Norrisia norrisi go by several common names including Norris’s topsnail and aptly kelp snail. They are medium sized snails, reaching diameters of 5.5 cm, with a dark red-brown shell and a bright red food (1). They are commonly found living among rocks or on kelp (1). While N. norrisi eat most types of brown algae, they preferentially feed on Macrocystis pyrifera (3). They exhibit diurnal migration while attached to kelp, migrating upwards towards more nutritious parts of the kelp during the day and moving downward during the night to avoid predation (3). Additionally, the morphology plays a large role in determining distribution of N. norrisi on kelp; with smaller snails more often found lower on kelp while larger snails are frequently found higher (2).
(1) Gotshall, D. W. (2005). Guide to marine invertebrates: Alaska to Baja California. Monterey, CA: Sea Challengers.
(2) Hartman H. (1983) The importance of body size in the subtidal ecology of Norrisia norrisii (Sowerby) (Gastropoda: Prosobranchia), Journal of Natural History, 17:3, 247-255,
(3) Wakefield, R. & Murray, S. Factors influencing food choice by the seaweed-eating marine snail Norrisia norrisi (Trochidae), Marine Biology (1998) 130: 631.
(Photograph) Lovell and Libby Langstroth © California Academy of Sciences.