Hydrobiologia 514: 7985, 2004.H. Kautsky & P. Snoeijs (eds), Biology of the Baltic Sea. 2004 Kluwer Academic Publishers. Printed in the Netherlands.
Food and habitat choice of the isopod Idotea baltica in thenortheastern Baltic Sea
Helen Orav-Kotta & Jonne KottaEstonian Marine Institute, University of Tartu, Marja 4d, 10617 Tallinn, EstoniaE-mail: firstname.lastname@example.org (Author for correspondence)
Key words: Baltic Sea, habitat choice, food choice, experiment, Fucus vesiculosus, Idotea baltica, Pilayellalittoralis
The isopod Idotea baltica is the most important benthic herbivore in the Baltic Sea. There exists a significantcorrelation between the distribution of the adult isopod and the belts of bladder wrack Fucus vesiculosus. However,following the eutrophication induced blooms of the filamentous macroalga Pilayella littoralis and the disappear-ance of F. vesiculosus a notable increase in idoteid abundances has been observed. The aim of this paper was(1) to evaluate experimentally whether F. vesiculosus provides either food, shelter or both to the isopods and(2) to estimate the role of associated filamentous algae in the habitat selection process. Amongst six abundantmacroalgal species, about 50% of isopod population was attracted to F. vesiculosus covered with the filamentousalgae P. littoralis. The majority of the remaining part of the population was either swimming freely or attractedto non-epiphytic P. littoralis. When both live algae and artificial substrata were provided, P. littoralis growing onartificial substrata was clearly preferred by the isopods over epiphyte-free F. vesiculosus. In the grazing experimentwhere I. baltica was allowed to choose between F. vesiculosus and P. littoralis the latter contributed practically100% of the diet of the isopod. The results indicate the importance of P. littoralis as a food item and F. vesiculosusas a shelter for I. baltica.
The euryhaline Idotea baltica (Pallas) is originally alittoral and sublittoral crustacean of tidal shores witha characteristic pattern of distribution correlated withtidal level, exposure and salinity variations of thehabitat. It has a wide, almost cosmopolitan distribu-tion, and its geographical races have been described(Naylor, 1955; Sywula, 1964). The species has alsopenetrated into the brackish waters of the Baltic Sea(Salemaa, 1978, 1979) where it is ranked among themost important necto-benthic herbivores (Schaffelkeet al., 1995).
Habitat choice of animals is determined by severalfactors such as food, shelter and access to mates. Ve-getation is crucial for herbivores both as a food sourceand as protection from predation (Puttman, 1986). Inthe Baltic Sea, I. baltica is common in the Fucus vesi-culosus L. and Cladophora glomerata (L.) Ktz. belts
(Jansson, 1974; Haahtela, 1984), but it also occurs onZostera marina L. dominated soft bottoms (Salemaa,1978; Bostrm & Bonsdorff, 1997). I. baltica is omni-vorous, eating benthic microalgae, filamentous algae,macroalgae, detritus, small invertebrates and even itsconspecifics (Naylor, 1955; Ravanko, 1969; Sywula,1964; Nicotri, 1980; Robertson & Mann, 1980; Franke& Janke, 1998). Despite its omnivory I. baltica isquite selective within food categories (Salemaa, 1978;Schaffelke et al., 1995; Jormalainen et al., 2001). Itsdietary choice involves the selection between differ-ent species (Schaffelke et al., 1995; Schramm et al.,1996), but likely between different parts of algal thalli(Salemaa, 1987). However, in the northern Baltic Sea,F. vesiculosus is considered as the main source offood for I. baltica (Salemaa, 1987; Jormalainen et al.,2001).
