Field observations on the rhythmic up-and-down movement ofNodilittorina exigua (Gastropoda: Littorinidae)

Field observations on the activity pattern ofNodilittorina exigua were carried out in various tidal conditions in various seasons. The snails were stationary when dry on rock surface and began to move just after being splashed. On the rising tide they crawled upward aggregating in the awash zone but were not active around the time of high tide. On the receding tide they crawled predominantly downward, again in the awash zone. Position of the snails was high in spring tide and low in neap tide, changing in parallel to the change of high tide level. They were far more active in the awash condition than in the exposed or in the submerged conditions. For onset and termination of the movement, predominant influence of endogenous factors was suggested, except for onset of upward crawling. The distance of upward movement was larger when the high-tide level was higher, while that of downward movement was related to the time of high tide. Such movement patterns enable the snails to decrease the period of submergence and increase the period of their stay in the awash zone, and help them avoid long-term drying.