Stalked tunicates Boltenia ovifera form biogenic habitat in the rocky subtidal zone of Nova Scotia

Abstract

The stalked tunicate Boltenia ovifera is widely distributed in the Arctic and North Atlantic oceans on rocky substrata at 10–300 m depth, although its ecological role in benthic communities is poorly understood. The distribution and abundance of B. ovifera were recorded at 10–100 m depth in towed-video transects in November 2011 and in June, July, and November 2012 off a wave-exposed headland near Halifax, Nova Scotia. Specimens also were collected at 30 m depth using SCUBA (44°26.88′N, 63°31.59′W) in September 2012. Areas dominated by B. ovifera had densities of 10–100 individuals m−2 on rocky substrata at 30–60 m depth. These tunicate beds often were associated with sparse kelp (Agarum clathratum) and turf-forming red algae. A generalized additive model indicated that depth, substrate type, and benthic algal type were strong predictors of tunicate abundance. Twenty-two epibiotic species were found on specimens of B. ovifera, including juvenile conspecifics. Filter-feeding macroinvertebrates, including anemones and soft corals, were more abundant in areas with B. ovifera than in areas without these tunicates. Our findings suggest that beds of B. ovifera can act as biogenic habitat to enhance local species richness in the rocky subtidal zone of Nova Scotia.