Marine Biology 39, 25-30 (1977) MARINE BIOLOGY 9 by Springer-Verlag 1977 Movement and Feeding Activity of Red Sea Urchins (Strongylocentrotus franciscanus) Adjacent to a Kelp Forest* J. E. Mattison, J. D. Trent, A. L Shanks, T. B. Akin and J. S. Pearse Coastal Marine Studies, University of California; Santa Cruz, California, USA Abstract Movement and feeding were studied in a population of red sea urchins, Strongylocen- trotus franciscanus (Agassiz, 1863), found within and immediately seaward of a kelp forest offshore from Santa Cruz, California, USA. Mean sea urchin movements varied from 7.5 cm/day inside the kelp forest to over 50 cm/day at 15 and 100 m outside the kelp forest. The percentage of sea urchins feeding decreased from 66% inside the kelp forest to 16 and 15% at 15 and 1OO m outside the kelp forest. These data indi- cate that movement by these sea urchins is a response to a low food supply. introduction kelp off Santa Cruz, California. MoRt of the sea urchins in this population oc- Several investigators have studied the curred along the seaward edge of the patterns of movement of regular echi- kelp forest, and those within or nearest noids. The timing and extent of movement to the kelp forest were among abundant have been related to avariety of environ- attached and drifting brown algae suit- mental factors, including water turbu- able for food. Beyond the edge of the lence (Ogden et al., 1973; Fletcher et al., kelp forest the rocky bottom appeared 1974), substratum (Sinclair, 1959; Kitch- relatively barren of brown algae, but ing and Ebling, 1961), light intensity there were still large numbers of sea (Millott, 1954; Lewis, 1958; Sharp and Gray, 1962; Pearse and Arch, 1969), di- urnal cycles (Thornton, 1956; Sinclair, 1959; Ebling et al., 1966; Dix, 1970; Pearse, 1972; Ogden et al., 1973), sea- sonal cycles (Elmhirst, 1922; Stott, 1931; Lewis, 1958; Moore et al., 1963), the presence of predators (Mauzey et al., 1968; Paine and Vadas, 1969), and social interactions among conspecifics (Dix, 1969; Pearse and Arch, 1969). In some cases, large numbers of sea urchins move as a grazing "front" which leaves barren- appearing areas behind (e.g. Leighton, 1971; Camp et al., 1973; reviewed by Lawrence, 1975). In these cases the availability of food is probably a major factor determining the extent of move- ment. We followed the movement of red sea urchins, Strongylocentrotus franciscanus, within and outside a forest of giant *Please address all correspondence and requests urchins present. Our studies presented here indicate that the movement of in- dividual sea urchins in this population was related to proximity of the kelp forest and available food, but did not reflect any mass directional migration. Materials and Methods Study Area The study area was in Monterey Bay on the east side of Santa Cruz Point (36~ 122~ (Fig. I). The sub- stratum in this area is soft shale with boulder outcrops and scattered sand channels. Although depth gradually in- creases from about 6 to 10 m, there was no obvious change in substratum from within the kelp forest to over 100 m sea- ward of the kelp forest. Density estimates of sea urchins, sea for reprints to Dr. J.S. Pearse, Coastal Marine stars, and brown algae in the study area Studies, University of California, Santa Cruz, were obtained during June 25-27, 1974, California 95064, USA. and are given in Table I. The dominant