Non-native solitary ascidian Ciona intestinalis (L.) depresses species richness

Abstract

Non-native ascidians are a dominant feature of many sessile marine communities throughout the world and may have negative effects on species diversity. We tested effects of the non-native Ciona intestinalis on the sessile invertebrate community in San Francisco Bay, where it occurs in dense aggregations. In particular, we compared species richness between PVC panels from which C. intestinalis were experimentally removed to panels with naturally dense C. intestinalis growth, using fouling panels of four sizes (between 49 cm2 and 1177 cm2) to measure the effect of C. intestinalis recruitment on species-area relationships. We initially deployed 120 fouling panels (30 of each size) at a site known to have dense populations of C. intestinalis, assigning these to three different treatments: (1) Experimental removal, whereby new recruits of C. intestinalis were removed on a weekly basis, pulling panels out of the water for a short time period to do so; (2) Manipulated control, whereby panels were removed from the water each week (as in the experimental removal) but without C. intestinalis removal; and (3) Unmanipulated control, which remained in the water throughout the experiment. After 4 months, all of the panels were collected and analyzed to estimate species richness and relative abundance (percent cover) of sessile invertebrates and of C. intestinalis. Across all panels, species richness was negatively correlated with C. intestinalis abundance. The removal of C. intestinalis produced communities with significantly higher species richness than the controls. The overall species composition of treated and control panels was also distinctly different, with many species occurring more often in the absence of C. intestinalis, while others occurred more often on C. intestinalis-dominated panels. These data suggest that C. intestinalis both depress local species diversity and alter community assembly processes to fundamentally change sessile community composition.