Microhabitat segregation of the amphipod genus Gammarus (Crustacea: Amphipoda) in the Northern Baltic Sea

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1 Mar Biol (21) 157: DOI 1.17/s x ORIGINAL PAPER Microhabitat segregation of the amphipod genus Gammarus (Crustacea: Amphipoda) in the Northern Baltic Sea Samuli Korpinen Mats Westerbom Received: 7 April 29 / Accepted: 6 October 29 / Published online: 22 October 29 Springer-Verlag 29 Abstract Closely related species may occupy very similar niches but are often found to diverge by one or more traits when they share the same habitat. Five indigenous and sympatric Gammarus species are characteristic for the Baltic rocky littoral ecosystem. Yet, the species-speciwc distribution of these sympatric gammarids has not been well studied in the northern Baltic Sea. This study was undertaken to assess the spatial distribution of gammarid amphipods along wave exposure and depth gradients to study whether they show segregation in their microhabitat use. We sampled 12 rocky sublittoral shores along a wave exposure gradient over a period of 5 years. Samples divered with respect to depth and macroalgal type. Three of the Wve gammarid species occurred mainly in diverent depth zones and among diverent macroalgae at the exposed shores. In contrast, on protected shores, where algal zonation is weaker, a link to zonation and macroalgal type was almost absent. Moreover, the microhabitat use was strongest during the reproductive seasons of the species. The observed microhabitats of the three gammarid species Wt well to their species-speciwc mean body sizes. Communicated by F. Bulleri. S. Korpinen M. Westerbom Tvärminne Zoological Station, 19 Hanko, Finland S. Korpinen (&) Helsinki Commission, Katajanokanlaituri 6B, 16 Helsinki, Finland samuli.korpinen@helcom.fi M. Westerbom Metsähallitus, Natural Heritage Services, Raseborgsvägen 9, 16 Ekenäs, Finland Introduction Species exploiting similar resources and sharing the same habitats often show divergence in one or several traits. This resource partitioning may arise from an avoidance of interference or exploitative competition (MacArthur and Levins 1967; Schoener 1983; Ciros-Pérez et al. 21; Guisande et al. 23), adaptations to optimal refuges from predation (Edgar 1983; DuVy and Hay 1991; Sotka et al. 1999; Corona et al. 2) or to abiotic environmental conditions (e.g. Santelices et al. 1982). Regardless of the reason, resource partitioning alleviates the coexistence of closely related species, often increasing species richness and biomass in a given area. Common variables of partitioning include microhabitat segregation (Olyslager and Williams 1993; MacNeil et al. 1999b; Jones et al. 22), food specialization (Pacala and Roughgarden 1982; Nakano and Furukawa-Tanaka 1994) and species-speciwc ontogenetic life cycle characteristics (Pinkster and Broodbakker 198; Kolding 1981). Closely related species often show divergence along such variables (e.g. Kolding and Fenchel 1981; Pacala and Roughgarden 1982; Nakano and Furukawa- Tanaka 1994; Ciros-Pérez et al. 21). Marine organisms in the littoral zone have often been shown to segregate to narrow depth zones (Connell 1961; Lewis 1964; Lubchenco 198) or to Wnd their niche within certain macroalgal species (Norderhaug 24). Although zonation is often attributed to sessile organisms, also mobile invertebrate fauna, such as amphipods and periwinkles, diverge in their depth distribution (Ingólfsson 1977; Williams 1995; Chavanich and Wilson 2). Gammarid amphipods are very common and important species in the rocky sublittoral of the northern Baltic Sea (Orav-Kotta et al. 29). Despite their very small interspeciwc diverences, that enable crossbreeding, Wve indigenous