Following coastal eutrophication, mass develop-ments of filamentous algal species have been observed
in the Baltic Sea during the last decades (Bonsdorffet al., 1997; Bck et al., 2000; Norkko et al., 2000;Vahteri et al., 2000; Lehvo & Bck, 2001). Thereexists circumstantial evidence that after the extensivegrowth of P. littoralis and the disappearance of F. vesi-culosus the density of idoteids has increased hundredsof times (Kotta et al., 2000). This suggests that otheralgae than F. vesiculosus would be the primary diet forthese isopods. The literature does not provide a def-inite answer if I. baltica will select F. vesiculosus forshelter or for food, because experiments consideringthe microhabitat choice of idoteids usually involvedonly a single macroalgal species (e.g. Salemaa, 1987;Merilaita & Jormalainen, 2000). Therefore, the aimof the present study was to show the importance ofsubstrate structure and food quality of different mac-roalgal species on the habitat selection of I. baltica.We hypothesized that when the density of filament-ous algae is low, i.e. in low eutrophicated condi-tions, F. vesiculosus provides both food and shelter forI. baltica. However, under eutrophicated conditions,filamentous algae are expected to be the prime foodfor I. baltica, and the occurrence of filamentous algaewill significantly modify the isopods habitat choice. Ineutrophicated conditions, filamentous algae may indir-ectly offer shelter for idoteids as their predators seemto avoid areas of with mass blooms of annual algae(Salemaa 1979, 1987).
Material and methods
Sampling of I. baltica was performed in Kiguste Bay,northeastern Baltic Sea in June 2000. Adult I. balt-ica (mean SE: 17 1 mm) were collected from ashallow hard bottom area, at 13 m of depth, withinF. vesiculosus fronds by shaking the algae.
Six different macroalgal species were used in theexperiments: the brown algae F. vesiculosus, Chordafilum (L.) Stackh. and Pilayella littoralis (L.) Kjellm.,the red alga Ceramium tenuicorne (Ktz.) Waern andthe green algae C. glomerata and Enteromorpha in-testinalis (L.) Nees. F. vesiculosus has a bush-likestructure, C. filum consists of coarse filaments and theother four species are fine filamentous algae. E. intest-inalis and C. glomerata are known to cause extensivemacroalgal blooms in shallow water and P. littoralisin the sublittoral (Bonsdorff et al., 1997; Bck et al.,2000; Norkko et al., 2000; Vahteri et al., 2000; Le-hvo & Bck, 2001). Macroalgae were collected at thesame site as the isopods. The lower and upper parts
of F. vesiculosus thallus differ in terms of their nu-tritional and protective qualities (Tuomi et al., 1989;Carlson, 1991). Therefore, we used only those speci-mens of F. vesiculosus, which had about equal sharesof younger apical and older basal thallus parts.
Four types of artificial substrata made of polypro-pylene, simulating plants and algae, were used: F. vesi-culosus, Furcellaria lumbricalis (Huds.) J.V. Lam-our., Z. marina and filamentous alga. The artificialF. lumbricalis was more fragile and its texture wasfiner compared with artificial F. vesiculosus. Z. mar-ina had narrow non-branching blades and filamentousalga represented the type of C. glomerata and P. lit-toralis. These artificial substrata represented the pre-valent morphological forms of plants and algae thatI. baltica may encounter in the study area.
The habitat choice of I. baltica was studied infive different experiments. In the first experiment weused F. vesiculosus, C. filum, C. glomerata, C. tenu-icorne, E. intestinalis and P. littoralis. Prior to theexperiment, the epiphytes growing on F. vesiculosuswere removed and some fronds of F. vesiculosus wereartificially covered with the epiphyte P. littoralis. Inthe second experiment the attractiveness of live andartificial F. vesiculosus was compared to artificialF. vesiculosus covered with epiphytes. If I. balticawould select live F. vesiculosus, this would indic-ate its importance in the diet of the isopod. On theother hand, if there was no significant difference inhabitat choice of I. baltica between live and artificialF. vesiculosus, then the alga would serve as a shelterfor I. baltica. This experiment also tests if epiphyticP. littoralis and C. glomerata will increase the attract-iveness of artificial F. vesiculosus as a substrate. Thethird experiment was designed to study the habitat se-lection of I. baltica in less eutrophicated conditions.To simulate this, epiphytic P. littoralis was removedfrom F. vesiculosus. Live and artificial F. vesiculosusand artificial F. vesiculosus covered with the epiphyteC. glomerata were provided to I. baltica. In the fourthexperiment, epiphyte-free live F. vesiculosus, artificialF. lumbricalis and artificial F. lumbricalis covered withepiphytic C. glomerata and P. littoralis were providedto I. baltica. F. lumbricalis is known to be an alternat-ive substrate for I. baltica in the case of scarcity ofF. vesiculosus (Kotta et al., 2000). This experimenttests the attractiveness of live F. vesiculosus in rela-tion to another algal morphology. This experiment alsoshows whether the occurrence of epiphytes will affectthe habitat selection of I. baltica. The fifth experimentincluded four types of artificial algae and live F. vesi-
culosus. The aim of the experiment was to demonstratethe role of algal morphology on the habitat selectionof I. baltica. This experiment also tests whether liveF. vesiculosus is favoured over artificial algae.
