Effects of predation and parasitism on climbing behavior of the marine snail, Cerithidea scalariformis

• We hypothesized that snail migration is an adaptation to minimize predation risk.
• Vertical migratory distance by intertidal snails depends upon water or tidal height.
• Predation is greatest near the substrate and declines with increasing water height.
• Parasitized snails climb substantially less than uninfected snails in crab presence.
• Both predation and parasitism can alter snail migratory behavior.

In mangrove marshes in Florida, the marine snail, Cerithidea scalariformis, exhibits a tidally-induced climbing behavior in which snails forage on the substrate during low tides, then climb aerial roots of mangrove trees on incoming and high tides. Under field and laboratory conditions, we tested the conventional explanation that snails vertically migrate to avoid predators, and because populations of C. scalariformis are parasitized by a guild of larval trematodes, we quantified the additive effects of parasitism on climbing behavior. We found that snail climbing distance was significantly and positively associated with water height, with snails migrating 2.7 times higher during high tide than low tide. Climbing distance depended on the species of trematode infection, and differences in migration distance between low and high tides were reduced for two of the four trematode species, suggesting that parasitism can affect climbing behavior. In field and laboratory experiments with tethered snails, individuals experienced significant levels of predation 0–8 cm above the substrate surface, which declined substantially at heights above 16 cm. A laboratory experiment showed that in the presence of blue crabs, Callinectes sapidus, unparasitized snails climbed 53% higher than parasitized snails, and 37% higher than they would without the presence of a predator. Surprisingly, infected snails climbed the same distance in the presence or absence of a blue crab, suggesting that parasitism may physiologically affect a snail's ability to respond to predators. Our data show that predation and parasitism are significant driving forces behind the climbing behavior of C. scalariformis whereby parasitic infection may affect their interactions with marsh predators. Reduced climbing distance may be an adaptive benefit for the parasite that keeps snails closer to the mud surface, providing favorable conditions for free-swimming cercariae to find a second intermediate host, since cercariae can only be transmitted to the next host when the snail host is inundated. Thus, the migratory behavior of C. scalariformis is a behavioral adaptation to avoid predation and that body snatchers, such as larval trematodes, can greatly modify this behavior to increase their own evolutionary fitness.