2 362 Mar Biol (21) 157: Gammarus species share the same shores (Bulnheim and Scholl 198; Kolding and Simonsen 1983). Gammarids have been found to diverge along salinity gradients (Fenchel and Kolding 1979) and reproductive periods (Kolding and Fenchel 1981). There are also observations of divergence in their depth distribution (Kolding 1981; Jazdzewski and Fronc 1982). Since interspeciwc matings among these species are fairly common in the laboratory, it has been suggested that avoidance of interspeciwc competition for mates is the mechanism driving the species to habitat segregation and life cycle diverences (Kolding 1986). We tested whether Wve northern Baltic gammarid species show microhabitat divergence, manifested as an association to diverent shore exposures, depth zones or an association to diverent macroalgal functional forms. Apart from interspeciwc competition, the distribution of gammarid species may also be explained by other factors, e.g. predation or wave shock where large individuals may Wnd refuge among robust macroalgae (Sotka et al. 1999; Norderhaug 24; Sotka 27). We compared the distribution of diverent sized individuals within macroalgal functional forms along a wave exposure gradient. We tested the following: (1) Densities of Gammarus species diver among macroalgal types, (2) densities of Gammarus species diver along the depth gradient and (3) body size diverences of gammarids result in diverent densities between algal types. These hypotheses were examined by using several years data from 12 shores, distributed along a wave exposure gradient. It is notable that biodiversity studies carried out in the northern Baltic Sea commonly recognize the Gammarus group only to the genus level, and few attempts exist where the species-speciwc distribution of all sympatric gammarids are studied. Given the changes in the Baltic Sea ecosystem, and introduction of new species (especially Gammarus tigrinus) to the ecosystem (Packalén et al. 28; Orav- Kotta et al. 29), a solid understanding of the behaviour of the Wve indigenous gammarid species is urgently needed. Materials and methods Study sites and sampling period The study was undertaken during at 12 islands in the western Gulf of Finland, northern Baltic Sea (Fig. 1). Samples were taken always at the same points on the islands, which are hereafter called shores. Because shore exposure to water motion is one of the most important abiotic factors avecting littoral communities, the evective fetch index was used to estimate shore speciwc wave exposure (Håkansson 1981). The index value consists of 15 cartographic measurements of distance (χ i ) and constant angles (γ i ) between the measurement lines, which are, 6, Fig. 1 Map of the study area. Arrows indicate the sampling sites. Site names with shore exposure index values (L f ): 1. Brännskär lagoon (L f = ), 2. Joskär (L f = 1), 3. Sundholmen (L f = 2), 4. Brännskär E (L f = 8), 5. Östra Kvarnskär (L f = 12), 6. Granbusken (L f = 12), 7. Långskär (L f = 33), 8. Långskär E 9 (L f = 33), 9. Spikarna (L f =35), 1. Västerbådan 9 (L f > 35), 11. Sankorna 9 (L f > 35), 12. Fårgrundet 9 (L f = 11). Shores denoted with 9 included mussel and red alga sampling only 12, 18, 24, 3, 36 and 42. χ i is the distance in kilometres to a shore or skerry along the line. The central axis ( ) is directed towards the strongest wind direction or the direction which gives the highest value. The index value is calculated using the formula: L f =(Σχ i cos γ i )/(Σ cos γ i ). A complete description of the method is disclosed in Håkansson (1981), and a Weld test of the method is presented in Ruuskanen et al. (1999). Samples were collected from diverent depths and algal morphological types (altogether 98 samples, including over 1, individuals that were distinguished to species): Wlamentous algae were mainly Pylaiella littoralis (L.) Kjellman and Cladophora glomerata (L.) Kützing, large fucoid alga Fucus vesiculosus L. and red turf-forming algae [mainly Ceramium tenuicorne Kützing (Waern) and Polysiphonia fucoides (Hudson) Greville]. The northern Baltic phytobenthic communities consist only of a dozen macroalgal species (Korpinen et al. 27). In addition to these samples, we also collected samples at 6 7 m depth, either in patches consisting solely of blue mussels (Mytilus trossulus Gould, n = 22) or red algae (mostly Ceramium tenuicorne,