In each of the five experiments, the habitat choiceof I. baltica was studied in nine replicate 100 l aquariawith a light regime similar to the field conditions.The aquaria received running seawater at a flow rateof 3 l h1. The water was taken from 2 m depthnearby Kiguste Marine Biological Laboratory. Ineach aquarium macroalgae and ten I. baltica speci-mens were placed. The algae were attached to theaquarium floor by pebbles. The coverage of algae andthe area without vegetation in the aquaria was 40%and 60%, respectively. The number of I. baltica ondifferent macroalgae and those swimming freely wererecorded every hour for 24 h. During dark periodsartificial red light was used during the observations.All water was exchanged and new testorganisms werecollected prior to each experiment. The survival ofI. baltica on average ( SE) 95% 3%. I. balticashowed no significant differences in habitat preferencebetween light and dark conditions and 24 h meanswere used in the statistical analyses of habitat choice.
A food preference experiment was performed withP. littoralis and F. vesiculosus, the two algal speciesthat were preferred as habitat by I. baltica. Freshlycollected P. littoralis (mean SE: 0.53 0.09 gdry weight) and F. vesiculosus (0.77 0.06 g dryweight) were placed into three replicate 100 l aquaria.Three control aquaria contained the same algae butno isopods. Prior to the experiment, the algae wereblotted dry and weighted to obtain the wet weight.Ten I. baltica specimens were added to each aquarium.The experiment was run for 48 hours after which wetand dry weights of the algae was determined. For dryweight, the algae were dried at 60 C for 48 hours. Thefeeding rate of the isopods was estimated in terms ofalgal dry weight changes. These values were correc-ted for the weight increment due to the photosyntheticactivity of the algae in the controls.
Analysis of variance (ANOVA) was performed toinvestigate the habitat choice of the isopods. The datawere transformed to obtain a normal distribution whennecessary. Prior to the analysis, Bartletts test was usedto check the assumption of homoscedasticity (Sokal &Rohlf, 1981).
Figure 1. Habitat choice of Idotea baltica, expressed as the meanpercentage ( SE) of the population associated with live Chordafilum, Fucus vesiculosus, Pilayella littoralis attached to F. vesi-culosus, Cladophora glomerata, Ceramium tenuicorne, Entero-morpha intestinalis, Pilayella littoralis or free-swimming.
Figure 2. Habitat choice of Idotea baltica, expressed as the meanpercentage ( SE) of the population associated with artificial Fucusvesiculosus, live Cladophora glomerata attached to artificial F. vesi-culosus, live Pilayella littoralis attached to artificial F. vesiculosus,live F. vesiculosus or free-swimming.
In the first habitat choice experiment including allmacroalgal species, I. baltica preferred F. vesiculosuscovered with P. littoralis as an epiphyte (32%) ornon-epiphytic P. littoralis (15%). On average 33%of I. baltica were swimming freely and 20% werefound on other algae of which 7% on the epiphyte-freeF. vesiculosus (ANOVA, p < 0.001; Fig. 1).
