Physical and biological processes influencing zonation patterns of a subtidal population of the marine snail ,

The population density and size distribution of the marine gastropod, Astraea (Lithopoma) undosa Wood 1828, at Bird Rock, Santa Catalina Island, CA, reveal an inverse relationship between population density and mean individual size, over a depth gradient. This trend may be correlated with physical and biological differences between habitats for parameters such as water motion, competitive interactions, and predation. The potential effect of hydrodynamic forces on the zonation patterns of Astraea undosa was tested in laboratory and field experiments. Based on theoretical predictions of the relationship between shear force and water velocity on different-sized snails, large snails are subjected to greater shear forces, as a result of water motion, than medium or small snails. Results of dislodgment experiments conducted in the laboratory indicated that for a given force per unit area, all snails dislodged at nearly the same frequency, with 50% of snails predicted to dislodge at about 4 m/s, and 100% of snails predicted to dislodge at about 8 m/s velocity. These results suggest that hydrodynamic forces may be an important factor in the shallowest subtidal zones. A factorial-designed caging experiment was used to test the effects of snail population density on growth rates of snails of three different size classes. For small and medium size classes, results indicated an inverse relationship between population density and growth rates, which was especially pronounced for smaller snails. These data, in conjunction with long-term patterns of population density and size distribution in the field, suggest that intraspecific competition also plays a role in determining size-specific zonation patterns. Tethering experiments, used to estimate predation rates in different algal-cover zones, suggest that there are no differences in survival rates among different snail size classes; however, survival rates differ among zones and may contribute further to the observed zonation patterns. Overall, data indicate that a combination of physical and biological processes controls the population *Corresponding author. Present address: School of Environmental and Marine Sciences, The University of Auckland, Auckland, New Zealand. Tel.: 164-9-373-7599, ext. 7418; fax: 164-9-373-7435. E-mail address: [email protected] (A.C. Alfaro) 0022-0981/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PI I : S0022-0981( 99 )00062-3 260 A.C. Alfaro, R.C. Carpenter / J. Exp. Mar. Biol. Ecol. 240 (1999) 259 –283 density and size-distribution of Astraea undosa over a depth gradient at Santa Catalina Island, California.  1999 Elsevier Science B.V. All rights reserved.