3 Mar Biol (21) 157: Polysiphonia fucoides and Furcellaria lumbicalis (Hudson) J. V. Lamouroux, n = 22) growing between the mussels. The sampling was done quantitatively using SCUBA diving and scraping the algae, Wlaments and mussels to mesh-bag frames of three sizes (areas: 18, 4 and 2, cm 2 ). The largest mesh frame was used exclusively for samples from F. vesiculosus to ensure that also large F. vesiculosus specimens could be included without losing gammarids. Samples from blue mussel beds were taken with the 4 cm 2 mesh frame while the 18 cm 2 sampler was used for the sampling of small F. vesiculosus and Wlamentous and red algae. A comparison of samples showed that the use of diverent sized mesh frames resulted in similar densities of gammarids. Immediately after collection, the samples were sorted and amphipods were stored in 4% formalin or alcohol. The Gammarus species were identiwed, their maturity and sex was determined (Lincoln 1979) and the body length was measured (from the tip of telson to the base of antennas) under a dissecting microscope (up to 5 magniwcation). Mature individuals were dewned as ones which had a developed brood pouch (females), visible calceoli (males) or whose body length was over 1 mm (G. oceanicus, G. locusta, G. duebeni) or 6 mm (G. salinus, G. zaddachi). These are average values for our study region and they are based on hundreds of males and females in precopula pairs. A small amount of older juveniles were identiwed by species according to Rygg (1974). Data analyses The data were classiwed into three categories of wave exposure (L f = 2 [sheltered, n =18], L f = 8 12 [moderately exposed, n =33], L f > 3 [exposed, n = 47]) (Fig. 1), four depth zones (<1 m [n = 5], 1 2 m [n = 13], 2 4 m [n = 1] and >4 m [n = 25]), three macroalgal types (F. vesiculosus [n =37], Wlamentous algae [n = 35], and red algal turfs [n = 24]) and four seasons (spring [n = 28], summer [n =4], autumn [n = 26], winter [n = 4]). Seasons were dewned by calendar weeks: winter (1 9), spring (1 22), summer (23 35) and autumn (36 52). The number of samples in years 1998, 2, 22, 23 and 24 were 19, 34, 24, 16 and 5, respectively. The signiwcance of depth, algal types, season and sampling year on the density of Gammarus species was assessed by a forward selection regression analysis (REG procedure in SAS statistical package 9.1). The three algal types were included in the analyses as dummy variables and the exact sampling depths, season and year as continuous variables. The analysis was done separately for the three exposure classes. The densities of the three species were transformed to logarithmic scale, whereas the density of unidentiwed juveniles was analysed untransformed. In a more detailed analysis of the evects of depth and algal type on the gammarid densities, also shore exposure was included in the model. The combinations of the factors algae and depth do not form a complete factorial model, because red algae did not occur in the shallowest depths or on the sheltered shores and F. vesiculosus did not occur at the deepest depth. Therefore, the depth and algal type were analysed in separate models with the shore exposure factor. However, the resulting model was not fully factorial, because the sheltered shores included only two of the four depth categories (<1 and 1 <2 m) and two of the three algal types (F. vesiculosus and Wlamentous algae). Moreover, the statistical model of densities of the species among the shores of diverent exposure to waves, depth zones/types of macroalgae was unbalanced. We ended up with a generalized linear mixed model (GLMM) (GLIMMIX macro in the SAS statistical package 9.1) with negative binomial distribution of data and each site as a random factor in the model. This model Wtted best with the data as seen in the normality of residuals, ratio of deviance and degrees of freedom, and when comparing interactions and main factors to observed data and its variance (Langsrud 23; Statsoft Inc. 28; Buckel 28). GLMMs have been found to evectively take into account unbalanced sample sizes across several sampling sites and produce safer estimates for the real diverences among tested factors than the general linear model (GLM) (Brandão et al. 24). The same model was used also to test the diverences in gammarid densities in the three seasons, spring, summer and autumn, by including either depth or algal type into the model. In this case, the model was run for exposed shores only, due to the insuycient number of samples for a season-wise statistical testing at sheltered and intermediate shores. Winter