast habitats in California. Watersipora species have been reported in California since 1963, and are now widely invasive in bays and marinas across the state. Watersipora was first observed in the rocky intertidal in 2014, and has continued to expand across rocky intertidal and subtidal habitats along the open coast. The recent success of this genus in the rocky intertidal, as well as its poorly understood invasion dynamics, are significant concerns for invasive species management, especially in marine protected areas and national parks where it may threaten native assemblages. We used a combination of community composition surveys based on photographed quadrats, abundance transects, and larval settlement plates in marinas, to assess and characterize the abundance of Watersipora species at two marina sites and four rocky intertidal sites in the San Francisco area.
Results/Conclusions:
In the intertidal, Watersipora spp. populations showed the capacity to rebound from seasonal die-back events. Additionally, Watersipora spp. abundance was not correlated with distance from San Francisco Bay outflow. These results are consistent with the hypothesis that established intertidal populations are mostly self-sustaining. Across all four intertidal sites, community surveys showed frequent overgrowth of Phragmatopoma californica reefworm tubes by Watersipora. Sponge/Watersipora interactions were also frequently observed, with overgrowth by both partners. At both marina sites, there was no difference in larval settlement on settling panels at different depths or different locations within marinas, which does not support larval preference for or differential success in varied microhabitats in marinas. Our data suggest that native assemblages may be threatened by Watersipora establishment, and that populations may expand rapidly via local site colonization. Colonization on the outer coast could be most effectively controlled by whole-site eradication measures performed between the post-winter decline and spring larval recruitment. Early detection and eradication efforts may be crucial to preventing further spread.