184 M
.G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
experiments. Finally, movement was measured over three periods of time 1 day, 1 week and 2 weeks to test the hypothesis that differences in patterns of movement among
species or among habitats are consistent, irrespective of the period over which they are measured.
2. Materials and methods
2.1. Study sites These experiments were done in the Cape Banks Scientific Marine Research Area,
Botany Bay, New South Wales, Australia Chapman, 1994. Six sites, each | 5 m 3 5 m, were selected around the shores of Cape Banks at the midshore level. The substratum
was primarily ‘bare’ rock probably coated with micro-algae; MacLulich, 1987, with patches of encrusting algae primarily Hildenbrandia rubra Sommerfelt Meneghini
and Ralfsia verrucosa Arescough J. Agardh and foliose macro-algae primarily Hormosira banksii Turner Decaisne and Corallina spp.. The rock-surface varied from
relatively smooth with few rock-pools from 5 cm to about 80 cm deep, to very complex, with numerous crevices, ledges, overhangs and pools. Three sites were chosen to
represent, subjectively, topographically simple sites with few pools, crevices or overhangs; the other three were complex. Cover of free-standing water during low tide
and of foliose macro-algae varied among sites, but were not correlated with topographic complexity. Each site had abundant A
. porcata, N. atramentosa and B. nanum. 2.2. Methods
The complexity of the rock-surface was measured in each site at three different spatial scales because the topographic complexity on these shores appears to vary at a hierarchy
of spatial scales. At the coarsest scale, the height of the substratum was measured using a dumpy level at 1-m intervals along each of five parallel 5-m transects through the site
i.e. 25 measures of height at 1-m spacing on a grid throughout each site. At an intermediate scale, height of the substratum was similarly measured at 50-cm intervals
along each of three 1-m transects, separated by 50 cm providing nine measures of height per grid. Data were collected from two grids, placed randomly in each site. At
the finest spatial scale cm, height of the substratum was measured at 5-cm intervals along each of 10 50-cm transects, separated by 5 cm. This was repeated in each of five
random grids per site. At each spatial scale, the index of topographic complexity for each grid was calculated as described in Underwood and Chapman 1989 and, for the
intermediate and small scales, the indices were averaged per site.
In addition, in each site, the proportion of standing water during low tide and the cover of foliose algae including canopy was estimated by measuring the length of each
of five 5-m parallel transects running through each site that lay over standing water or foliose algae. These values were averaged to give one mean measure per site. Finally,
along the same transects, the relative amount of substratum was calculated as the ratio of
M .G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
185
the total length TL of a tape measure laid flush with the substratum to the 5-m linear length LL of the transect TL:LL.
A few days prior to the start of each experiment, | 30–60 snails of each species in each site were individually marked using numbered plastic labels stuck onto the shells.
Previous studies have shown little disturbance associated with similar handling and marking Chapman, 1986. Nevertheless, the snails were left undisturbed for a few days
prior to the start of each experiment. Movement was measured as the linear distances and directions displaced by individual snails over three different periods of time; 1 day
two tidal cycles, 1 week and 2 weeks, for each of three experiments May, August and September, 1998. These measures were calculated from the subsequent positions of
snails on the shore, measured in situ from two fixed points in each site Underwood, 1977 with no further disturbance or handling. Numbers of marked snails varied among
species and sites according to the proportions expected to be recovered in each site expected sample size was n 5 20; see Chapman, 1986. Because hypotheses were about
variation among species and sites over different periods of time and not specifically about changes in movement over time, the same pool of marked animals of snails was
available for recovery after 1 day, 1 week and 2 weeks. Nevertheless, because not all snails were found on each occasion, samples were different each time.
To test the hypothesis that the different species responded differently to features of habitat, on eight occasions during the three experiments, the use of three different
microhabitats by each species was measured by counting the marked animals of each species in and out of three different microhabitats: in crevices or overhangs, in contact
with or under the canopy of foliose macro-algae or in water 1 cm deep.
3. Results