was not included in this model because of ice and diycult weather conditions. Only sexually mature individuals were included in this model. The Gammarus individuals, both juveniles and mature, were further divided into three size classes: <9 mm (small), 9 15 mm (mid-sized) and >15 mm (large). In this data set, we used a small number of juveniles which were identiwed to species (see previous section). A generalized linear model (SAS GENMOD procedure, SAS statistical package 9.1) was used to test whether densities of the three size classes divered among the macroalgal morphological types and whether this depended on species or the three shore exposure categories. Pairwise comparisons were done by Wald s a posteriori test. Results Densities of Gammarus species At the twelve shores examined, Wve Gammarus species were found, although three of them (Gammarus zaddachi

4 364 Mar Biol (21) 157: Sexton, Gammarus oceanicus Segerstråle, and Gammarus salinus Spooner) dominated. Gammarus locusta L. occurred only sporadically and Gammarus duebeni Liljeborg were restricted to rock pools above the water surface. They were found in samples only during very high water levels. This study concentrates on G. zaddachi, G. oceanicus, and G. salinus and excludes G. locusta due to its sparse occurrence and G. duebeni due to its obvious habitat segregation (Segerstråle 1969). The three species occurred among diverent algal morphological types and in diverent depth zones. Mature individuals of G. zaddachi were common among F. vesiculosus in sheltered locations (Fig. 2a; Table 1). On moderately exposed shores, they were more abundant among Wlamentous algae and F. vesiculosus than among red algae (Fig. 2a, b; Table 1; pairwise comparisons: P <.5). On exposed shores, G. zaddachi was abundant on all algal types (Fig. 2c), and no seasonal evects on algal type were seen (Season Algae: F 3, 33 =1.1, P >.1). G. zaddachi was prevalent close to the surface; at less than 1 m depth on exposed shores and less than 2 m depth on moderately exposed shores (Fig. 3b, c; Table 1). No clear depth zonation was found on the sheltered shores (Fig. 3a). On exposed shores, near-surface occurrence was found only in spring and autumn, while in summer there was no diverence among the depth zones (Season Depth: F 6, 29 =7.4, P <.1). According to the regression analysis on data from the exposed shores and the studied variables, shallow depth alone explained the density of this species (Table 2). On all the shores, mature individuals of G. oceanicus were most dense among F. vesiculosus (Fig. 2; Table 1), which forms a distinctive belt with understory and epiphytic algae at the intermediate depth zone (1 4 m). This association to F. vesiculosus did not vary among seasons on the exposed shores (Season Algae: F 3, 46 = 1.7, P >.1). G. oceanicus occurred predominantly at the 1 4 m depth range and had very low densities in the shallowest and deepest depth zones (Fig. 3; Table 1). When analysed separately for the exposed shores, no seasonal pattern was found (Season Depth: F 6, 42 =.3, P >.1). In the regression analysis, the density of G. oceanicus was dependent on F. vesiculosus, whereas depth had no evect (Table 2). G. salinus dominated the deepest habitat where it was very abundant within red algae and blue mussels. On the exposed shores, the species lived mostly within red algae or F. vesiculosus (Fig. 2c; Table 1) and at depths exceeding 4m (Fig.3c; Table 1). G. salinus had stronger association to red algae in the spring than in the summer or autumn (Season Algae: F 3, 33 =4.2, P <.5). The species depth distribution on the exposed shores did not vary signiwcantly among the seasons, but a slightly stronger, statistically nonsigniwcant, zonation was found during the spring and autumn (Season Depth: F 6, 29 =1.3, P >.1). According A Density (ind. m -2 ) B Density (ind. m -2 ) C Density (ind. m -2 ) G.zaddachi G.oceanicus G.salinus SHELTERED Filamentous MODERATELY EXPOSED Fucus Red algae Filamentous Fucus EXPOSED Red algae Filamentous Fucus Fig. 2 Densities of Gammarus zaddachi, G. oceanicus and G. salinus among Fucus vesiculosus, Wlamentous algae and red algal turfs on (a) sheltered, (b) moderately exposed and (c) exposed shores over all seasons. Values are least square means ( SE) of the model. Juvenile individuals were omitted from the data. Note the diverent scales on the vertical axes to the regression analysis, the highest densities of the species were found at deeper depths (Table 2). Within mussel beds at 6 7 m depth, G. salinus was almost the sole

