Oecologia (2005) 143: 402–411 DOI 10.1007/s00442-004-1819-5

P L AN T A N IM A L I NT E R AC TI O NS

Petri Kemppainen Æ Solveig van Nes Æ Christofer Ceder Kerstin Johannesson

Refuge function of marine algae complicates selection in an intertidal snail

Received: 16 September 2004 / Accepted: 20 December 2004 / Published online: 15 February 2005
Springer-Verlag 2005

Abstract Species with restricted gene flow often show trait-shifts from one type of environment to another. In those rock-dwelling marine gastropods that lack larval dispersal, size generally decreases in wave-exposed habitats reducing risk of dislodgement, while increases in less exposed habitats to resist crab-crushing. In Littorina fabalis, however, snails of moderately exposed shores are generally much larger (11–14 mm) than snails of sheltered shores (5–8 mm). Observations from the White Sea (where crabs are not present) indicate that in the absence of crabs snails are small (6–7 mm) in both habitats. We assumed that the optimal size for L. fabalis in the absence of crabs is less than 8 mm, and thus that increased size in moderately exposed habitats in areas with crabs might be a response to crab predation. In a crab-rich area (Sweden) we showed that crab predation is an important mortality factor for this snail species in both sheltered and moderately exposed habitats. In sheltered habitats, snails were relatively more protected from crab-predation when dwelling on their habitual substrate, fucoid algae, than if experimentally tethered to rocks below the algae. This showed that algae function as snail refuges. Snail dislodgement increased, however, with wave exposure but tethering snails in moderately exposed habitats showed that large snails survived equally well on rocks under the algae as in the canopy of the algae. Thus in sheltered habitats a small snail size is favored, probably due to life-history reasons, while increased risk of being dislodged from the algae refuges promotes a large size in moderately exposed habitats. This study shows an example of selection of a trait depends on complex interactions of different factors (lifehistory optimization, crab predation, wave induced dislodgement and algal refuges). P. Kemppainen Æ S. van Nes Æ C. Ceder Æ K. Johannesson (&) Department of Marine Ecology, Tja¨rno¨ Marine Biological Laboratory, Go¨teborg University, 452 96, Sweden E-mail: [email protected] Tel.: +46-52-668600 Fax: +46-52-668607

Keywords Littorina fabalis Æ Size polymorphism Æ Habitat heterogeneity Æ Hydrodynamic force Æ Size-selective predation

Introduction As a result of divergent natural selection, species living in heterogeneous environments often show habitatlinked morphological variation in adaptive traits (Levins 1968; Hedrick et al. 1976; Endler 1977; Hedrick 1986). Such shifting traits seem particularly common in species with a restricted gene flow among populations (Johannesson et al. 1993; Nevo 1998; Filchak et al. 2000; Schluter 2000; Hendry et al. 2002). In rocky-shore snails, trait-shifts have been extensively studied, both with respect to patterns and mechanisms, and it is evident that many traits change in parallel in different species, given the same circumstances of environmental variation (Boulding and van Alstyne 1993; Kirby et al. 1994; Johannesson 2003; Rochette et al. 2003), supporting natural selection as the main component of variation for these traits. Adult shell-size is one trait that in general varies with habitat in direct developing rocky-shore gastropods with limited dispersal and this variation is largely inherited in several species (Janson 1982; Johannesson and Tatarenkov 1997; Tatarenkov and Johannesson 1998; Rochette et al. 2003). For instance, in rock-dwelling littorinid gastropods shell size is generally negatively correlated with wave exposure (Littorina saxatilis: Sundberg 1988; Reid 1996; Hull 1998; Johannesson et al. 1993; L. arcana: Reid 1996; L. compressa: Heller 1976; Reid 1996; L. obtusata: Goodwin and Fish 1977; Reimchen 1982; Trussell et al. 1993; Reid 1996; L. sitkana: Behrens Yamada 1992; L. subrotundata: Reid 1996 [less complete evidence]). This trend is commonly explained by intense crab predation in sheltered and moderately exposed habitats promoting large and robust shells (Elner and Raffaelli 1980; Johannesson 1986; Boulding

