A morphometric approach to the geographic variation of the terrestrial isopod species Armadillo tuberculatus (Isopoda: Oniscidea)

""The terrestrial isopod species Armadillo tuberculatus Vogl, 1876 (Crustacea, Isopoda, Oniscidea) is a widely polymorphic species distributed in the south-central Aegean region (Greece) with a different morph on each island. Variation
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  Section of Animal Biology, Department of Biology, University of Patras, Patra, Greece A morphometric approach to the geographic variation of the terrestrial isopod species Armadillo tuberculatus  (Isopoda: Oniscidea) M.  Kamilari  and S.  Sfenthourakis Abstract The terrestrial isopod species  Armadillo tuberculatus  Vogl, 1876 (Crustacea, Isopoda, Oniscidea) is a widely polymorphic species distributed in thesouth-central Aegean region (Greece) with a different morph on each island. Variation consists in coloration, size of cuticular tubercules, shape of telson and the shape of the male first pleopod exopodite (secondary sexual character of taxonomic importance). We studied the allometric growthof a cuticular tubercule in 17 populations (for both male and female individuals) and the shape variation of the first male pleopod exopodite in 10populations using Elliptic Fourier Analysis, in order to test for patterns of intraspecific variation and possible relationships between morphs. Inaddition, Thin Plate Spline analysis was used for the calculation of the minimum bending energy between different exopodite shapes, which wasthen used for estimating the minimum spanning network (MSN) connecting them. The different allometric growth rates of the tubercule amongisland groups were significantly related to island latitude and climatic factors. On the other hand, the clustering of islands and the MSN based onmale exopodite shape differences were not related to the palaeogeography of the Aegean region or to the present geographic distances of islands.These results are interpreted as evidence for non-adaptive radiation of the morphs. Key words:  Morphometry – Elliptic Fourier Analysis – intraspecific variation – Aegean palaeogeography – allometric growth – Thin Plate Spline – minimum bending energy Introduction Taxonomic classification of organisms and understanding thediversity of life were both based till recently on descriptions of morphological forms (Adams et al. 2004). Despite the infor-mation offered by modern molecular analyses, morphologystill plays a crucial role in the evaluation of patterns of diversification. Besides qualitative characters, important evi-dence resides often in the shape of morphological characters.Several morphometric methods have been developed thatprovide useful tools in the attempt to describe and compareshapes of whole organisms or parts of them. These methodshave the potential to resolve variation at many levels, as theycan be applied to samples representing whole species, localgeographic populations, developmental stages, genetic and   ⁄   orenvironmental effects, even individuals (Rohlf and Marcus1993).The shape of a biological character is often rather complexand this can lead to loss of crucial information if simple linearmorphometric data are used. The development of newmorphometric approaches over the last 20 years take underconsideration the whole geometry of the studied structure, andprovide more accurate descriptions of morphological forms.Two main approaches are available in relation to the nature of the descriptors: landmark approaches and outline approaches.Landmarks are specific points of the biological structure thatare supposed to be equivalent or homologous between eachspecimen. Thus, shape variation among samples can beassessed from the differences in the geometry of landmarkconfigurations. Even though many aspects of shape can becharacterized by a small number of landmark points, theremay be additional information about the form in betweenlandmarks that is not incorporated into landmark analyses(McLellan and Endler 1998).With outline approaches, the shape is reduced to the outlineof the structure that is defined by a set of constructed pointslocated on the outline. Outlines are then fitted by mathematicalfunctions and the parameters of these functions can beemployed to compare individual shapes (Rohlf 1990). EllipticFourier Analysis (EFA) applies to closed contours in a two-dimensional plane. It consists of decomposing a curve into asum of harmonically related ellipses. EFA has been effectivelyapplied to the analysis of various biological shapes in animals(Rohlf and Archie 