FORAGING AND SPAWNING RHYTHMS OF THE PULMONATE LIMPET SIPHONARIA SIRIUS (PILSBRY): SWITCHING OF ACTIVITY PERIOD BY A DIURNAL FORAGER

Foraging and spawning rhythms of the intertidal pulmonate limpetSiphonaria sirius were studied at Shirahama, Wakayama Prefecture,Japan. Continuous 24 h observations were made five times, andtwo series of daytimeobservations were conducted for over 10days almost successively. This species inhabited the lower intertidalzones and had a diurnal habit in spring to autumn. At springtide most marked individuals showed foraging activity when awashand submerged both in ebb and flood tides before and after thetime of lowest water. When the tide progressed towards neaps,the activity peaks became ca. 30 min later per day and smaller.A new activity peak appeared early in the morning at neap tidein waxinghalf moon, and in summer, many adult limpets copulatedduring the morning activity. Simultaneous spawning by such limpetsoccurred one or two days after the peak of copulation. S. siriusswitched the activity period from late evening back to earlymorning at neap tide. The adaptive significances of such activitypatterns are discussed focusing on the responses against daytimedesiccation and nocturnal aquatic predators. (Received 5 September 1994; accepted 30 October 1994)