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and Van Alstyne 1993; Boulding et al. 1999), while in wave-exposed habitats, small shell-size enables snails to escape friction from waves by hiding in crevices (Underwood and McFadyen 1983; Denny et al. 1985; Trussell et al. 1993). In the direct developing epiphytic littorinid, L. fabalis, size variation is different; shell-size decreases with wave-exposure as expected from moderately exposed shores to heavily exposed rocky shores (in areas where a rare exposed-shore morph is present; Rola´n and Templado 1987), while comparing sheltered shores and moderately exposed shores, shell-size instead increases (by 50–100%) with wave-exposure (Goodwin and Fish 1977; Reimchen 1981; Tatarenkov and Johannesson 1994; Reid 1996). This is despite the fact that L. obtusata, a sibling species of L. fabalis, shows a slight decrease in size from sheltered to moderately exposed habitats (Goodwin and Fish 1997; Reimchen 1982; Reid 1996). Both species dwell mostly in the canopies of fucoid algae where they graze directly on the algal thallus (L. obtusata: Reid 1996) or on epiphytic micro-algae (L. fabalis: Norton et al. 1990; Reid 1996) in contrast to most other littorinids, which predominantly stay on rocky surfaces (Reid 1996). The ‘‘Large-Moderate’’ morph (hereafter LM-morph) of L. fabalis present in moderately exposed habitats, and the ‘‘Small-Sheltered’’ morph (hereafter SS-morph) present in sheltered habitats, have a wide distribution over the shores of western Europe (Reimchen 1981, 1982; Reid 1996; Tatarenkov and Johannesson 1999). The small morph of heavily exposed shores is however only common in NW Spain (Rola´n and Templado 1987; Reid 1996) and will not be considered further here. Besides the disparity in size trends between L. obtusata and L. fabalis, there is also a general size difference between these species, such that L. obtusata is even larger than the large morph of L. fabalis (Goodwin and Fish 1977; Reimchen 1981, 1982; Reid 1996). This is most likely explained by different life-history optimizations (Williams 1992); in the North Sea, L. obtusata takes up to 2 years to reach sexual maturation and lives the subsequent 2–3 years to reproduce even more (Fretter and Graham 1980; Williams 1992) while L. fabalis usually only reproduces one season and few survive more than 1 year (Williams 1992, 1996). The shorter lifespan of L. fabalis probably promotes earlier maturation and thus a small size compared to L. obtusata. The latter instead delay maturation to increase female fecundity by growing large females. Crab predation and wave exposure are viewed as the two most important selective forces shaping the morphology and size of marine gastropods (e.g., Reimchen 1982; Underwood and McFadyen 1983; Etter 1989; Williams 1992; Boulding et al. 1999). The White Sea, where shore crabs are not present, makes an interesting comparison to the North Sea. Similar to elsewhere, L. obtusata is larger and lives longer than L. fabalis in the White Sea (Mikhailova, personal communication). On the other hand, L. fabalis is similar in size (average

adult size 6–7 mm) to the SS morph of Western Europe in both sheltered and moderately exposed habitats (M. Fokin, personal communication). This explicitly suggests that crab predation is a major factor structuring the size variation of L. fabalis in Western Europe, and that an adult size of 6–8 mm is probably favored for lifehistory reasons. An earlier study showed that the adults of the SS morph may easily be crushed by medium-sized (40– 60 mm carapace width) shore crabs (Carcinus maenas), while adults of the LM-morph are much more resistant to these attacks (Reimchen 1982). As crab predation seems to be an equally important mortality factor in both sheltered and moderately exposed habitats (this study) we propose that, the presence of a third factor, fucoid algae (besides crab predation and wave exposure), changes the effect of the two other factors by functioning as a refuge from crab predation. We predict that snails of L. fabalis are more protected from crab predation in the canopies of fucoid algae compared to the situation when, after being dislodged from the algae, they dwell on the rocks under the algae, and that dislodgement due to wave action increases with wave exposure. Thus, the overall risk of predation should increase with wave exposure and select for a larger shellsize in moderately exposed habitats, while in sheltered habitats shell-size should be small, reflecting early maturation of a relatively short-lived intertidal snail species. In the present study we test the predictions of differential survival and dislodgement of snails in habitats of different exposure and our results support the explanatory model outlined above.

Materials and methods Study area and collection of animals Field experiments were conducted on three small islands (Lo¨kholmen, La˚ngholmen and Salto¨; 0.3–1.5 km across), near Tja¨rno¨ Marine Biological Laboratory on the Swedish West-coast (5852¢N; 1109¢E). Snails used in the laboratory experiments were also obtained from these islands. Sampling-sites and experimental sites are referred to as either ‘‘moderately exposed’’ or ‘‘sheltered’’ to wave action (in still more exposed shores no fucoid algae are present and no snails of L. fabalis). Both habitat types are boulder shores with dense stands of fucoid macroalgae such as Fucus spiralis, F. vesiculosus and F. serratus, but sheltered sites were also occupied by the fucoid algae Ascophyllum nodosum, which is particularly sensitive to wave-exposure and an indicator of wave-protection (Lewis 1972). Indeed, we used the presence/absence of A. nodosum to define habitats as sheltered or moderately exposed, and we found that in all sites this distinction also separated habitats with large and small adults of L. fabalis. From the abundance of Ascophyllum and from geomorphological characteristics of the shore, we actively avoided sampling from within