1984; Ferson et al. 1985; Liu et al. 1996;Laurie et al. 1997; Garnier et al. 2005) and plants (Iwata et al.2002; Yoshioka et al. 2004; Camargo Neto et al. 2006). Theadvantages of outline analysis by no means diminish the meritsof landmark approaches, collectively termed geometric mor-phometrics, that have proved to be powerful tools forcomparing shapes at different taxonomic levels, including theintraspecific level, suggesting that such methods could beparticularly useful for detecting and quantifying hiddenmorphological polymorphism (e.g. Corti et al. 2001; Montiet al. 2001; Bertin et al. 2002). In fact, the two approachescould be used complementarily, taking advantage of thevariety of tools offered by each one in order to maximizeinformation gained on shape variation. In this work, we use acombination of these approaches in a study of the geograph-ical variation of a terrestrial isopod.The terrestrial isopod species  Armadillo tuberculatus  Vogl,1876 (Crustacea, Isopoda, Oniscidea) is a widely polymorphicspecies distributed in the south-central Aegean region(Fig. S1), an area of great topographical and palaeogeograph-ical complexity and environmental heterogeneity (Sfenthoura-kis 1996). A relatively large number of studies have beenconducted on several aspects of the biogeography and differ-entiation of species within this insular area (e.g. Sfenthourakis1996; Welter-Schultes and Williams 1999; Dennis et al. 2000;Fattorini 2002; Hausdorf and Hennig 2005; Triantis et al.2008; Trichas et al. 2008) which is an important naturallaboratory for research on evolution, historical and ecologicalbiogeography. The palaeogeography of the region is quitewell known, with a sequence of repeated fragmentation and   2009 Blackwell Verlag GmbHAccepted on 15 July 2008J Zool Syst Evol Res doi: 10.1111/j.1439-0469.2008.00510.x J Zool Syst Evol Res (2009)  47 (3), 219–226  re-connections of islands, starting from the middle Mioceneand continued till the glacial periods of the Pleistocene (seeFig. S2). The current topography, with a large number of islands lying in between two mainland source areas, provides avaluable template for investigating several aspects of bioticprocesses, with geographic variation playing an important rolein the patterns observed among a variety of taxa (e.g. Battin1992; Simaiakis and Mylonas 2006; Douris et al. 2007).Terrestrial isopods are organisms with a low potential forover-sea dispersal, present in almost all Aegean habitat typesand represented therein by a relatively large number of species(Sfenthourakis 1996). As a consequence, they can serve asmodels for several kinds of studies regarding insular biogeog-raphy and evolution. In fact, the terrestrial isopod fauna of central and southern Aegean is well known and a variety of analyses have been conducted using this group as studymaterial (Sfenthourakis 1991, 1996; Sfenthourakis et al. 1999,2004; Hausdorf and Hennig 2005).As far as  A. tuberculatus  is concerned, every island or smallgroup of islands within its distributional range hosts a differentmorphotype (Sfenthourakis 1991; Schmalfuss 1996). Thevariation is exhibited more apparently in coloration, size of cuticular tubercules, shape of telson in both males and femalesand shape of adult male first pleopod exopodite. The lattercharacter is a secondary sexual character usually ascribed withtaxonomic importance in terrestrial isopods. Until now, thiswide polymorphism has been described mostly in qualitativeterms. In this paper, we use a quantitative approach in order toobtain a more objective description of certain aspects of thismorphological diversity and to detect possible geographicpatterns of variation that could provide evidence of the taxon  sradiation. We focus on the two major aspects of variation inthis species, namely the allometric growth of cuticular tuber-cules for both male and females, and the shape of the firstpleopod exopodite of males. Our aim was to investigatewhether variation in these two characters can be adequatelyassessed by using morphometric methods and if the patternsexhibited by the two characters are congruent.As Sfenthourakis (1991) has tentatively shown, the tuber-cules of the cuticle exhibit allometric growth whose rate variesamong populations. We use a traditional morphometricapproach for the study of allometry in the tubercules amongstthe populations from 17 islands. We investigate also shapevariation in the adult male first pleopod exopodite through theuse of EFA. In addition, we use Geometric Morphometrics(thin plate spline, TPS analysis) in order to calculate theminimum bending