The Influence of Recruitment, Competition, and Benthic Predation on Spatial Distributions of Three Species of Kelp Forest Gastropods (Trochidae: Tegula)

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Abstract
Factors influencing the bathymetric segregation of three species of turban snails (Tegula) were examined in Monterey Bay, California. Two of the three species are abundant: Tegula brunnea occurs in shallow water (0—6 m), and T. pulligo occurs in deep water (7—12 m). Tegula montereyi occurs at intermediate and deep depths but is relatively rare. I examined the effects of larval recruitment, interspecific competition, and benthic predation on the maintenance of these patterns. The distribution of recently settled juvenile snails coincides closely with adult distribution, indicating that planktonic larvae of the three species may settle differentially to different depths. Juveniles of the two abundant species occur in different microhabitats at the extremes of their adult ranges: juvenile Tegula brunnea occur in the low—intertidal/shallow—subtidal zone on algae—covered rock substrate, and juvenile T. pulligo occur in deep water (11—12 m) on shell—fragment rubble. T. montereyi juveniles are less abundant than those of the other two species, depth and microhabitat distributions are similar to those of T. pulligo. Competition among the three species is not strong; neither food nor space limitation appears to occur in nature. The effects of competition on patterns of movement and habitat use in the field and on growth and reproductive development in the laboratory showed that interspecific and intraspecific effects were not significantly different. In addition, densities at which competition would have a strong effect are probably much higher than those that occur naturally. It is unlikely that competition is responsible for the bathymetric segregation of the three species. The major predators of Tegula are benthic invertebrates and demersal fishes. Tegula have two spatial refuges predators: kelp plants (which allow snails to stay off the primary substratum and away from predators) and dense benthic vegetation (predators cannot detect and capture snails as easily in areas of dense vegetation). Benthic predation pressure increases with increasing substrate depth because (1) the cover of protective benthic vegetation decreases significantly, and (2) predators are more abundant in deeper water. Higher predation pressure in deeper water limits adult Tegula brunnea to shallower depths. Individual T. brunnea spend more time on the kelp forest floor than do individuals of the other two species causing them to be killed at a disproportionately higher rate in deeper water. Benthic predation is also responsible for the concentration of T. pulligo on kelp plants in deeper depths, but cannot explain why this species is not more abundant in shallow depths. The rarity of T. montereyi is not caused by predation, since this species is the best defended of the three against starfish. For both species, patterns and intensities of larval settlement may be of primary importance. Further information on the larval biology of these species is needed.