Variability in trail-following and aggregation in Nodilittorina unifasciata Gray

During low tide, the small periwinkles, Nodilittorina unifasciata Gray, an typically aggregated on intertidal shores in New South Wales, Australia. Previous work has documented considerable spatial and temporal variability in patterns of aggregation. in this study, aspects of behaviour of the snails that may lead to aggregation are examined in field and laboratory experiments. Although small snails were more Likely to be found in aggregations than were large snails, this pattern was influenced by the height on the shore and possibly by exposure to waves. There was also considerable individual variability in behaviour; many animals changed their pattern of dispersion (from solitary to aggregated and vice versa) from one low tide to the next. N. unifasciata appeared to recognise and respond to features of habitat as sites for aggregation, but, more importantly, aggregated in sites that previously contained an aggregation. They may move into these sites by following conspecific trails, but this could not be demonstrated in the field. Trail-following was common under laboratory conditions, although not all snails followed trails and trail-following varied in individuals from time to time. This behaviour was also influenced by environmental conditions, e.g., period of emersion. N. unifasciata did not, however, follow old trails and usually followed trails in the direction they were laid down. Most animals are therefore unlikely to return to sites of aggregation along outbound trails. These data are used to propose a model for aggregation of N. unifasciata which can explain how snails tend to aggregate in the same sites. (C) 1998 Elsevier Science B.V.