energy between the different exopoditeshapes, which is then used for the estimation of the minimumspanning network (MSN) connecting them. The latter isevaluated for a possible geographical structure that couldrelate to the established palaegeography of the Aegean regionor to other known environmental factors.In addition to linear measurements for the study of allometric growth, the combination of two morphometricapproaches for shape analysis (EFA and geometric morpho-metrics) and the use of an MSN based on the minimumbending energies among shape configurations provide a novelmethodological framework that can have a wide use in thestudy of geographical variation in morphological characters. Materials and Methods A total of 578 (170 males and 408 females) individuals of  A. tuberculatus  were collected from 17 islands (Table S1) of the centraland southern Aegean region (see map in Fig. S1). Sampling wasperformed by hand and specimens were preserved in 95% ethanolsolution. All individuals (578) were used in the analysis of allometry,whereas only adult male specimens (114) were used in the shapeanalysis of the first pleopod exopodite, in order to make comparisonsbetween fully formed structures.Linear measurements of the frontal line width that stands as a proxymeasurement of animal size were obtained from each individual. Theheight of one prominent tubercule on the lateral part of the seventhpereionite of each individual was measured under a stereoscope(Fig. 1). All measurements were obtained in micrometric units at thesame magnification. The srcinal values were log 10  transformed and asimple regression analysis was conducted for each population. Theslopes of regression lines, giving the allometric growth rate, between allpairs of populations were compared using the respective test in Zar(1984:292–295).For comparative purposes, the allometric growth of tubercules wasfurther studied using also the normalization technique suggested byLleonart et al. (2000) that removes the allometric effects of body sizefrom the analysis. According to these authors, this normalizationprocedure completely removes all the information related to size, notonly scaling all individuals to the same size, but also adjusting theirshape to that they would have in the new size. This procedure was (a) (b) Fig. 1. An adult (a) and a young(b) individual from Anafi island.Measurements of frontal width andtubercule height are depicted 220  Kamilari  and  Sfenthourakis J Zool Syst Evol Res (2009)  47 (3), 219–226   2009 Blackwell Verlag GmbH  recorded as one of the most efficient methods in allometric studies(Reist 1985). With this procedure we obtained a transformed theoret-ical value ( Y    i  ) of the tubercule height ( Y  ) for each individual using theequation Y    i  ¼ Y   i  X  m  X  i   b where  Y    i  is the theoretical value of   Y  ,  X  i  ,  Y  i   are the pair of measurement values made on the individual,  X  m  is the mean size of thepopulation, and  b  is a constant, estimated as the slope of the standardregression line of each population.This way, any particular observed point ( X  i  ,  Y  i  ) is converted to atheoretical point ( X  m ,  Y    i  ) and all prior observations are normalized toa unique  X  m . The normalized  Y   values of each population  s individualswere subjected to  anova , as residuals were homoscedastic.We applied nonparametric Spearman  s rank correlation coefficientin order to test the relation of   Y  * with the latitude of islands. A linearregression was also applied on these two variables in order to check fora linear trend in the variation of   Y  * with latitude. We usednonparametric Spearman  s rank correlation coefficient testing therelation of   Y  * with temperature, annual precipitation and monthlyaverage precipitation ( worldclim  database, Hijmans et al. 2005).The shape of adult male first pleopode exopodite is rather simple(see Fig. 2) with very few distinguished homologous points that couldbe used as landmarks. We were able to locate only landmarks of type 2(maxima of curvature) and type 3 (extrema points), according to theterminology of Bookstein (1991). Therefore, we preferred to assess theshape differences of exopodites using outline analysis (Bookstein1997). Exopodites were carefully removed from each male individual.Outlines studied correspond to the two-dimensional projection of theleft ventral view of the character (Fig. 2). Shapes of exopodites fromKasos and Karpathos islands were very similar; so, these two islandswere considered as one population referred as KAP (see Fig. 2). Adigital camera was