Down the up staircase: Equatorward march of a cold-water ascidian and broader implications for invasion ecology

Abstract

Aim While warming temperatures are expected to facilitate the poleward movement of species previously restricted to more equatorial waters, the arrival and persistence of cold-water species in more equatorward waters are relatively unprecedented. The native north-east Pacific ascidian Corella inflata Huntsman, 1912, has spread southward and invaded new regions along the North American Pacific coast, a rare example of a marine species moving towards the equator. Here, we document C. inflata's equatorward movement and potential impact, assess several hypotheses for its spread and consider implications for invasion ecology. Location North American Pacific Coast (Puget Sound to San Francisco Bay). Methods We recorded the southward movement of C. inflata by compiling records from the literature, conducting informal searches and conducting standardized fouling community surveys at sites from the southern border of C. inflata's native range in Puget Sound to San Francisco Bay. Within San Francisco Bay, we recorded C. inflata's arrival and potential impact by conducting standardized surveys across an 18-year period encompassing the invasion. Results First collected outside its native range in Coos Bay, Oregon, in 2004, C. inflata was subsequently detected 1,000 km south of its historical distribution by 2008 in San Francisco and Humboldt Bays, California. At times, this large, conspicuous solitary ascidian became locally abundant in San Francisco Bay fouling communities, which showed significant shifts in species composition and relative abundance at invaded sites concomitant with C. inflata's arrival. Main conclusions The recent southward movement of C. inflata runs counter to global warming expectations and may be linked to undetected changes in vessel traffic patterns, rather than natural range expansion. However, an understanding of the one or more processes that would serve to explain the equatorward movement of this cold-water affinity marine species remains elusive.