Abstract. We investigated the hypothesis that predation risk affects mating decisions in the intertidal snail Littorina plena in Bamfield Inlet, Northeast Pacific. First, we conducted a field tethering experiment to test the assumption that mating pairs of snails are more susceptible to predation than solitary individuals, and then performed a laboratory experiment to quantify the effect of predation threat on the propensity of snails to form mating pairs. Our results support the hypothesis, in that ''mating pairs'' were more frequently killed than single snails in the field, and snails were less likely to form mating pairs in the laboratory when simulated predation risk was high (chemical cues from crushed conspecifics were added to the water) than when it was low (no risk cues were added to the water). In contrast to several earlier studies, we found no effect of individual size on snail susceptibility to predation, perhaps because our two size classes were contiguous and snails within them were not dissimilar enough. The results of the behavioral experiment were consistent with this lack of individual size effect on snail vulnerability; both size classes of snails showed a significant and similar tendency to decrease mating when predation risk was high. Taken together, the results of this and recent studies indicate that predators can considerably affect the behavior of littorinid snails, including their movement patterns, feeding, and reproduction. We argue that greater consideration should be given to how marine invertebrates trade off predation risk and activities related to reproduction.Additional key words: gastropod, copulatory behavior, intertidal ecology In 1990, Lima and Dill published an influential review demonstrating that an array of animal species adaptively trade off the risk of predation with various activities. In such cases, behavioral responses to a general threat stimulus are not inflexible, but rather vary depending on the magnitude of the threat as well as the cost of the response; costs can be in terms of energy and/or lost opportunities (e.g., feeding, reproduction). For example, many animals readily compromise safety from predators to obtain food when the value of a given feeding opportunity is high (e.g., when starved) or when the risk of predation is low (e.g., when cover is close by). Lima and Dill's review also revealed two marked biases in the literature. First, of the B250 studies reviewed that focused on particular organisms, o3% involved marine invertebrates. Second, onlyB6% of organism-based studies investigated how predation risk affects reproductive behavior; most studies addressed aspects of feeding and sociality, as well as defense responses (i.e., escape, vigilance, group size, mobbing) to variable predation threat. The scarcity of studies involving reproductive behaviors and predation risk may reflect a greater difficulty of quantifying, in many systems, the impact of predation on reproduction decisions than on feeding decisions. However, because courtship and reproduct...

Canalization-the evolutionary loss of the capacity of organisms to develop different phenotypes in different environments-is an evolutionary phenomenon suspected to occur widely, although examples in natural populations are elusive. Because behavior is typically a highly flexible component of an individual's phenotype, it provides fertile ground for studying the evolution of canalization. Here we report how snail populations exposed for different lengths of time to a predatory crab introduced from Europe to America exhibit different degrees of canalization of an adaptive antipredator behavior: soft tissue withdrawal, measured as angular retraction depth. Where crab-snail contact is shortest (60 years), snails showed the highest behavioral flexibility. Where crabs invaded 110 years ago, snails showed significantly less behavioral flexibility, and where the interaction is ancient (Europe), snails exhibited highly canalized behavior. Selection therefore appears to have acted rapidly to increase canalization in wild snail populations, leading ultimately to the hard-wired behavior seen in European conspecifics.

Activity patterns of sub-tidal free-roaming green crabs, Carcinus maenas (L., 1758), were monitored in real time using radio-acoustic positional telemetry in New Harbour, Nova Scotia, Canada. Activity patterns were monitored relative to time of day and tidal height. Periodogram analysis showed activity was circatidally rhythmic, as activity increased with rising tides and decreased with falling tides. Potential proximate and ultimate causes of observed circatidal rhythmicity in C. maenas are discussed. In contrast to other studies, no circadian rhythm was observed in crab activity. This is likely because other studies have been conducted in the intertidal zone, where desiccation and predation stress favour reduced activity during daylight hours. This study provides the first demonstration of circatidal rhythmicity in sub-tidal free-roaming C. maenas. RÉSUMÉLes modèles d'activité des crabes verts subtidaux errants, Carcinus maenas (L., 1758), ont été déterminés en temps réel, en utilisant la télémétrie radio-acoustique positionnelle à New Harbour, Nouvelle-Ecosse, Canada. Les modèles d'activité étaient suivis par rapport au moment de la journée et à la hauteur de la marée. L'analyse du périodogramme a montré que l'activité était calquée sur le rythme des marées, car l'activité augmentait avec les marées montantes et diminuait avec les marées descendantes. Les causes potentielles directes et indirectes de cette rythmicité circatidale observée chez C. maenas sont discutées. Contrairement à d'autres études, aucun rythme circadien n'a été observé dans l'activité du crabe. Ceci s'explique probablement parce que les autres études ont été menées dans la zone intertidale, où le stress dû à la dessiccation et à la prédation favorise une activité réduite pendant la journée. Cette étude fournit la première démonstration de la rythmicité circatidale chez les C. maenas subtidaux errants.

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