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.J. Jeffery J. Exp. Mar. Biol. Ecol. 252 2000 15 –26
1. Introduction
Larval supply, which is influenced by hydrodynamics, has been shown to determine temporal variability within barnacle populations Farrell et al., 1991; Roughgarden et al.,
1991, 1994; Pineda, 1994a,b, whereas spatial variability has been linked to larval behavioural responses and availability of substratum during settlement Gaines and
Roughgarden, 1985; Bushek, 1988; Minchinton and Scheibling, 1993; Pineda, 1994a. Most models on settlement of open marine populations emphasise free space as the main
determinant of demography Roughgarden et al., 1985, 1988; Roughgarden and Iwasa, 1986; Possingham and Roughgarden, 1990, but they do not recognise that larval
behaviour may also influence settlement.
In fact, many previous studies on barnacles have emphasised that amount of free space available determines numbers of larvae that settle. In separate experiments on
Semibalanus and Chthamalus spp., Bertness et al. 1992 and Pineda 1994a, respectively, found that patch size determined numbers of barnacles settling; smaller
patches had greater settlement. Alternatively, Gaines and Roughgarden 1985 and Minchinton and Scheibling 1993 showed that there was more settlement with increased
availability of substratum for S
. balanoides and Balanus glandula, respectively. Pineda and Caswell 1997 further suggested that differences in availability of substratum may
be responsible for variations in settlement at different times, so that there would be an intensification of settlement when suitable substratum became scarce.
While this may be true for species with large numbers of larvae available and limited free space on the substratum, it is not known whether similar processes operate in
species that have abundant free space and relatively few larvae. In fact, other site- specific characteristics, such as local hydrodynamics, may also influence larval supply
and determine spatial variability in species where larvae are sparsely distributed and where there is much space available for settlement. Larval behaviour when settling
Raimondi, 1991 may also be important, especially when species are gregarious. Hence, the presence of conspecific adults may define the numbers of gregarious larvae settling
and will depend on the degree of exposure to adults at the perimeter of patches Minchinton, 1997. Abundance of conspecifics may also play a primary role. This is
contrary to the previously expressed models of free space where variability in recruitment is said to be driven by the amount of substratum available.
Per unit area of settlement has been used to standardise data and to define the relationship between free space and recruitment Sousa, 1984; Gaines and Roughgarden,
1985; Le Tourneaux and Bourget, 1988; Raimondi, 1990; Minchinton and Scheibling, 1993. For gregarious species, the conspecific component of settlement must also be
recognised. Hence, the perimeter of a patch, which can be used as a function of the numbers of adults present Raimondi, 1990; Minchinton, 1997, must be incorporated in
analyses when gregarious barnacles are settling within aggregations of barnacles. Therefore, it would be expected that, in different-shaped patches of equal area, more
larvae would be expected to settle in patches with longer perimeters of adult barnacles Raimondi, 1990. Further, it would be predicted that, if different-sized patches were
used, similar numbers of larvae would settle per unit perimeter. Alternatively, it would be predicted that more larvae would settle proportionally in the smallest patches in
C .J. Jeffery J. Exp. Mar. Biol. Ecol. 252 2000 15 –26
17
response to more concentrated conspecific cues Minchinton, 1997. It may be, however, that reduced area of patch will intensify settlement Pineda and Caswell, 1997 despite
other influences. At Cape Banks, Botany Bay, New South Wales, Australia, contrary to previously
documented cases, few larvae of the small gregarious barnacle Chamaesipho tasmanica arrive and settle relative to the space available Jeffery, 1997. The barnacles often form
continuous sheets of joined individuals or occur in isolated groups of joined or separate individuals on mid-littoral areas. In the more wave-exposed areas at Cape Banks, a
patchy distribution is prevalent whereas, in sheltered areas, cleared spaces may be scattered throughout aggregations of barnacles. Despite the amount of space differing
from place to place, there were consistent patterns in numbers of cyprids arriving from 1990 to 1993. Although the numbers of cyprids varied among places, they were
correlated with the distributions of juveniles on the substratum Jeffery, 1997; Jeffery and Underwood, 2000. Obviously, the suggestions of Pineda and Caswell 1997 that
consistent differences in rate of settlement might be explained by the proportion of the substratum available, cannot be substantiated by Chamaesipho. Other processes must
also be operating.
For example, because Chamaesipho settle gregariously in response to the presence of adults Otaiza, 1989; Jeffery, 1997, if different-sized patches were cleared within
aggregations of barnacles, it was hypothesised that the degree of exposure to adults along perimeters of patches, rather than the amount of space available, would influence
settlement of cyprids. It was therefore predicted that in plots of 6, 3 or 1.5 cm diameter cleared daily of newly-settled barnacles, there would be proportionally greatest
settlement in the smallest plots with the greatest relative exposure to adults along perimeters. This study therefore tested the hypotheses that increased availability of
substratum and or exposure to conspecific adults would influence settlement of Chamaesipho.
If free space contributed to numbers settling, it would be expected that there would be no differences in numbers settling per unit area in patches of different sizes. Alter-
natively, if adults around the perimeter of a patch influence settlement, there should be no differences in settlement per unit length of perimeter around patches of different
sizes.
2. Materials and methods