5 Mar Biol (21) 157: Table 1 Statistical analyses by generalized mixed models of the densities of three Gammarus species and their juveniles in sheltered, moderately exposed and exposed shores (A) within three types of macroalgae (Fucus vesiculosus, Wlamentous algae and red algae) and (B) along a depth gradient ndf, ddf G. zaddachi G. oceanicus G. salinus Juveniles F P F P F P F P A. Algae Exposure 2, Algae 2, < < Exp. Alg. 3, < Shore 1 <.1 <.1 <.1 <.1 B. Depth Exposure 2, Depth 3, Exp. Dep. 4, Shore 1 <.1 N.S. N.S. <.1 The signiwcances of the random factor Shore are based on χ 2 statistics. The densities are presented in Figs. 2 and 3 gammarid species; only some rare individuals of Calliopius laevisculus Krøyer and Gammarus oceanicus were found (Fig. 4). The densities of G. salinus did not diver between Mytilus trossulus or red algal patches within the mussel beds (χ 2 = 3., df =3, P >.1). On moderately exposed or sheltered shores, where red algae only occurred sparsely or not at all, G. salinus was abundant in the Gammarus assemblage within stands of F. vesiculosus (Fig. 2a, b). Juvenile Gammarus individuals (unidentiwed to species) living within Wlamentous and red algae were up to 1 times more abundant on exposed shores than on moderately exposed and sheltered shores (Exposure Algae in Table 1). On exposed shores, juvenile density was on average highest within Wlamentous algae (4,288 ind. m 2 ) and lower within red algae (1,15 ind. m 2 ) and F. vesiculosus (142 ind. m 2 ). On the moderately exposed shores, the densities were lower, particularly within red algae, and the highest density was found within Wlamentous algae. Also, the regression analysis indicated that Wlamentous algae host the highest density of juveniles (Table 2). Juveniles had higher densities at less than 4 m depth than deeper down (depth in Table 1). Body lengths within macroalgal types G. oceanicus was clearly the largest gammarid species in body length (ANOVA: divergence between mature males: F 2, 74 = 99, P <.1; mature females: F 2, 67 =217, P <.1). The mean length of a mature G. oceanicus male was 22. SE.6 mm and for mature female, it was mm. G. zaddachi and G. salinus were of similar length: males 11.8 SE.5 and 12.9 SE.6, respectively, and females 1. SE.3 and 1.5 SE.3, respectively. All the species were larger on sheltered shores (ANOVA: males: F 2, 74 = 8.5, P <.1; females: F 2, 67 = 22, P <.1) than on moderate and exposed shores. Densities of diverent sized amphipods, both juvenile and mature individuals, among the algal types were assessed for the sheltered, moderately exposed and exposed shores (Fig. 5). According to the analysis, small-sized gammarids were denser among Wlamentous algae or red algal turfs than among F. vesiculosus (Algae Size: χ 2 =3.1, df =4, P <.1; Fig. 5). The densities of the size groups divered also among the three shore exposure classes (Exposure Algae Size: χ 2 =12.1, df =6, P =.6; Fig. 5). On sheltered shores, small G. salinus and G. zaddachi were more abundant among Wlamentous algae than F. vesiculosus (Wald s test: P <.1 for both), whereas densities of small G. oceanicus individuals did not diver among the two algal types (Fig. 5a c). Densities of midsized and large gammarids did not diver among the algal types. On moderately exposed shores, small and mid-sized G. salinus and G. zaddachi were denser among Wlamentous algae than among F. vesiculosus or red algal turfs (Wald s tests: P <.1 for small and mid-sized G. zaddach and small G. salinus and P <.1 for mid-sized G. salinus; Fig. 5d f). Densities of small and mid-sized G. oceanicus did not diver among the algal types, whereas large G. oceanicus were signiwcantly denser among F. vesiculosus than Wlamentous algae (P <.5) or red algae (P <.1) (Fig. 5d). Large G. salinus and G. zaddachi showed no diverence in their densities among the algal types (Fig. 5e, f). On exposed shores, densities of small gammarids, independent on species, were higher among Wlamentous algae or red algal turfs than F. vesiculosus (Wald s tests: P <.5, <.1 and P <.1 for G. oceanicus, G. salinus and G. zaddachi, respectively). The density of mid-sized