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in, or close to intermediate zones between the two morphs (Tatarenkov and Johannesson 1999). We used the width of the aperture (the distance from the junction between the lip and the body whorl to the furthest point of the anterior lip) as an indicator of overall size, as this measurement is more easily defined than shell height in this flat-spired species. Crab predation Old and repaired shell injuries of gastropod shells are thought to stand in proportion to the amount of shellbreaking predation on the populations (Raffaelli 1978; Reimchen 1982). Reimchen (1982) found repaired shell injuries to be very common in populations of L. fabalis in tidal shores of Britain and Ireland ranging up to 50% of the populations. Reimchen (1982) argued that the injuries were predominantly caused by unsuccessful attacks from C. maenas, since (1) the damages were indistinguishable from those seen in laboratory experiments with this organism, (2) other potential (rare) predators, such as intertidal fish, do not crush but instead swallow the entire shells, and (3) damages caused by the physical forces of waves and rolling boulders were not likely to have caused the damages since shell injuries were more common on sheltered shores than on exposed. To show that C. maenas is an important predator of L. fabalis, also at the Swedish west-coast, we conducted a similar survey of shell damages as Reimchen (1982). We collected 150 adult snails from four sheltered and four moderately exposed shores and calculated the proportion of shells with repaired shell injuries. Shell damages between sheltered and moderately exposed shores were tested with a one-way ANOVA. In addition, we estimated crab densities and sizes in both sheltered and moderately exposed areas. Crabs were captured at equal effort in two moderately exposed and two sheltered shores on two of the islands (Lo¨kholmen and 1 week later La˚ngholmen) at about 0.5 m depth. Width of the carapace was used as an estimate of crab size. We used traps that were able to catch crabs larger than approximately 33 mm in carapace width (smaller crabs were able to move through the netting). The traps were baited with 85 g of thawed herring that were placed in perforated tubes that allowed crabs to sense, but not eat, the bait. After 20 h, the trapped crabs were counted and measured. We used a three-way ANOVA to test for differences between moderately exposed and sheltered shores of the two islands. In this test, island was treated as a random factor orthogonal to the fixed factor exposure. The random factor site was nested under the interaction of exposure and island. Tethering experiments In two consecutive experiments, the consequences of snail dislodgement from the plant fronds in terms of

altered predation risk from crabs was simulated by tethering snails to the bottom substratum (stones) and as a comparison to canopies of fucoid algae. Tethering experiments have successfully been applied to investigate crab predation pressure in earlier experiments on marine snails (e.g. Williams 1992; Behrens-Yamada and Boulding 1996; Boulding et al. 1999). In this experiment, we predicted that the frond structure of fucoid algae constitutes a refuge from crab predation and thus that snails would survive better in the fronds than on the bottom. In the first experiment LM-morphs and SS-morphs were tethered to algae and stones beneath the algae in their respective habitat, that is, LM-morph in moderately exposed sites and SS-morph in sheltered sites. This test investigated the effect of crab attacks, while remaining in the canopy as compared to being dropped on to the rocky bottom in the snail’s own habitat. Moderately exposed and sheltered habitats are likely to differ in several aspects, and, in particular, in the degree of wave exposure. A difference in crab attack rate between the morphs in this experiment could therefore be due either to differences between the habitats or to differences between the two snail morphs, or both. To separate effects of morph and habitat differences, we conducted a second experiment in which we tethered both morphs in the same habitat. We used moderately exposed shores for this experiment, because it seemed more relevant to tether snails to the bottom substratum in the habitat where dislodgement was most likely to happen. We sampled 120 individuals of each morph for the first experiment. Only adult snails were used in the experiment. Adults (sexually mature individuals) were distinguished from juveniles on the basis of their morphology; juveniles have thinner shells than adults and the lip of the aperture extends outside the rest of the whorl (Reid 1996). The snails were individually marked using numbered, 2-mm-diameter, plastic tags that were glued to the shell. The tether, consisting of nylon monofilament fishing line, was attached under the plastic tag. Three moderately exposed and three sheltered test sites were chosen on Lo¨kholmen and La˚ngholmen. In each test site, four groups of five snails each were tethered to the canopy of different individual algae of F. vesiculosus and four groups were attached to large stones. From the five snails, the proportion of attacked snails was calculated and used as the dependent variable in the analysis. We assumed that all snails that were either partly damaged or completely gone had been attacked. One of the groups at each site was placed under a cage excluding predators from access to the snails. This was to estimate the rate at which snails disappeared by other factors than predation. The cages may, however, also have constrained some physical factor of the environment such as waves. Physical effects were, on the other hand, probably not that important since fewer snails had disappeared or been damaged in exposed than sheltered habitats, even though exposed shores were subjected to heavy wave action during the experiment. The snails were checked after 1 day and 5 days of exposure to crab predation. The

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results were analyzed in a three-way ANOVA with the factors morph, position (fixed orthogonal factors) and site (nested under position). In the second experiment we changed the mode of tethering. Holes were drilled through the lips of the shells with a 0.6-mm drill, through which the tether (fishing line) was tied to the snails. The tags were tied to the fishing line. We changed the mode of tethering since this method was less time-consuming than the first one. The snails were checked after 2 days and 5 days of tethering, instead of 1 day and 5 days. We also changed the experimental setup so that site would include only one combination of morph and position, which made the interpretation more general. To get enough sites, we included a close-by island, Salto¨, in this experiment. No control group was used in this experiment, the reason being that only one of the control animals (1.7%) in the first experiment was missing after 5 days of tethering. In addition, a tethering experiment, where the method of drilling holes also was used, showed that all control animals were untouched after 74 days of tethering, except for one (Williams 1992). In both experiments, the snails that obviously had been damaged by the experimental setup were discarded. Some snails were, for instance crushed by the stone to which they were tethered. In addition, some replicates could not be found, and therefore the analyses were made on unbalanced data. Since we had measured size of all the tethered snails, we could analyze the relationship between size and the probability of being attacked by a crab. Here we used linear regressions that were applied both for separate morphs (data from the first and second experiments), and for pooled morphs (data from the second experiment only; these data could be pooled since the combinations involved the same times and habitats). The snails were divided into different size-classes in a way that each class had approximately the same number of snails. The dependent variable was the proportion of snails being attacked, that is, the number of attacked snails divided by the total number of snails in each size-group. Wave simulation and dislodgement of snails We hypothesized that snails living in our moderately exposed sites were at a higher risk of being dislodged than snails living in our sheltered sites. We initially attempted to test this in a field experiment, but it proved to be technically difficult due to the strong physical forces acting on the moderately exposed shores, and we were thus constrained to use a laboratory test. The effect of waves was simulated in a tank (105·550·40 cm) using a sled constructed from reinforcement rods and plywood that could run smoothly back and forth in the tank. Shock cords were attached to one side of the sled and the other side was anchored with a rope. As the sled was pulled back by means of the rope and released, both the backwash and the swash of a breaking wave could be