applied to a binocular stereoscope and images of exopodites were obtained. Outlines were automatically extracted fromthe digital image using conventional image processing software. Thepart of the exopodite that bears the pleopodal lung was removed whenthe outline was drawn as shape variation is exhibited only at the apicalpart of the pleopod exopodite. The images were then processed usingthe  shape  software (Iwata and Ukai 2002). This program calculates thenormalized elliptic Fourier descriptors (EFDs), in accordance with theprocedures suggested by Kuhl and Giardina (1982). The program usesa particularly simple way of obtaining Fourier coefficients of a chain-encoded contour as well as bounds on the error of such a represen-tation, and also an intuitive way of normalizing the Fourier coefficientsusing a harmonic, elliptic description of the contour. The resultingFourier descriptors are invariant with rotation, dilation andtranslation of the contour, and also with the starting point on thecontour, but do not lose information about shape. The normalizationslead to the degeneration of the first, second and third coefficient of thefirst harmonic (Kuhl and Giardina 1982). Thus, for  N   harmonics, thereare 4 N   )  3 non-trivial normalized coefficients (Rohlf and Archie 1984;Ferson et al. 1985; Tatsuta et al. 2001, 2004). The coefficients of thelower order Fourier harmonics correspond to the overall shape and thehigher order harmonics correspond to smaller details of the outline.The overall outline was correctly reconstructed with 20 harmonics.In any case, the same results, with only minor numerical differences inscore values, were obtained by the use of more harmonics, but giventhat harmonics of higher rank describe small details of outlines and aregenerally subject to high measurement error there is no point toinclude more variables that provide no relevant information. Inaddition, as our total sample includes 114 individuals from 10populations, the use of a larger number of shape variables would leadto flaws in the statistical analyses. Overall difference in exopodite shapewas assessed using General Discriminant Analysis (using lineardiscriminant functions) based on the Fourier coefficients. Each set of 77 coefficients was treated as a multivariate point representing theoutline of the character. We used the Mahalanobis generalized distance( D 2 ) as input to cluster analysis (UPGMA) in order to group thevarious island populations according to their differences in exopoditeshape.Next, we used  shape  to calculate the mean shape configuration of exopodites of each population with the 20 harmonics, on which atotal of 40 semilandmarks (Bookstein 1997) equally spaced wereposted along the contour using T ps D ig  software (v. 1.37; F.J. Rohlf,NY State University). Superimposition and size adjustment wasperformed by means of IMP C oord G en6  software (H.D. Sheets,Department of Physics Canisius College, NY, USA,  sheets/morphsoft.html/). In order to evaluate the mini-mum bending energy between the different exopodite shapes, we usedthe thin plate spline algorithm (T ps S plin  software, v. 1.20; F.J. Rohlf,NY State University). Cited software are available at TPS functions are used in order tofit the differences in the position of landmarks in one characterrelative to their position in another, and the effect is visualized as acontinuous deformation (Bookstein 1989). Minimum bending energyyields the strength of the deformation. After TPS analysis between allpairs of populations, we selected the minimum values of the minimumbending energy for each pair forming a triangular matrix. Weprocessed this matrix with  minspnet  software, provided by LaurentExcoffier in the website:, for the estimation of the MSN connecting the populations. Theaffine and non-affine transformations between mean shape configura-tions were plotted on the MSN. In order to compare results from thetwo methods (EFA and geometric morphometrics), we performedFig. 