Urchin searchin’: Red sea urchins and drift kelp found at 284 m depth in the mesophotic zone

Abstract

Red sea urchins (Mesocentrotus franciscanus) are kelp-associated ecosystem engineers found in rocky habitats throughout the North Pacific from Baja California, Mexico, to Japan. Red sea urchins depend on kelp detritus, herein ‘drift’, for nutrition; in open coast loca-tions (e.g., California) sea urchin abundance declines precipitously with depth outside the kelp forest owing to a lack of drift and habitat. In the Salish Sea, a region of the Northeast Pacific characterized by steep, glacier-carved rocky reefs, red sea urchins have been reported to 125 m depth. Considering the natural history of this species, we predicted red sea urchins could be found deeper than 125 m in areas with hard substrate and abundant drift. We paired submersible and scuba transects to search for deep red sea urchins and quantified availability of drift to sea urchins from the mesophotic (290 m) to macrophyte zones (<20 m). In addition, we quantified the rate of drift capture by sea urchins at the upper boundary of the mesophotic zone to assess patterns of food availability. A red sea urchin was observed at 284 m, more than doubling this species’ known depth range. Red sea urchins and drift were observed in all depth zones surveyed and 24% of sea urchins had captured drift. Drift capture across depth varied between sites and contrasts observations from other deep regions, such as Dume Submarine Canyon, on the Southern California coast. Red sea urchins captured abundant drift (15.4 g drift per urchin per day) at the edge of the mesophotic zone, implying that red sea urchins could facilitate a considerable nutritional subsidy to local benthos. These observations open new avenues for research as the sheltered, rugose coastline in the Salish Sea is representative of >90% of rocky shorelines in the Northeast Pacific from Baja California to Alaska, suggesting a major portion of the red sea urchin’s habitat, and natural history, remains to be explored.