EFFECT OF GRAZING GASTROPOD ON ALGAE 
15 
found during this experiment, covering 80- 
100% of the substratum in control areas and ap¬ 
proximately 50% in test areas (those containing 
B. nanum) after 8 weeks growth (Fig. 1). Over 
the experimental period, H. banksii regrowth 
was greatest after 3 weeks; subsequently the 
number of H. banksii plants decreased. 
The results of the analysis of variance show 
that B. nanum exhibited a significant (F = 5.90; 
df = 1,18; p = 0.026) inhibition on the recolon¬ 
isation of 5*. lomentaria . No significant effect of 
B. nanum upon E. intestinal is (F = 3.15; df = 
1,18; P = 0.084) recolonisation or upon total 
organic matter (F = 0.2; df = 1,18; P = 0.646) 
was seen in this experiment. Insufficient num¬ 
bers of H. banksii were found in the recolon¬ 
isation experiment for analysis. 
DISCUSSION 
In this study B. nanum exhibited a significant 
negative effect on the recolonisation of S. lom¬ 
entaria over the intertidal platform during the 
period mid April to early June. This phenom¬ 
enon may also indirectly alter the recolonisation 
of other species of algae (Lubchenco 1983, 
Underwood 1980, Underwood and Jernakoff 
1981). It has been previously noted (Quinn & 
Ryan 1989, Underwood 1980, 1984) that B. 
nanum grazes on ephemeral algae, such as Ulva 
spp., and on microalgae. Quinn & Ryan (1989) 
also observed B. nanum grazing upon E. intes- 
tinalis and S. lomentaria in a study conducted 
during winter and spring, but no evidence was 
given that B. nanum is able to restrict numbers 
of colonizing S. lomentaria. 
In the present study, no direct significant ef¬ 
fects of B. nanum upon E. intestinal is or H. 
banksii were observed. It is feasible, however, 
that grazers may reduce the numbers of the most 
competitively aggressive algal species. An initial 
recolonisation of the perennial H. banksii , 
which subsequently became overgrown by the 
opportunistic ephemeral species S. lomentaria 
and E. intestinalis , suggests that grazing gastro¬ 
pods such as B. nanum may enhance the recolon¬ 
isation of H. banksii over a longer period. 
Despite the reduction in the number of S. lom¬ 
entaria in areas where B. nanum was present, no 
significant reduction of total organic matter was 
observed. Since the organic matter was meas¬ 
ured in the central region of each experimental 
area, this latter result may reflect the patchy 
nature of the recolonisation of the algae rather 
than an effect of grazing. A longer study may 
clarify these points. 
Experimental areas were bounded by a layer 
of antifouling paint (adjusted to 25% CuSo 4 ). 
The toxic effects of copper are believed to repel 
gastropods and prevent them from crossing a 
barrier of such paint (Cubit 1984). Antifouling 
paint was used in preference to cages in order to 
alleviate cage effects such as a reduction in wave 
and wind action, shading and harbouring of 
water and food particles (Cubit 1984, Under¬ 
wood 1980). These microclimatic changes may 
also be compounded by the growth of algae on 
cages and by the presence of roofs on cages (Un¬ 
derwood 1980). Antifouling paint did not pre¬ 
vent B. nanum leaving the enclosures, and 
missing animals were replaced when necessary, 
although no B . nanum entered the control areas. 
Loss of animals from test areas was greater in the 
lower zones and may have been due to increased 
wave action and immersion in these zones. 
In conclusion, B. nanum has been seen to 
exert a significant effect over the recolonisation 
of the ephemeral brown alga Scytosiphon lomen¬ 
taria in the late autumn to early winter period in 
Victoria. B. nanum thus plays a recognizable 
role in the structure of this intertidal com¬ 
munity. 
ACKNOWLEDGEMENTS 
We thank B. Newton for typing the manu¬ 
script. 
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