simulated in a consistent way. We consider this a realistic way of simulating the effects of waves as the force imposed on an organism by a breaking wave and postbreaking flows is due to both the water’s velocity and its acceleration (Denny et al. 1985). Three different levels of exposure were used a, b, and c by varying the number of shock cords, a being the lowest levels and c being the highest. Five plants in three rows (15 plants total) were attached to the sled at a distance of 15 cm between each plant. Five snails were allowed to attach to two plants (replicates), which were positioned as number two and four of the middle row. Eight additional plants thus surrounded each replicate plant. This was done to simulate field conditions where a snail most likely may be dislodged not only by drag from water currents but also by contact with other branches of the plant or adjacent plants. Plants from moderately exposed habitats were used for LM-morph snails and plants from sheltered shores were used for SS-morph snails. In the experiment, every trial (consisting of one level of exposure and one morph) was initiated by slowly moving the sled, back and forth, ten times so that the snails would become accustomed to water movements. After this the sled was pulled backwards and released at increasing distances. After eight of these pre-pulls, the sled was pulled back and released ten times at its maximum distance (=120 cm). Thereafter, the replicate plants were removed from the sled and the number of snails that had been dislodged was counted. Exactly when snails had been dislodged was not assessed. New replicate plants and snails were used for every trial. The whole experiment was repeated twice (during 2 days) and time of the experiment was used as an additional random orthogonal factor (besides morph and exposure) in a three-way ANOVA. A Student–Newman–Keuls test (SNK) was performed to distinguish how snails of different morphs were dislodged at different exposures. The probability of snail dislodgement is dependent in part on the maximum acceleration of the water. This can be quantified with a modified dynamometer originally designed by Jones and Demetropoulos (1968) and further improved by Bell and Denny (1994). Two of these modified dynamometers (hereby referred to as maximum velocity recorders [MVRs]) were built according to the design of Bell and Denny (1994). We used the MVRs to roughly calibrate the exposure scale generated by the water flow in the tank by comparing the recordings from the tank with those occurring naturally on exposed and sheltered shores. As the deflections on the MVR is just a measure of the maximum extension of a spring during a trial, we recalculated our MVR data to flow-speed (m s
2) using the equation:
E 10 þ 3:16 ; Flow speed ¼ 4:8 where E is the maximum extension of the spring in the MVR during a trial. This equation is from a study by Gamfeldt (unpublished) in which he dragged MVRs

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behind a boat and registered the speed of the boat with a GPS. Speed of the boat was then plotted against the maximum extension of the spring to which the above equation was fitted. Recordings in the tank were obtained by attaching two MVRs (replicates) to a sled with algae using the same procedure as in the experiment with snails. Every level of exposure was replicated two times to see if the water flows that were generated in the tank were stable. We analysed the differences between the three levels of exposure with a two-way ANOVA in which trial was a random factor nested under the fixed factor exposure. Prior to this we also checked that the choice of plants (exposed or sheltered) did not have any effect on the level of exposure. This test was conducted by dragging and releasing the sled only once at each trial. Every trial was, in turn, replicated three times for three levels of exposure. A mean from the two attached MVRs was used as the dependent variable in the following two-way ANOVA (factors plant, fixed, and exposure, random). Recordings in the field were made during a day with strong wind (‡10 m s
1) of a direction (SW) that generates maximum wave action in the study area. The MVR was left in the water fixed 90 towards the waves, at a depth of approximately 10 cm below the average water level, for 5 min at each time. Five recordings were made on sheltered shores and seven on moderately exposed shores and these were compared with a one-factor ANOVA. Statistics Prior to ANOVA, we checked all data for homogeneity of variances with Cochran’s test (Underwood 1997) and data-sets with heterogeneous variances were transformed. To increase the statistical power of the analysis, all factors were tested against pooled estimates of mean squares each time the original error terms proved to be highly insignificant (P>0.25; Underwood 1997). A posteriori tests (Student–Newman–Keuls) were performed when appropriate following Underwood (1997).

Table 1 Crab (Carcinus maenas) abundances and average sizes in wave-exposed and sheltered habitats Source of variation

Exposure (E) Island (I) E·I Site (I, E) Residual (R) Pooled (P)

df

1 1 1 4 16 20

Crab abundance

Crab size

MS

P

MS

P

459 805 1110 268

0.64 0.19 0.13 0.7 483 440

4.68 17.3 11.5 8.8

0.64 0.16 0.25 0.4 8.11 8.25

Error term

E·I P P R

Dependent: number of trapped crabs and carapace width

than 35, but crabs of this size could pass through the netting of the trap and the presence of small crabs could not be estimated. Mean crab carapace width for captured crabs ranged from 44.5 mm to 49.7 mm for the different sites, and mean numbers trapped varied between 19 and 57. After 20 h of trapping, no factor or interaction between factors, had significant effects on average captures of crabs or on average carapace widths (Table 1). That is, equally many crabs of about equal size distributions were trapped at moderately exposed and sheltered sites (Fig. 1). Tethering experiments For 25% of the snails that had been attacked shell fragments were still left attached to the tether, indicating a crushing predator (i.e., crabs). The remaining 75% were completely removed and tethers broken. It is most likely that also these snails were lost by predator attacks, as in our control cage excluding predators, only one snail (1.7%) was lost over 5 days of experiment. As argued earlier, crabs are the dominating snail predators, although we cannot exclude some impact by fishes and birds.

