GEOGRAPHIC ORIENTATION, TIME AND MUDSNAIL PHOTOTAXIS

Abstract

1. Mudsnails, Nassarius obsoletus, collected at Chappoquoit marsh, West Falmouth, Massachusetts, were observed in apparatus designed to assay samples of forty snails both mornings and afternoons for their activity, directional preference, and sign and strength of phototaxis. 2. Assays were made during two to three months over each of four summers (1972, 1974, 1975 and 1976). 3. Activity was less, and variances among choices of directions was greater, during afternoons than mornings. 4. A mean monthly pattern was described for the difference between AM and PM activities for all snails; for only negatively phototactic snails a comparable monthly pattern for AM was highly significantly correlated with one for PM. The monthly pattern of negatively phototatic snails negatively correlated with the monthly pattern of total snail activity. 5. The strength of negative phototaxis, as well as differential influences between right and left lights, differed with geographic oriention of the snails and indicated an integration within the snails of response to light and to a horizontally polarized subtle geophysical field. 6. For positively phototactic snails a highly significant positive correlation between the AM and PM patterns of directional choices occurred only when PM compass patterns were rotated 90° clockwise relative to AM ones, with a similarly highly significant negative correlation occurring for 270°. 7. This 90° shift is interpreted to result from orientation to geomagnetism employing a "rotating magnetic sensor" in the snails which is normally used for sun-compass compensation.