Daily variability in height-related settlement-pattern in ...

J. Mar. Biol. Ass. U.K. (2000), 80, 545^546 Printed in the United Kingdom

Daily variability in height-related settlement-pattern in an intertidal barnacle Takashi Noda*, Kotaro Fukushima, Tsukasa Mori and Shuhei Ban *Faculty of Fisheries, Hokkaido University, 3-1-1 Minato-cho, Hakodate 041-8611, Japan. E-mail: noda@pop.¢sh.hokudai.ac.jp To examine the e¡ects of submergence time, free-space availability, and larval supply on height-related di¡erences in settlement density of Semibalanus cariosus, we monitored the daily settlement density in plots where all benthos were removed daily and in natural-community plots, which were left undisturbed, at two sites with di¡erent tidal heights: the centre and the upper limit of S. cariosus zone, during one settlement season. Settlement density was higher at the low-tidal site, where settler mortality was low, than at the mid-tidal site, where settler mortality was high. The between-site di¡erences in settlement was determined by larval preference for height and not immersion time. The settlement preference for height was masked on days with high larval supply, since suitable settlement sites in lower height were saturated by settlers. For intertidal sessile organisms, settlement at di¡erent tidal levels often results in di¡erent mortality levels (e.g. Carroll, 1996; Miyamoto et al., 1999). Consequently the population dynamics of these organisms may be strongly a¡ected by the vertical distribution of settlement. However, relatively few studies have examined the mechanisms determining this distribution (e.g. Raimondi, 1988). A settlers' vertical distribution can be determined by at least three important factors: (1) the height-dependant submergencetime, which is related to larval availability (e.g. Minchinton & Scheibling1991, 1993); (2) settlement preference (e.g. LeTourneux & Bourget, 1988; Raimondi, 1990); and (3) the free-space availability (e.g. Bertness et al., 1992; Minchinton & Scheibling, 1993; Pineda, 1994). To examine the e¡ects of these three factors on tidal height di¡erences in settlement density, we monitored the daily settlement variability of the barnacle Semibalanus cariosus at two sites with di¡erent tidal heights: the centre of the S. cariosus zone and the upper limit of their zone. Speci¢c questions addressed were: (1) Does survival rates of settlers di¡er between the two sites? (2) Does settlement density di¡er between the two sites? (3) Does immersion time determine the di¡erence in settlement density between the two sites? (4) Do larval supply and free-space availability a¡ect the between-site di¡erence in settlement density? Sampling was carried out in the intertidal zone of an exposed rocky shore at Usujiri (41857'N 140858'E), southern Hokkaido, Japan. Settlement was monitored at two sites (i.e. a mid-tidal site and low-tidal site) on a small rock (10.7 m), where the predator, Nucella was not found. The low-tidal site was 23 cm lower than the mid-tidal site. The horizontal distance between the two sites was 50 cm. Rock-surfaces were smooth and horizontal at both sites. A few S. cariosus adults occurred on the sites (cover 510%). The height of the low-tidal site corresponded to the middle of the vertical distribution of settled S. cariosus, while the mid-tidal site is near the upper limit. The distribution and abundance of sessile organisms at each site (i.e. mid-tidal site and low-tidal site) were representative for each tidal level. Four adjoining permanent 55 cm plots were placed at each site. Two diagonal plots were designated as `community-removal plots', in which all benthos were removed, while the other two were `natural community plots', which were undisturbed. In the community removal plots, all organisms were removed from the rock with a scalpel blade or forceps and a wire brush on each sampling date after photographs were taken of the plots. This Journal of the Marine Biological Association of the United Kingdom (2000)