Results Crab predation Repaired shell injuries varied from 43% to 63% in the eight sampled sites, indicating a considerable effect of crab predation on different populations of L. fabalis. In addition, shell injuries were significantly higher on moderately exposed compared to sheltered shores (49% vs. 58%; F1,6=8.0; P=0.030) indicating a stronger impact of crab predation in moderately exposed shores. A total of 749 C. maenas were trapped. Of these, 2.9% (22) had carapaces that were 60 mm or wider, the largest measuring 68 mm. About 20% (146) of the crabs were smaller than 40 mm and only 2% (15) were smaller

Fig. 1 Mean number and size (carapace width) of captured crabs (C. maenas) from moderately exposed (e) and sheltered (s) sites (mean ± SE). No significant differences were found between habitats or between islands (Table 1)

407 Table 2 Differences in attack rates between snails tethered to algae and stone Source of variation

df

After 1 day MS

After 5 days MS

P

Error term P

Result of the first experiment in which we tethered LE-morph in exposed habitats and SS-morph in sheltered habitatsa Position (P) 1 0.15 0.023 0.09 0.14 Habitat (H) 1 0.51 <0.001 2.78 <0.001 P·H 1 0.3 0.002 0.09 0.14 Site (H) 4 0.03 0.4 0.02 0.77 P·Site (H) 4 0.01 0.84 0.01 0.89 Residual (R) 24 0.03 0.05 Pooled (Po) 32 0.03 0.04 Source of variation

df

After 2 days MS

After 5 days MS

P

Po Po Po R R

Error term P

b

Result of the second experiment in which we tethered both morphs in exposed habitats Morph, (M) 1 0.4 0.007 0.71 Position, (P) 1 0.08 0.24 0.15 M·P 1 0.08 0.24 0.01 Site (P, M) 8 0.05 0.6 0.06 Residual (R) 23 0.06 Pooled (Po) 31 0.06

0.001 0.11 0.68 0.36 0.05 0.06

Po Po Po R

Data analyzed with a three-factor ANOVA. Significant results are indicated in bold Dependent: proportion attacked b Dependent: proportion attacked2 a

In the first experiment, when the two morphs were tethered in their respective habitat, positioned in and beneath the plant canopy, site was highly insignificant as was the interaction between position and site (Table 2). Their mean squares were therefore pooled with the residual, and the remaining factors and interactions were tested against the pooled value. We found a significant interaction between position and habitat after 1 day of exposure to crab predation (Table 2). This was caused by SS-morph snails in sheltered habitats being more attacked on stones than in the canopy of algae, while LM-morphs in moderately exposed habitat were attacked at low rates in both positions (Fig. 2a). Thus, the canopy constitutes a refuge from crab attacks at least for SS-morphs in sheltered habitats, whereas LM-morphs seem to be resistant against crabs regardless of position. This suggests that for SS-morphs being dislodged by waves from the algal canopy mortality increases, while for the LM-morph in the moderately exposed habitats,

dislodgement does not significantly increase the risk of crab attacks. After 5 days, neither position nor the interaction between habitat and position was significant any more (Table 2). Instead, attack rate was substantially higher in sheltered (72%) as compared to exposed (17%) habitats (Table 2; Fig. 2a). This could perhaps reflect different vulnerability of the two morphs to crab attacks, but more likely it may have been a habitat-related effect. Even if crab densities and sizes were equal, opportunities for foraging may have been lower at moderately exposed sites owing to more pronounced wave-action. In the second tethering experiment, we separated the effect of morph from confounding effects of habitat by tethering both morphs in the same habitat (the moderately exposed habitat). As variances were heterogeneous (Cobs=0.461> Ccrit=0.392 and Cobs= 0.408> Ccrit=0.392 after 2 days and 5 days, respectively) we power-transformed the data and this reduced

Fig. 2 a Attack rates when LM- and SS-morphs of L. fabalis were tethered to their own habitats. The interaction between habitat and position was significant after 1 day of tethering but not after 5 days (Table 2). b Attack rates when LM- and SS-morph snails of L.

fabalis were tethered in moderately exposed habitats. LM-morphs were less attacked after both 2 days and 5 days of tethering (Table 2). Numbers above bars indicate how many days the snails were tethered. Bars indicate SE

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0.028x+0.45, r2=0.033, F1,6=0.21, P=0.67; b SS-morph:
0.024x+0.40, F1,6=0.18, P=0.69, r2=0.028; LM-morph:
0.0048x+0,079, r2=0.0035, F1,6=0.021, P=0.89; pooled:
0.072x+0.67, r2=0.35, F1,15=7.6, P=0.016)

