Physical and biological processes influencing zonation patterns of a subtidal population of the marine snail, Astraea (Lithopoma) undosa Wood 1828

The population density and size distribution of the marine gastropod, Astra ea (Lithopoma) undosa Wood 1828, at Bird Rock, Santa Catalina Island, CA, r eveal an inverse relationship between population density and mean individua l size, over a depth gradient. This trend may be correlated with physical a nd biological differences between habitats for parameters such as water mot ion, competitive interactions, and predation. The potential effect of hydrodynamic forces on the zonation patterns of Ast raea undosa was tested in laboratory and field experiments. Based on theore tical predictions of the relationship between shear force and water velocit y on different-sized snails, large snails are subjected to greater shear fo rces, as a result of water motion, than medium or small snails. Results of dislodgment experiments conducted in the laboratory indicated that for a gi ven 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 snail s 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 snai l population density on growth rates of snails of three different size clas ses. For small and medium size classes, results indicated an inverse relati onship between population density and growth rates, which was especially pr onounced for smaller snails. These data, in conjunction with long-term patt erns of population density and size distribution in the field, suggest that intraspecific competition also plays a role in determining size-specific z onation 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 an d may contribute further to the observed zonation patterns. Overall, data i ndicate that a combination of physical and biological processes controls th e population density and size-distribution of Astraea undosa over a depth g radient at Santa Catalina Island, California. (C) 1999 Elsevier Science B.V . All rights reserved.