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

Alexander T Lowe,Aaron We Galloway

doi:10.7773/cm.v46i4.3156

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 locations (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 (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.

Highlights

Sea urchins are important ecosystem engineers throughout the world’s oceans due to their effects on community structure and food webs as grazers and habitat providers (Harrold IntroducciónLos erizos de mar son importantes ingenieros de ecosistemas en los océanos del mundo debido a sus efectos en la estructura de la comunidad y las redes alimentarias comoScreened via Similarity Check powered by iThenticate Open AccessCiencias Marinas, Vol 46, No 4, 2020 and Reed 1985, Hartney and Grorud 2002, Nishizaki and Ackerman 2007, Ling et al 2015, Lowe et al 2015)
From these observations we put forth 3 hypothetical scenarios to contrast patterns of drift availability and red sea urchin distribution based largely on bathymetry and water motion (Fig. 1). These scenarios reflect habitats represented by our study sites, yet broadly apply across the habitat continuum and lead to specific testable hypotheses on sea urchin–drift interactions. We present these observations as the basis for expanded research of red sea urchins across their range
Published accounts have reported red sea urchins to 125 m in the central Salish Sea (Mortensen 1943), yet maximum water column depths within the Salish Sea exceed 300 m depth–depths equal to those found on the continental slope many kilometers from the open ocean coast

Summary

Introduction

Sea urchins are important ecosystem engineers throughout the world’s oceans due to their effects on community structure and food webs as grazers and habitat providers (Harrold IntroducciónLos erizos de mar son importantes ingenieros de ecosistemas en los océanos del mundo debido a sus efectos en la estructura de la comunidad y las redes alimentarias comoScreened via Similarity Check powered by iThenticate Open AccessCiencias Marinas, Vol 46, No 4, 2020 and Reed 1985, Hartney and Grorud 2002, Nishizaki and Ackerman 2007, Ling et al 2015, Lowe et al 2015). The export of macrophyte biomass can support sea urchins hundreds of kilometers from the location of macrophyte production and alter their effects on local benthic ecosystems (Vanderklift and Kendrick 2005). Unlike other species of sea urchins, M. franciscanus rarely aggregate on accumulations of detrital kelp, ‘drift’, but instead remain sedentary and catch drift transported by water motion or gravity (Britton-Simmons et al 2009). The sedentary drift catching behavior and messy feeding may facilitate trophic transfer of kelp carbon to benthic food webs (Dethier et al 2019, Yorke et al 2019) and alter transport of carbon to the deep sea (Wernberg and Filbee-Dexter 2018), below the euphotic zone, where algal biomass is limited

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Conclusion