2. Mean shape configurationof male pleopod exopodites I fromthe 10 populations under analysis(with pleopodal lung removed – see text), as produced by  shape software Morphometrics in an isopod species 221J Zool Syst Evol Res (2009)  47 (3), 219–226   2009 Blackwell Verlag GmbH  another cluster analysis of the 10 populations, this time usingProcrustes distances and Procrustes angles obtained by using TPSanalysis.We used Mantel test to compare the matrix with the minimumcurrent geographical straight distances among all pairs of islands withthe Mahalanobis generalized distances and minimum bending energiesamong their populations. In addition, the same test was used for acomparison between the matrices of palaeogeographical inter-islandsdistances (Tortonian–Messinian, Pliocene and the last Pleistocene searegression, i.e. 18 000 Ma) and the matrices of Mahalanobis general-ized distances and minimum bending energies.Finally, we used Spearman nonparametric rank correlation coeffi-cient in order to test the relation of the shape of the character withannual precipitation, monthly average precipitation, temperature(WORLDCLIM database, Hijmans et al. 2005) and with the latitudeof islands. For this analysis, we used PCA factor scores (factor 1–4)from a PCA analysis conducted on the 77 Fourier coefficients. Each setof 77 coefficients was treated as a multivariate point representing theoutline of the character. Results Allometry of tubercules Simple regression of the log 10 -transformed values of tuber-cule height on body size for all individuals pooled de-monstrates a significant allometric relationship betweenthe tubercule height and body size ( r  = 0.18,  n  = 578,p < 0.0001). The majority of populations (11) show eitherpositive ( b  > 1) or negative ( b  < 1) allometry, whereas forCrete the slope has a negative value ( b  < 0), meaning that inthis population the absolute value of the tubercule  s visibleheight decreases with animal size (Table S2). Six populationshave a statistically non-significant or marginally significantrelationship. Comparison of regression lines of the 11populations with a significant relationship shows severalsignificant differences of allometric growth among popula-tions (Table S3).The  anova  of normalized  Y   values ( Y  *) of the tuberculeheight among all 17 populations, with Bonferroni correction atthe 95% confidence level, indicates a high between-groupheterogeneity ( F   = 87, d.f. = 577, p < 0.0001). Other meth-ods tested (e.g.Tukey  s, least significant differences (LSD)) givethe same results. Nine homogenous groups are formed(Table S4). Spearman nonparametric rank correlation coeffi-cient revealed correlations with both annual precipitation( r an-pr  =  ) 0.599, p < 0.0001) and monthly average precipita-tion (r m-pr  =  ) 0.598, p < 0.0001) but not with temperature( r T   =  ) 0.75, p > 0.05). It is noteworthy that the  Y  * values of thecuticulartubercules  heightincreasesalongasouth-to-northlatitudinalgradient,asshownbySpearmannonparametricrankcorrelationcoefficient( r lat  = 0.47,p < 0.0001).Asimplelinearregressionof   Y  *on latitudealso shows astatistically significantrelationship between the variables at the 99% confidence level( r  = 0.47, p < 0.001, see Fig. 3). Variation in the shape of male exopodite I A discriminant analysis of the 77 elliptic Fourier coefficientsfor each first male exopodite results in a high discriminationbetween the 10 populations used (Wilks   k  = 0.00, v 2 = 1185.598, p < 0.0001). The first six canonical axes arestatistically significant, with the first two axes explainingalmost half (49%) of the variance (Fig. 4). All individualstested are classified correctly in their respective populations(correct classification = 100% for all populations).Cluster analysis (Fig. 5) based on Mahalanobis distances(Table S5), suggests that the 10 populations form three majorgroups; however, each group hosts populations that areneither geographically nor palaeogeographically related (seeDiscussion). It has to be noted that the same clusters areformed when using the Procrustes distances or Procrustesangles returned from the geometric morphometric (TPS)analysis. Furthermore, the MSN (Fig. 6) based on the mini-mum bending energy for the first male pleopod exopodite alsoreveals a pattern that is consistent neither with the establishedpalaeogeography of the region (see Fig. S2) nor with thecurrent geographical placement of islands. Mantel tests forcomparisons of the matrix of inter-island current geographicaldistances (cgd) with those of Mahalanobis distances ( D 2 ) andminimum bending energies (mbe) among their populations areboth non-significant ( D 2 :  Z  cgd  =  ) 0.087,  t cgd  =  ) 0.551,p > 0.05; mbe:  Z  cgd  =  ) 0.269,  t cgd  =  ) 1.594, p > 0.05).Likewise the comparison of the matrices of Mahalan-obis distances and minimum bending energies with thepalaeogeographical distances of islands during the Tortonian– Messinian [(Tor), ( D 2 :  Z  Tor  =  ) 0.111,  t Tor  =  ) 0.775,p > 0.05; mbe:  Z  Tor  =  ) 0.155,  t Tor  =  ) 1.2, p > 0.05)],Pliocene [(Plio) ( D 2 :  Z  Plio  =  ) 0.258,  t Plio  =  ) 1.356,p > 0.05; mbe:  Z  Plio  = 0.074,  t Plio  = 0.393, p > 0.05)] andlast Pleistocene sea