In the second experiment, in which I. balticawere given a choice of live and artificial epiphyte-free F. vesiculosus, the artificial alga covered eitherby C. glomerata or P. littoralis, I. baltica significantlypreferred the artificial F. vesiculosus with attachedP. littoralis (51%) over the live F. vesiculosus (7%)(ANOVA, p < 0.001). There was no significant dif-
Figure 3. Habitat choice of Idotea baltica, expressed as the meanpercentage ( SE) of the population associated with artificialFucus vesiculosus, live Cladophora glomerata attached to artificialF. vesiculosus, live F. vesiculosus or free-swimming.
ference in the isopod density between epiphyte-freelive and artificial F. vesiculosus (ANOVA, p > 0.05;Fig. 2).
The third experiment was the same as the second,but the artificial alga covered with the epiphyte P. lit-toralis was removed. As a consequence, half of theI. baltica were swimming freely (50%). Similar num-bers were observed on live and artificial F. vesiculosus(18% and 23%, respectively), and the lowest numberson artificial F. vesiculosus covered with C. glomerata(10%) (ANOVA, p < 0.01; Fig. 3).
In the fourth experiment, a design similar to thesecond experiment was used. The artificial F. vesicu-losus was replaced by artificial Furcellaria lumbricaliswith a more fragile and finer texture. The habitatchoice of I. baltica resembled the result of the secondexperiment, but the artificial F. lumbricalis was muchmore attractive to I. baltica than the live F. vesicu-losus. Similar to the previous experiments, I. balticawere mostly attracted to P. littoralis, now attached toartificial F. lumbricalis (52%) (ANOVA, p < 0.001;Fig. 4).
In the last experiment of habitat choice, all livemacroalgae were removed except for F. vesiculosus.The majority of I. baltica were either swimming (43%)or stayed on the artificial F. lumbricalis (38%) (AN-OVA, p < 0.001). There was no significant differencein the habitat preference between the live and artificialF. vesiculosus (10% and 6%, respectively; Fig. 5).
In the feeding experiment, I. baltica practicallyonly consumed P. littoralis. The grazing rate of I. balt-ica was on average ( SE) 0.332 0.077 g dry weightalga per gram I. baltica per day. The average daily pro-duction of P. littoralis was 2.0% 0.1% based on dry
Figure 4. Habitat choice of Idotea baltica, expressed as the meanpercentage ( SE) of the population associated with artificial Fur-cellaria lumbricalis, live Cladophora glomerata attached to arti-ficial F. lumbricalis, live Pilayella littoralis attached to artificialF. lumbricalis, live Fucus vesiculosus or free-swimming.
Figure 5. Habitat choice of Idotea baltica, expressed as the meanpercentage ( SE) of the population associated with live Fucus vesi-culosus, artificial Furcellaria lumbricalis, artificial Zostera marina,artificial filamentous alga or free-swimming.
weight. Taking into account the initial weight of thealga and the density of isopods, the grazing pressureof I. baltica on P. littoralis was 400% of the algal pro-duction. Also the net production rate of F. vesiculosusexceeded its consumption rate by the isopods. The av-erage daily production of F. vesiculosus was 0.6% 0.1% based on dry weight.
We found that Idotea baltica preferred Pilayella lit-toralis and Fucus vesiculosus as habitat over the otherstudied algal species. The robust, flat thalli of fucoidsprovided the isopod mainly shelter, but was of littlevalue as food. The epiphytic macroalga P. littoralis
significantly added to the habitat quality of F. vesicu-losus, indicating that P. littoralis was more attractivefor I. baltica site than F. vesiculosus. This was alsosupported by the feeding choice experiment, whichshowed that P. littoralis constituted an essential partof the diet of the isopod I. baltica.
Previous studies have suggested that the habitatchoice of isopods is mainly a function of algal mor-phology (e.g. Nicotri, 1980; Hacker & Madin, 1991).Despite a relatively low food value, I. baltica is at-tracted to large, tough, branched algae to which thedorsiventrally flattened animal body is well adap-ted. However, recent studies have demonstrated thatgrazers prefer to feed on filamentous algae (Shacklock& Doyle, 1983; DAntonio, 1985; Worm & Som-mer, 2000). Hence epiphytic food resources should bethe prime factor to determine the presence of idoteidsin macrovegetation (Bostrm & Mattila, 1999; Paviaet al., 1999). These findings correspond to field ob-servations of positive relationships between epiphyteload and grazer density (Kotta et al., 2000; Worm &Sommer, 2000).
