Reference: Biol. Bull 168: 214-221. (April, 1985) LONG-LASTING SUBSTRATE MARKING IN THE COLLECTIVE HOMING OF THE GASTROPOD NER1TA TEXTILIS G. CHELAZZI, P. DELLA SANTINA, AND M. VANNINI Istituto di Zoologia dell'Universitd, 50125 Firenzc, Italy ABSTRACT Field observations and experiments were conducted on the intertidal gastropod Nerita textilis Gmelin along the Somalian coast to determine if its rhythmical mass-homing includes the detection of durable substrate marking as well as short-term trail-following. The snails' first response to displacement is a zonal orientation compensating for the vertical component of experimental shifting. The homing performance of symmetrically transferred animals supports the hypothesis that a marked area is present in and below the aggregation site, detectable by the homer snails 24 hours after its deposition by spontaneously moving conspecifics. No specific marking of different collective homes resulted from these experiments. INTRODUCTION Experimental evidence shows the importance of trail-following in the orientation of molluscs toward goals of different ecological significance, including rest sites (Newell, 1979; Underwood, 1979). The Indo-Pacific intertidal gastropod Nerita textilis Gmelin performs looped feeding excursions whose homeward branch partly overlaps its outward path. Moreover, arena tests show inter-individual short-term retracing as well (Chelazzi el al., 1983). Under high population density this species shows markedly rhythmical clustering during all high tides and low tides occurring between about midnight and noon (Vannini and Chelazzi, 1978). This rhythmical aggregation is controlled by external (tidal) factors and spatial interactions between members of the population (Chelazzi et al., 1984). Collective homing of N. textilis and other gregarious intertidal gastropods (Moulton, 1962; Magnus and Haacker, 1968; Willoughby, 1973) would seem to require not only a capacity to follow freshly deposited mucous trails but also the ability to detect long-lasting chemical cues, including a durable system of trails connecting feeding and resting places, as well as the marking of collective homes. The capacity to follow durable mucous trails and the use of stable chemical labeling of the rest site are both present in some solitary-homer intertidal gastropods (Funke, 1968; Cook, 1969). The displacement experiments reported in this paper were designed to verify if N. textilis homes not only through the use of short-term trail-following, but also by detection of long-lasting chemical cues. MATERIALS AND METHODS Experiments and observations were conducted at different sites along the Benadir coast (Somalia), whose morphology and intertidal ecology have been described elsewhere (Chelazzi and Vannini, 1980a). Observations on natural behavior were performed using photography at set intervals. Received 12 May 1984; accepted 18 January 1985. 214
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