Fig. 3 Proportion attacked snails as a function of snail size in the first (a) and second (b) tethering experiment. Each point represents a size-group consisting of eight to sixteen individuals (a SS-morph: y=
0.10x+1.4, F1,5=11, P=0.021, r2=0.69; LM-morph:

y= y= y= y=

the heterogeneity below the critical values (0.272 and 0.360, respectively). In this experiment, the effect of both site and the interaction between position and site were highly insignificant and we pooled mean squares of these factors as before (Table 2). The position of a snail (on stone or in algae) had no effect on the proportion of attacked snails after 2 days or 5 days and we found no significant interaction between morph and position (Table 2). After both 2 days and 5 days of tethering, however, a significant effect of morph was found, with the SS-morph being more frequently attacked than the LEmorph in both positions (Table 2, Fig. 2b). There was a large difference in attack rates between the first and the second experiment. Only 14% had been attacked in the second experiment compared to 56% in the first experiment. This was most likely an effect of weather conditions creating different water levels in this almost atidal area (maximum tidal range 0.3 m). During the first experiment, the fucoid belt was submerged most of the time as the daily mean water level was above normal during a period of southwest wind and low air pressure. During the second experiment, however, a high air-pressure and north to east winds resulted in an extremely low water level and the fucoids remained emerged most of the time of the experiment, resulting in fewer crab attacks, which most likely also resulted in a low power of the experiment. The probability of being attacked decreased with increasing size for snails of the SS-morph while not for the LM-morph in the first experiment (Fig. 3a). In the second experiment, none of the morphs indicated a sizeeffect when analysed separately, but after pooling data for the two morphs to get a wider size interval, we found a significant effect of size (Fig. 3b). This effect might, however, be confounded by differences between morphs other than size.

water flow (plant: F1,14=0.85, P=0.16 [pooled error term]; exposure*morph: F1,12=0.16, P=0.85) and thus an effect of snail morph would not be confounded by type of plant. The three different levels of exposure a, b and c that were used in the dislodgement experiment corresponded to a flow velocity of 0.90±0.06 m s
1, 1.26±0.07 m s
1 and 1.46±0.09 m s
1, respectively

Dislodgement of snails The choice of plant (moderately exposed or sheltered) in the dislodgement experiment had no significant effect on

Fig. 4 a Water flows in the tank used in the wave simulation experiment (open circles). Level ‘‘a’’ is the lowest flow and ‘‘c’’ is the highest. Filled circles represent values that were obtained from the field during a moderately windy day of SW winds. b Dislodgement of LM and SS-morph snails of L. fabalis from plants of F. vesiculosus at different water flows (see (a)). There was a significant interaction between morph and exposure (Table 3). Bars indicate SE

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(±95% C.I.; Fig. 4a). There was no difference in flow velocity between the two different trials (F2,6=0.47, P=0.71), thus water flows that were generated in the tank were consistent. The lowest level, a, was somewhat higher than the water flow recorded on the sheltered shore during a windy day (0.78 m s
1), whereas the average flow on the moderately exposed shore during the same day (1.64 m s
1) was higher than level c (Fig. 4a). That is, exposures generated in the tank were within the limits of the exposure on sheltered and moderately exposed L. fabalis habitats during a windy day. When we tested dislodgement rate of snails in different wave flows, time of experiment had no significant effect on dislodgement rate between morphs, but as expected, the overall rate of dislodgement increased with exposure though depending on an interaction between morph and exposure (Table 3; Fig. 4b). An SNK test revealed that more LM-morphs were dislodged at high exposure, c, compared to medium (b: Dobs=0.4> Dcrit=0.29) and low exposure (a: Dobs=0.35> Dcrit=0.24), whereas no difference was found between high (a) and medium (b) levels (Dobs=0
Discussion The algal-dwelling littorinid species, L. fabalis, shows a complicated pattern of size variation that deviates from what is general seen in direct developing littorinid snails, including the sister species L. obtusata. Reimchen (1982) Table 3 Differences in the degree of snail dislodgement in different experimental exposures (low, intermediate and high; see text for details) and for different morphs of Littorina fabalis (LM and SS) attached to plants of Fucus vesiculosus Source of variation

df

MS

P

Error term

Experiment (E) Morph (M) Exposure (Ex) E·M E·Ex M·Ex E·M·Ex Residual (R) Pooled (P)

1 1 2 1 2 2 2 12 16

0.007 0.06 0.152 0.107 0.012 0.105 0.012 0.03 0.025

0.65 0.59 0.011 0.057 0.69 0.037 0.69

R E·M P P R P R

The experiment was replicated two times (factor experiment). Data analyzed with a three-factor ANOVA. Significant results are indicated in bold, Dependent: proportion dislodged snails