It is likely that the increasing epiphyte load in re-cent decades has triggered a substantial shift in thehabitat selection of I. baltica. In less eutrophicatedconditions, with only few epiphytes present, I. balticaseems to select canopy-forming macroalga to increaseprotection from predators (Stoner, 1980; Main, 1987).With an increasing epiphyte load, I. baltica is attrac-ted to fully overgrown macrovegetation (Kotta et al.,2000; Worm & Sommer, 2000), but also to free-floating macroalgal mats consisting of the ephemeralalgae (Norkko et al., 2000). This selection may be ad-vantageous to isopods as fish seem to avoid biotopesdominated by ephemeral algae (Salemaa, 1979, 1987).Following the bloom of ephemeral algae the density ofI. baltica has notably increased (Salemaa, 1979; Kan-gas et al., 1982; Arrontes, 1990; Duffy, 1990; Kottaet al., 2000). Consequently, isopods may escape pred-ation pressure, which would explain their occurrencein more exposed microhabitat.
In the present study we found that I. baltica washighly selective in its food choice of epiphytic mac-roalgae. Compared to P. littoralis, C. glomerata signi-ficantly reduced the habitat value for I. baltica. LiveC. glomerata has thick cell walls and is likely to beresistant to herbivory (Birch et al., 1983; Gabrielsonet al., 1983; Paalme et al., 2002). Furthermore, adultsI. baltica did not seem to be well adapted to crawlalong C. glomerata; in some cases they were observedto become trapped into the filaments and died there.
During the experiments, P. littoralis was in an earlystage of decomposition in the study area during theexperiment (Paalme et al., 2002). The decomposing al-gae probably had less resistant cell walls and high con-centrations of nitrogen and phosphorus, which makesthe alga more attractive to mesoherbivores (Mann,1988). In the light of high selectivity of I. baltica,the generally found poor correlation between epiphyteload and isopod density (Nicotri, 1980; Pavia et al.,1999) might be explained by an unfavourable spe-cies composition of the epiphytes. Seasonal changesin epiphyte species composition may further mask arelationship.
In the habitat choice experiment it was found thatI. baltica preferred less robust algae than F. vesicu-losus. Following the mass bloom of P. littoralis andthe decline of F. vesiculosus in the northern Baltic Sea(Haahtela, 1981; Kangas et al., 1982; Salemaa, 1987),the majority of I. baltica switched into an alternat-ive substrate, i.e. Furcellaria lumbricalis (Kotta et al.,2000). As compared to F. vesiculosus, this alga has afiner thallus. However, under less eutrophicated con-ditions F. lumbricalis has a low coverage of epiphytesand food availability for I. baltica is low
The conclusion of this study is that under more eu-trophicated conditions the habitat choice of I. balticais mainly due to the attractiveness of the algae as afood. It is likely that when the coverage of epiphytesis low (little eutrophication), and the active search forfood considerably increases the risk to be eaten bypredators, the habitat choice of isopod is more determ-ined by the structural characteristics of the substrateoffering the best protection against the predator. If epi-phytes are effectively removed in eutrophicated condi-tions, I. baltica may have a positive effect on perennialmacroalgae (Brawley & Adey, 1981; Robertson & Lu-cas, 1983; Brawley & Fei, 1987; Williams & Seed,1992; Jernakoff & Nielsen, 1996). Thus, isopods actas stabilisers of the macroalgal communities by com-pensating nutrient effects on ephemeral algal growth(Neckles et al., 1993; Williams & Ruckelshaus, 1993;Jernakoff et al., 1996).
This research was supported by the Estonian Gov-ernmental Programme (grants nos 0200792s98 and0182578s03) and the Estonian Science Foundation(grant no 5103).
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