treatment can completely remove all barnacles from rocks (T. Noda, personal observation). The natural community plots were undisturbed. To quantify the daily settlement densities and mortality of settlers, the plots were photographed at low tide during daylight from 15 April to 29 June 1995, except when bad weather prevented photographing. Consequently, photographs were taken over 47 d at the low-tidal site and 48 d at the mid-tidal site. In the plots, daily settlement densities and number of settlers lost daily were counted. It was determined which individuals settled and were lost within the previous 24 h based on comparisons with photographs taken 24 h earlier. The daily settlement densities and daily mortality were estimated only when photographs were taken on consecutive days. In addition the settlement sites of metamorphosed barnacles and cyprids were determined from photographs of community-removal plots. Daily mean mortality of new settlers was lower at the low-tidal site (0.084 0.016), located at the centre of the S. cariosus zone, than at the mid-tidal site (0.181 0.027), located at the upper limit of the S. cariosus zone (Mann ^ Whitney U-test, Uˆ85, Pˆ0.0053). Settlement densities were higher at the low-tidal site than at the mid-tidal site (Wilcoxon signed-rank test, natural community plots: Zˆ73.724, P50.05; community-removal plots: Zˆ72.417, P50.001). This height-related di¡erence in settlement density appeared to have been caused not by di¡erent immersion times but by the settlement preference of larvae; in community-removal plots, settlement density per period of immersion (settlers cm72 h71) was higher at the low-tidal site (Wilcoxon signed-rank test, Zˆ72.982, P50.01; Figure 1A), suggesting that larvae preferred the low-tidal site. For sessile organisms, including S. cariosus, that can not move after settlement, therefore, natural selection might be expected to produce the tidal height-selection by larvae (Raimondi, 1988). On days with low larval supply, the low-tidal site held more settlers than the mid-tidal site (Figure 1B), suggesting that these between-site di¡erences in settlement re£ected settlement preference. However, on days with high larval supply, the low-tidal site held similar density of settlers as the mid-tidal site (Figure 1B). This fact suggested that the preferred height was saturated by settlers, and that, as a result, settlement also occurred at a less preferred height. Saturation of settlers on a heavy settlement day appeared not to have been caused by the shortage of free space at the lowtidal height (Figure 1B). Free-space in the natural-community

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Figure 1. Daily variability in height-related settlement-pattern of Semibalanus cariosus: (A) regression analysis of the daily settlement density in natural community plots per period of immersion (settlers cm72 h71) at mid-tidal site ( y ) against that at low-tidal site (x). Dotted line indicates settlement density per period of immersion is equal at each site. (B) Regression analysis of the daily settlement density in natural community plots ( y ), against the daily mean settlement density in removal plots (x). Low-tidal site: yˆ70.20 ‡ 0.56x, r2 ˆ0.48; mid-tidal site: yˆ70.47 ‡ 1.22x, r2 ˆ0.69. Analysis of covariance (ANCOVA) showed that both slopes and elevations di¡ered signi¢cantly between the two regressions (dfˆ1,33, slopes, Pˆ0.004; elevation: Pˆ0.034). (C) Regression analysis of the daily settlement density in community-removal plots ( y) against the daily mean settlement density in removal plots (x): low-tidal site: yˆ0.35 ‡ 0.99x, r2 ˆ0.87; mid-tidal site: yˆ70.31 ‡ 1.72x, r2 ˆ0.92. ANCOVA showed that both slopes and elevations di¡ered signi¢cantly between the two regressions (dfˆ1,33, slopes, P50.0001; elevation, P50.001).

Table 1. Settlement of Semibalanus cariosus in the community removal plots. Total number of larvae settled, proportion of area which larvae settled repeatedly, and proportion of number of larvae settled repeatedly in the area, in low and mid intertidal zones, during the sampling period. Tidal zone

Total number of larvae settled Proportion of area which larvae settled repeatedly (%) Proportion of number of larvae settled repeatedly in the area (%)

Low

Mid

4062

734

5.9

1.7

62.3

68.8

plots ranged from 48 to 78% of the low-tidal site during the study period (T. Noda, unpublished data). Alternatively, saturation of settlers may have been caused by the shortage of suitable settlement sites (Raimondi, 1990). More than 60% of settlement occurred repeatedly in a limited area of a small depression (5.9% of the total quadrat area at low-tidal site) of the communityremoval plots (Table 1), suggesting that larvae strongly preferred the small area. If the favourable height was occupied by settlers, laterarriving larvae had two options: (1) settling at a less favourable height (Minchinton & Scheibling, 1993); or (2) seeking a favourable height. Cyprids of S. cariosus chose the former (Figure 1B,C), possibly because a prolonged larval period results in reduced post-settlement growth and survival (e.g. West & Costlow, 1987; Pechenik et al., 1993). In conclusion, settlement density of S. cariosus at Usujiri, was higher in the centre of the S. cariosus zone where settler mortality was low, than on upper limit of the S. cariosus zone where mortality was high. These height-related di¡erences in settlement density may have been primarily determined by settlement preference of the larvae, daily suitable-settlement site availability, and daily larval supply. Journal of the Marine Biological Association of the United Kingdom (2000)

We thank A. Paturusi and Dr J.R. Bower for correcting the English text, Y. Miyamoto, and anonymous referees for their constructive comments on the manuscript.

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