argued that the small size and early maturation of snails in sheltered habitats is a result of a more intense selection from small crabs in sheltered compared to moderately exposed habitats, as adult snails have thicker shells than juveniles. The observation that snails of both habitat types are small in the White Sea where shore crabs are absent (M. Fokin, personal communication), suggests an alternative explanation. Nevertheless, similar sizes of snails in habitats of different exposure in the White Sea suggests that crab predation is a key factor contributing to the size polymorphism in populations of L. fabalis outside the White Sea. To explain the reversed size trend in L. fabalis populations of western Europe assuming that a small size is favored in the absence of crabs, we propose that if the canopy of fucoid algae constitutes a refuge from predatory crabs and the risk of dislodgement from the canopy increases with exposure, larger and less susceptible snails of L. fabalis would be favored by selection in moderately exposed habitats. Our model makes at least three explicit predictions: (1) the risk of crab attacks should be lower when snails are dwelling on fucoid algae compared to the substrate below the algae; (2) large snails (LM-morphs) should be more resistant to crab predation than small snails (SS-morphs) and (3) the risk of dislodgement from fucoid algae should be higher at moderately exposed habitats than sheltered habitats. In this study, we found support for all three predictions. Fewer SS-morphs were attacked when tethered to algal canopy than to stones in the first tethering experiment and the same tendency was present in the second experiment (Table 2, Fig. 2). In the second tethering experiment, where both morphs were tethered to moderately exposed habitats, LM-morphs were more resistant against crab attacks than SS-morphs (Table 2, Fig. 2b) and also the probability of crab attacks in the tethering experiments decreased with snail size when tested with linear regressions (Fig. 3). Finally we demonstrated that overall the risk of dislodgement increased with wave exposure and thus the refuge function of algae becomes less effective in moderately exposed compared to sheltered habitats (Table 3, Fig 4). Insofar our model is based on the assumption that a small size for this annual species is favored by selection in the absence of crab predation (as indicated by White Sea populations). This assumption is supported by the fact that a short life-span is also, in general, considered to select for early maturation (and small size) as this reduces the costs of prolonged growth and delayed reproduction (Blanckenhorn 2000). As L. fabalis reproduces more or less continuously from early spring throughout the summer (Reid 1996), and most individuals do not survive more than one reproductive season (data from western Europe; Williams 1992, 1996), the cost of delayed maturity may not be compensated by the higher fecundity gained at a larger size. In Sweden, juvenile snails of both morphs grow, on average, 2 mm per month, but at about 6-mm size, snails of SS-morph retard growth and become sexually mature, while snails

410

of LM-morph continue to grow until a size of about 10 mm (Tatarenkov and Johannesson 1998). This delays reproduction by about 2 months in the LM-morph, which probably is a severe cost in terms of lost reproduction. The rationale for L. fabalis being small when crab-predation is less intense seems thus indeed a consequence of early maturation optimizing the reproductive output in this short-lived species. Size, in direct developing intertidal snails, is generally dependent on the interaction of crab predation, selecting for larger size in sheltered and moderately exposed habitats, and wave-induced dislodgement, selecting for smaller size in exposed habitats. Size variation in L. fabalis is, however, a more complicated matter and we suggest that adult size in this species is dependent on the interaction of four factors: 1. Life history optimization selecting for early maturation and small size. 2. Crab predation, selecting for large size. 3. Fucoid algae functioning as refuges from crab predation. 4. Wave-induced dislodgement, reducing the effects of the refuge function in moderately exposed habitats. Support for the interplay of factors 2–4 is given in the present study and references herein, while an indirect supported for the first factor is obtained from size variation in a crab-free environment (the White Sea). Acknowledgements Financial support for this study was provided by the Swedish Research Council (grant to KJ). The experiments conducted in this study comply with the current laws of the country in which they were performed. We thank three anonymous referees for useful comments.

References Behrens Yamada S (1992) Niche relationships in northeastern Pacific littorines. In: Grahame J, Mill PJ, Reid DG (eds) Proceedings of the third international symposium on littorinid biology. The Malacological Society of London, London, pp 281–291 Behrens Yamada S, Boulding EG (1996) The role of highly mobile crab predators in the intertidal zonation of their gastropod prey. J Exp Mar Biol Ecol 204:59–83 Bell EC, Denny MW (1994) Quantifying ‘‘wave exposure’’: a simple device for recording maximum velocity and results of its use at several field studies. J Exp Mar Biol Ecol 181:9–29 Blanckenhorn WU (2000) The evolution of body size: what keeps organisms small?. Quart Rev Biol 75:385–407 Boulding EG, Van Alstyne KL (1993) Mechanisms of differential survival and growth of two species of Littorina on waveexposed and protected shores. J Exp Mar Biol Ecol 169:139– 166 Boulding EG, Holst M, Pilon V (1999) Changes in selection in gastropod shell size and thickness with wave-exposure in Northeastern Pacific shores. J Exp Mar Biol Ecol 232:217–239 Denny MW, Daniel TL, Koehl MAR (1985) Mechanical limits to size in wave-swept organisms. Ecol Monogr 55:69–102 Elner RW, Raffaelli DG (1980) Interactions between two marine snails Littorina rudis Maton and Littorina nigrolineata Grey, a predator Carcinus maenas (L) and a parasite, Microphallus similis Ja¨gerskiold. J Exp Mar Biol Ecol 43:151–160