6 366 Mar Biol (21) 157: A <1m 1- <2m B <1m 1- <2m 2- <4m C >4m <1m 1- <2m 2- <4m >4m Fig. 3 Densities of Gammarus zaddachi, G. oceanicus and G. salinus along the depth gradient on (a) sheltered, (b) moderately exposed and (c) exposed shores over all seasons. Values are least square means ( SE) of the model. Juvenile individuals were omitted from the data. Note the diverent scales on the horizontal axes G. oceanicus did not diver among the algal types, whereas G. salinus and G. zaddachi were densest among Wlamentous algae (Wald s tests: P <.1 for both compared to F. vesiculosus; Fig. 5h, i). Large G. salinus were most abundant among red algae (P <.5 compared to F. vesiculosus; Fig. 5h), but the densities of the large individuals of the other two species did not diver among the algal types (Fig. 5g, i). Discussion -2 Density (ind. m ) G.zaddachi G.oceanicus G.salinus In line with our Wrst and second hypotheses, our results suggest that on wave-exposed shores, the Gammarus species occur in species-speciwc microhabitats set by the depth gradient and the zonation of macroalgae and blue mussels. Gammarid zonation has been found previously in the southern and central Baltic Sea (Kolding 1981; Jazdzewski and Fronc 1982), in the eastern coast of North America (Stephenson and Stephenson 1972; Chavanich and Wilson 2) and in Iceland (Ingólfsson 1977). Common for all these studies, and our results, is the overlap in the zonation or microhabitat use between at least two species. In this study, we show the densities of Gammarus species pooled across all the seasons, which has led to high variation in the densities (Figs. 2, 3, 5). Despite this variation, the diverences between microhabitats are statistically signiwcant, which strengthens our conclusions. Many of the previous studies have not compared the microhabitat segregation by several variables, such as algal morphological types, mussels and shore exposure. We found that the association of gammarid species to depth and macroalgal type is species speciwc. G. oceanicus was more associated to F. vesiculosus than to depth and the species had a wide depth range on the shore, rexecting zonation of its host alga. In contrast, G. zaddachi had an apparent depth zonation; the species was very dense in less than 1 m water depth, but very few in numbers in deeper water. G. salinus was clearly densest at deeper depths, where there was no diverence between their density among blue mussels or red algae. The microhabitat divergence with the two sibling species G. zaddachi and G. salinus (sensu Bulnheim and Scholl 198) was not maintained over all seasons. The conwnement to microhabitats was strongest in spring and autumn, while during the summer, their densities were more similar among algal types or depth zones. Moreover, the seasonwise microhabitat use was clearer between depth zones for G. zaddachi than algal types, which strengthens our previous conclusions of the characteristics of the G. zaddachi and G. salinus microhabitats. Kolding and Fenchel (1981) hypothesized that the Gammarus species avoid interspeciwc competition for mates by displaced breeding time. Kolding (1981) reiterated the hypothesis by suggesting that breeding time is also linked to vertical migrations on the shore. He concluded that the ecologically selective forces must be particularly strong for G. zaddachi and G. salinus which have highly overlapping breeding times in the Baltic Sea. Both species have two generations per year; egg-carrying females are found in the central and northern Baltic Sea in March June and August October (Kolding 1981; S. Korpinen, unpublished). Thus, the microhabitat divergence in spring and autumn, but not in the summer, which we found in this study, Wts well with the previous hypothesis (Kolding and Fenchel 1981; Kolding 1981). Our study is the Wrst to statistically show this detailed microhabitat pattern. However, an ultimate explanation for such seasonal diverences may be the wide summer distribution of the brown alga Pylaiella littoralis, which is a known food source for gammarids and a common epiphyte on F. vesiculosus (Orav-Kotta et al. 29). Its dominance during June July in

