Rapidly Entrainable Tidal Vertical Migrations in the Salt Marsh Snail Littoraria irrorata

Abstract

The salt marsh periwinkleLittoraria irrorata (Say) remains on the substratum during low tide but climbs above the water on stalks ofSpartina alterniflora Loisel during high tide. Rhythmic tidal migrations may allowL. irrorata to avoid predators such as blue crabsCallinectes sapidus Rathbun that forage when the marsh is inundated. These tidal rhythms may be driven by endogenous clocks or they may be easily entrained. Snails with flexible and entrainable climbing rhythms may be able to avoid predators in unpredictable environments (e.g., when water unexpectedly covers the substratum as in storm surges). We tested the behavioral response ofL. irrorata to different simulated tidal regimes in the laboratory, and the effect of remaining above mean high water (MHW) on snail survivorship in a smallS. alterniflora salt marsh. In laboratory mesocosms, vertical snail position was measured under constant water levels, simulated tidal cycles, and simulated tidal cycles 180° out of phase (reversed). Under constant water levels, snails ceased to migrate vertically after 1 d. When exposed to tidal and reversed tidal cycles, snails migrated in synchrony with the appropriate simulated rhythm.L. irrorata entrained quickly to differing tidal cycles and maintained their position above the water surfce when water levels were high. In a field experiment, snails were tethered toS. alteriflora plants near the substratum and above MHW in the marsh for 1 wk to assess survival. Survival of snails tethered above MHW was sigificantly greater than for snails tethered at the base of plants; no snails in control cages died. Rapid alteration of tidal vertical migrations may allowL. irrorata to avoid predators that forage when water inundates the marsh predictably or unexpectedly.