Endler JA (1977) Geographic variation, speciation and clines. Princeton University Press, Princeton Etter RJ (1989) Life history variation in the intertidal snail Nucella lapillus across a wave-exposure gradient. Ecology 70:1857–1876 Filchak KE, Roethele JB, Feder JL (2000) Natural selection and sympatric divergence in the apple maggot Rhagoletis pomonella. Nature 407:739–742 Fretter V, Graham A (1980) The prosobranch molluscs of Britain and Denmark, part 5: marine Littorinacea. J Mollusc Stud Suppl 7:243–284 Goodwin BJ, Fish JD (1977) Inter- and intraspecific variation in Littorina obtusata and L. mariae (Gastropoda: Prosobranchia). J Mollusc Stud 42:241–254 Hedrick PW (1986) Genetic polymorphism in heterogeneous environments: a decade later. Annu Rev Ecol Syst 17:535–566 Hedrick PW, Ginevan ME, Ewing EP (1976) Genetic polymorphism in heterogeneous environments. Annu Rev Ecol Syst 7:1–32 Heller J (1976) The effects of exposure and predation on the shells of two British winkles. J Zool Lond 179:201–213 Hendry AP, Taylor EB, McPhail JD (2002) Adaptive divergence and the balance between selection and gene flow: Lake and stream stickleback in the misty system. Evolution 56:1199–1216 Hull SL (1998) Assortative mating between two distinct microallopatric populations of Littorina saxatilis (Olivi) on the northeast coast of England. Hydrobiologia 378:79–88 Janson K (1982) Genetic and environmental effects on the growth rate of Littorina saxatilis. Mar Biol 69:73–78 Johannesson B (1986) Shell morphology of Littorina saxatilis (Olivi): the relative importance of physical factors and predation. J Exp Mar Biol Ecol 102:183–195 Johannesson K (2003) Evolution in Littorina: ecology matters. J Sea Res 49:107–117 Johannesson K, Tatarenkov A (1997) Allozyme variation in a snail (Littorina saxatilis): deconfounding the effects of microhabitat and gene flow. Evolution 51:402–409 Johannesson K, Johannesson B, Rolan-Alvarez E (1993) Morphological-differentiation and genetic cohesiveness over a microenvironmental gradient in the marine snail, Littorina saxatilis. Evolution 47:1770–1787 Jones WE, Demetropoulos A (1968) Exposure to wave-action: measurements of an important ecological parameter on rocky shores on Anglesey. J Exp Mar Biol Ecol 2:46–63 Kirby RR, Bayne BL, Berry RJ (1994) Phenotypic variation along a cline in allozyme and karyotype frequencies, and its relationship with habitat, in the dog-whelk Nucella lapillus, L. Biol J Linnean Soc 53:255–275 Levins R (1968) Evolution in changing environments. Princeton University Press, Princeton Lewis JR (1972) The ecology of rocky shores. The English Universities Press, London Nevo E (1998) Molecular evolution and ecological stress at global, regional and local scales: The Israeli perspective. J Exp Zool 282:95–119 Norton TA, Hawkins SJ, Manley NL, Williams GA, Watson DC (1990) Scraping a living: a review of littorinid grazing. Hydrobiologia 193:117–138 Raffaelli DG, Hughes RN (1978) The effects of crevise size and availability on populations of Littorina rudis and Littorina neritoides. J Anim Ecol 47:71–83 Reid DG (1996) Systematics and evolution of Littorina. Ray Society, London Reimchen TE (1981) Microgeographical variation in Littorina mariae Sacci and Rastelli and a taxonomic consideration. Conchol Soc Great Britain Ireland 30:341–350 Reimchen TE (1982) Shell size divergence in Littorina mariae and L. obtusata and predation by crabs. Can J Zool 60:687–695 Rochette R, Dunmall K, Dill LM (2003) The effect of life-history variation on the population size structure of a rocky intertidal snail (Littorina sitkana). J Sea Res 49:119–132 Rola´n E, Templado J (1987) Consideraciones sobre el complejo Littorina obtusata-mariae (Mollusca, Gastropoda, Littorinidae) en el noroeste de la peninsula iberica. Thalassas 5:71–85

411 Schluter D (2000) The ecology of adaptive radiation. Oxford University Press, London Sundberg P (1988) Microgeographic variation in shell characters of Littorina saxatilis (Olivi): a question mainly of size. Biol J Linnean Soc 35:169–184 Tatarenkov A, Johannesson K (1994) Habitat related allozyme variation in a microgeographical scale in the marine snail Littorina mariae (Prosobranchia: Littorinacea). Biol J Linnean Soc 53:105–125 Tatarenkov A, Johannesson K (1998) Evidence of a reproductive barrier between two forms of marine periwinkle Littorina fabalis (Gastropoda). Biol J Linnean Soc 63:349–365 Tatarenkov A, Johannesson K (1999) Micro- and macrogeographical allozyme variation in Littorina fabalis; do sheltered and exposed forms hybridize?. Biol J Linnean Soc 67:199–212

Trussell GC, Johnson AS, Rudolph SG, Gilfillan ES (1993) Resistance to dislodgment: habitat and size-specific differences in morphology and tenacity in an intertidal snail. Mar EcolProg Series 100:135–144 Underwood AJ (1997) Experiments in ecology: their logical design and interpretation using analysis of variance. Cambridge University Press, Cambridge Underwood AJ, McFadyen KE (1983) Ecology of the intertidal snail Littorina acutispera Smith. J Exp Mar Biol Ecol 66:169–197 Williams GA (1992) The effects of predation on the life histories of Littorina obtusata and Littorina mariae. J Mar Biol Assoc UK 72:403–416 Williams GA (1996) Seasonal variation in a low shore Fucus serratus (Fucales, Phaeophyta) population and its epiphytic fauna. Hydrobiologia 326/327:191–197