7 Mar Biol (21) 157: Table 2 Regression analyses on the evect of year, season, depth, F. vesiculosus, Wlamentous algae and red algal turfs on the density of the three species of Gammarus and their juveniles on the exposed shores Exposed shores G. zaddachi G. oceanicus G. salinus Juveniles Model F 3, 42 =4.1** F 5, 4 = 17.7*** F 4, 41 = 13.2*** F 3, 42 = 1.9*** Total R Year.61**.34*.19 NS 691*** Season.49 NS 1.27** 1.28*** 589 NS Depth.48*.6*** F. vesiculosus.77 NS 4.9*** 1.22* Filamentous algae 2,656** Red algal turfs In addition to F value, probability and R 2 value of the overall model, each parameter is given with its slope and probability. Parameters which did not have explanatory value (P >.9) were not included in the model. Asterisks denote signiwcance levels: (P <.1), *(P <.5), ** (P <.1), *** (P <.1), NS denotes P >.1-2 Density (ind. m ) G.salinus G.oceanicus G.zaddachi C.laevisculus Blue mussel Red algae Fig. 4 Densities (mean SE) of the gammarid amphipods Gammarus salinus, G. oceanicus, G. zaddachi, and Calliopius laevisculus within mussel beds at 6 7 m depth in patches dominated either by blue mussels (Mytilus trossulus) or by red algae (mainly Ceramium tenuicorne, Polysiphonia nigrescens and Furcellaria lumbricalis) all the depth zones may break down the gammarid microhabitat segregation. During the absence of this alga, the gammarid zonation was particularly distinctive. Such a break down of amphipod alga association has been shown to happen in times of high epiphytic biomass (Johnson and Scheibling 1987). G. zaddachi and G salinus have only a small diverence in body size on exposed shores and, as shown in this study, both species live abundantly among Wlamentous and Wnely branching algae independent of body size. Of the two species, G. zaddachi occurred in shallow water, mainly among dense bed-forming Wlamentous algae, and G. salinus lived in deep water where Wnely branching red algae or mussel beds predominate. The small interstitial spaces between the blue mussels, which grow very small in this area (Westerbom et al. 22), may over a suitable refuge only for a small species. Based on these arguments, it seems likely that the sibling species G. zaddachi and G. salinus have partitioned the shore vertically in order to avoid resource competition or interspeciwc pairings (Kolding 1986), but both species have also adapted to living in microhabitats which Wt well to their body size. This is supported by the result that the densities of these species followed primarily the depth gradient and only secondarily microhabitat structure. Although we did not test for interspeciwc competition, such competitive interactions have been shown to be prevalent in gammarid assemblages (Fenchel and Kolding 1979; Dick and Elwood 1992; MacNeil et al. 1999b). For example, the isolation of G. duebeni to rock pools in the Baltic Sea has been suggested to result from severe competition with other Gammarus species (Segerstråle 1969). Previous studies on gammarids algal host preferences have explained a threefold mechanism in host selection: to live among one s main food source (Poore 24), to live in a refuge from predation (Sotka et al. 1999; Norderhaug 24; hereafter predation hypothesis ), or to seek for shelter from dislodging by wave shock (Sotka 27; hereafter wave shock hypothesis ). Sometimes these three strategies are combined (Edgar 1983; DuVy and Hay 1991; Sotka 27). However, because the Gammarus species are known omnivores, the food hypothesis cannot alone determine their microhabitat selection. Hacker and Steneck (199) discuss reasons for diverential microhabitat selection and emphasize the roles of interstitial space and branch width in determining the size spectrum of amphipods living within an alga (see also Gee and Warvick 1994). They found that unbranched and branched Wlamentous algae attract smallsized amphipods and corticated macroalgae attract largesized amphipods. In this study, we found support for this result and for our third hypothesis; small-sized amphipods were most abundant among Wlamentous algae and red algal turfs. Since there were no or only weak species-speciwc

