Mortality, behavior and the effects of predators on the intertidal distribution of littorinid gastropods

Predators can affect the vertical distribution of mobile intertidal inverte brates in two ways: they can (1) cause greater mortality of prey at certain intertidal levels, and (2) induce prey to seek safer intertidal areas. In this study, we investigate whether low-intertidal and subtidal predators af fect the intertidal distribution of two congeneric species of small herbivo rous gastropods of northeastern Pacific shores, Littorina sitkana Philippi 1846, and L. scutulata Could 1849. In particular, we tested the hypothesis that predators affect the distribution of these snails by inducing them to seek higher and safer intertidal areas. On a wave-sheltered shore in Barkle y Sound, British Columbia, L. sitkana and L. scutulata were both killed by predatory crabs (e.g., Cancer productus) more frequently when tethered near the lower limit of their intertidal distribution (approximate to 1 m) than when tethered where they were most common (approximate to 2.5 m), suggesti ng that high mortality rates are partly responsible For the lower-limit of these snails' intertidal distribution. However, two field mark-recapture ex periments indicated that the snails' behavioral response to predation risk also influences their distribution. Tn the first experiment, snails from th e 2.5-m level (low risk) transplanted to the 1.0-m level (high risk) displa yed a strong and consistent tendency to move shoreward, especially L. sitka na, some traveling 10-15 m in 2-3 days to regain their original level. Thes e shoreward movements were especially precise in the northern part of the s tudy area, where predation rates on tethered snails were greatest. Furtherm ore, larger more vulnerable snails were more strongly oriented shoreward th an smaller individuals, indicating that antipredator behavior might also co ntribute to intertidal size gradients in these species. In the second mark- recapture experiment, we manipulated predation risk using small cages and f ound that snails exposed to the odors of C. productus crabs foraging on con specific and heterospecific snails displayed more precise (L. sitkana and L . scutulata) and longer (L. sitkana) shoreward movements than snails held i n control conditions. These results provide the first experimental evidence that antipredator behavior may contribute to the intertidal distribution p atterns of littorinids. (C) 2000 Elsevier Science B.V. All rights reserved.