This study was conducted from December 1979 to December 1981 to determine the diet of Siphonaria diemenensis in the high and low intertidal zone on the rocky shore at Griffith Point, San Remo, Victoria.
Gut contents analysis indicated that in both zones the limpets were feeding primarily on encrusting brown algae (e.g. Ralfsia verrucosa and Scytosiphon lomentaria). In an experimental manipulation, limpets were excluded from 15 x 15cm areas of rock to determine the change in algae cover in the presence and absence of limpets. Two runs of this experiment were conducted in Zone 1 from February to September and from September to December in 1980. The change in algae cover during the experiment was generally greater in the exclusion plots. This supports conclusions from the gut contents analysis, that encrusting brown algae form the major part of the diet of S.diemenensis.
A growth experiment was conducted from 5 September to 15 November 1981 in Zone 1. There were 12 enclosures (15 x 15cm) with 15 limpets in each. Three enclosures were assigned to each of the 4 treatments; Ralfsia, Scytosiphon, scraped or bare. At the end of the experiment, 5 individuals were collected from each enclosure and their shell lengths and dry tissue weights measured. The mean shell length and dry tissue weights were greater in the Ralfsia and Scytosiphon treatments compared to the bare and scraped indicating that the presence of brown algae can be beneficial to this species of limpet.
Algal abundances were monitored throughout this study by analysing photographs of permanently marked quadrats (50 x 50cm) at each of the study sites (3 sites in Zone 1 and 2 sites in Zone 2). Photographs were taken at monthly intervals from December 1979 to December 1981. In Zone 1, there was a clear seasonal trend in algae cover with a minimum cover in summer (January to March) and a maximum in spring (August to December). In Zone 2, there was no seasonal trend but a 100% cover of encrusting brown algae at all times.
Individual adults were marked in both zones in April 1980 (42 in Zone 1 and 43 in Zone 2) and again in January 1981 (104 in Zone 1 and 66 in Zone 2) by cementing a small numbered label to the shell with a clear epoxy resin. The positions of marked individuals were measured regularly over a period of 8 weeks in 1980 and 1981. Over 70% of individuals were observed to home back to their original positions after 8 weeks with the remainder moving to new positions. Limpets were observed to begin to move at least 1 hour before being exposed to air on a receding tide, and to cease movement between 1 and 2 hours after they were uncovered.