8 368 Mar Biol (21) 157: G. oceanicus G. salinus G. zaddachi A B C 2 Fucus Filamentous Red turfs SHELTERED Density (ind. m ) D E G H I F EXPOSED MODERATELY EXPOSED Fig. 5 Densities of small (<9 mm), mid-sized (9 15 mm) and large (>15 mm) Gammarus amphipods (juveniles and adults) among F. vesiculosus, Wlamentous algae and red algal turfs on sheltered (a c), moderately exposed (d f) and exposed (g i) shores over all seasons. Values are least square means ( SE) of the model. Note the diverent scales on the vertical axes diverences in this association, the reason for this microhabitat selection of small individuals is not necessarily interspeciwc competition. Because the separate analysis on (unidentiwed) juvenile Gammarus species showed that in general juveniles had higher densities than adults within Wlamentous algae and red algal turfs on exposed shores compared to the bushes of F. vesiculosus, the size-associated assortation may be limited to juveniles only and not to small-sized mature individuals. Such an assortative association has previously been shown for gammarids in Wlamentous alga Cladophora glomerata compared to F. vesiculosus at moderately exposed shores in the northern Baltic Sea (Kraufvelin and Salovius 24). On the other hand, according to the predation and wave shock hypotheses, large individuals should avoid Wlamentous or Wnely branching algae and choose F. vesiculosus. In line with this, mid-sized and large G. oceanicus, the largest of the species, were found tightly associated with F. vesiculosus, the only fucoid alga in the study area, which overs a wide array of diverent sized refuges. However, large individuals of G. zaddachi and G. salinus were not denser among F. vesiculosus than among Wlamentous algae or red algal turfs, which is in contrast to the expectations of our third hypothesis. Thus, it seems likely that although juvenile Gammarus have adapted to Wnd shelter from predation or wave shock, the algal associations of mature Gammarus amphipods in the northern Baltic Sea are not primarily determined by refuge from predation or wave force, but are species speciwc and may thus rexect also other pressures, such as interspeciwc competition.

9 Mar Biol (21) 157: The densities of Gammarus species along the depth gradient and among algal types divered clearly on the shores exposed to strong wave forces, but on the sheltered shores, where mussel beds were absent (c.f. Westerbom and Jattu 26) and algae grew only close to the water surface, the three species co-existed within F. vesiculosus and Wlamentous algae. Such a co-existence may rexect reduced interspeciwc competition. Low interspeciwc competition has been commonly attributed to strong predation pressure (Menge and Sutherland 1976, 1987), which has been predicted to be stronger in sheltered localities than on exposed shores, both by theoretical and empirical work (Robles and Robb 1993; Nielsen 21). The main predators of gammarids are Decapod shrimps, several bird species and numerous Wsh species (MacNeil et al. 1999a). Moreover, on sheltered shores, the body sizes of G. zaddachi and, in particular, G. salinus were signiwcantly larger than on exposed shores. The greater adult body sizes of the two sibling species in sheltered localities may also point to high predation pressure on smaller individuals. Thus, there is a signiwcant diverence in microhabitat use between the two shore types almost isolation between the two species on exposed shores and co-existence on sheltered shores which may indicate character displacement between the two species triggered by interspeciwc competition. However, determining such an ecological adaptation should be based on further experimental evidence. Acknowledgments The authors are grateful to the stav of the Tvärminne Zoological Station for their kind help during the Weld season and to Walter and Andrée de Nottbeck Foundation for Wnancial support. Special thanks are given to Dr. Patrik Kraufvelin for a critical review. We appreciate the constructive comments from Dr Fabio Bulleri and three anonymous referees that helped us to clarify the manuscript. 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