The feeding ecology of the intertidal limpet, Siphonaria diemenensis at Griffith Point, Victoria.

Brief description

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.

Lineage

Statement: GUT CONTENTS: Twenty, actively feeding adults were collected from Zone 1 and 2 on falling tides in July and December 1980. The limpets were immediately fixed in 10% formalin and later dissected and gut contents analysed under a binocular microscope. There was no quantitative dietary analysis, just a general qualitative description. EXCLUSION EXPERIMENT: Limpets were removed from 15 x 15cm enclosures that were made using steel mesh as described in the parent record. A control enclosure containing limpets was placed on the adjacent rock. Colour photographs were taken at monthly intervals of each replicate treatment and control enclosure and the change in percent cover of algal was estimated. GROWTH EXPERIMENT: Three enclosures were assigned to each of the 4 treatments, which were (1) 'bare', no macroalgae on substrate, (2) 'Ralfsia', substrate had an 80% cover of encrusting brown algae, (3) 'Scraped', all encrusting algae were removed and (4) 'Scytosiphon', substrate had an 80% cover of erect fronds of Scytosiphon lomentaria. At the end of the experiment, 5 individuals were collected from each enclosure and their shell lengths and dry tissue weights measured. ALGAL ABUNDANCES: The percentage cover of bare rock, ralfsoid crust and any other macroalgae was determined from the photographs using an image analyser. ACTIVITY PATTERNS: The positions of individual limpets were determined after an individual had stopped moving, by measuring its distance from 2 permanently fixed screws. The proportion of limpets homing to their original position and the proportion of limpets homing to new positions were recorded. Qualitative descriptions of the limpet activity patterns were made from regular observations over the experimental period.

Notes

Credit
Littlejohn, Murray, Dr