regression (18 000 Ma) [(Pleis)( D 2 :  Z  Pleis  =  ) 0.234,  t Pleis  =  ) 1.544, p > 0.05; mbe: Z  Pleis  =  ) 0.003,  t Pleis  = 0.022, p > 0.05)] were non-signifi-cant. Spearman nonparametric rank correlation coefficientrevealed no correlation with latitude, temperature, annualprecipitation and monthly average precipitation for all factorscores tested (all p > 0.05). Discussion The use of morphometric analysis has provided useful insightsinto the patterns of variation in the terrestrial isopod species A. tuberculatus . Multivariate quantitative approaches to mor-phology have not been used for terrestrial isopods, in contrastto other organisms, such as aquatic isopods (Bertin et al.2002), decapods (Mariappan and Balasundram 2004), insects(Rohlf and Archie 1984; Luebke et al. 1988; Monti et al. 2001;Tatsuta et al. 2001, 2004; Garnier et al. 2005) and, mostly,vertebrates (e.g. Cavalcanti et al. 1999; Marcus et al. 2000;Swiderski et al. 2000; Corti et al. 2001; Loy et al. 2001;Albertson et al. 2003; Guill et al. 2003; Johnson et al.2005; Koumoundouros et al. 2005; Monteiro and Dos Reis Fig. 3. The linear regression of each individuals   Y  * on the latitude(transformed to numeric form) of the islands hosting the respectivepopulation ( r  = 0.47, p < 0.0001) 222  Kamilari  and  Sfenthourakis J Zool Syst Evol Res (2009)  47 (3), 219–226   2009 Blackwell Verlag GmbH  2005; Monteiro et al. 2005). It is true that classical morpho-metrics using linear measurements cannot describe efficientlymany morphological features of these soft-bodied crustaceans,due to the   flexibility   of their cuticle and the lack of adequaterecognizable homologous points on their pleopodal append-ages. Nevertheless, modern methods of geometric morpho-metrics and outline analysis can override this obstacle. In thisstudy, we explored the potential of such analyses, with positiveresults regarding population discrimination. In addition, thehardened epicuticle of the species studied allowed for aclassical approach to the allometric growth of its ornamenta-tion (tubercules) using simple linear measurements.The various cuticular protuberances (ridges, tubercules,spines, etc.) present on many terrestrial isopod species havebeen considered either as anti-adhesive structures facilitatingthe movement of these organisms within soil crevices or asanti-predator mechanisms (Schmalfuss 1975). The tuberculesof   A. tuberculatus , in particular, have been explained as apossible adaptation against predation by gekkos, especially in juveniles, whose structures are more prominent (Schmalfuss1975). Nevertheless, we have found that populations fromdifferent islands exhibit different rates of tubercule growth,with some retaining pronounced tubercules even in adultstages. Furthermore, there is an apparent trend for reducedtuberculation in adults towards the south–south-eastern partsof the species   distribution correlated also with climatic factorsthat follow a similar geographical trend. Taking into accountthe fact that the fauna of possible predators is more or lessuniform throughout this distributional range, tuberculationgrowth patterns should be considered as responses to abioticenvironmental factors. These animals live mainly in drycalcareous rocky habitats, where humidity can become animportant limiting factor. Juveniles are generally found underrocks, in relatively sheltered micro-habitats, but larger indi-viduals that move around in more exposed areas should bemore protected against drought. The reduction in tuberculeheight by size is obviously related to the thickening of thecuticle in adults. This is especially evident in the case of Crete,where the tubercles seem to become smaller with increasinganimal size, a fact that can only be explained by the thickeningof the cuticular space in between tubercules. Moreover, takinginto account that the sympatrically distributed congenericspecies  A. officinalis , that has a thick cuticle but lackstuberculation, is not restricted to rocky habitats, we couldassume that the tubercules in  A. tuberculatus  play a morecomplex role, related to both life under rocks and climaticfactors. Further studies on the ecology and life history of thespecies are needed to clarify this role.On the other hand, the variation in the shape of the malefirst pleopod exopodite does not seem to reflect any geograph-ically or ecologically consistent pattern. It is possible torecognize certain groups of islands hosting populations withsimilar pleopod shapes, but these groups do not consist of neighbouring islands or of islands more recently connected in 2015 –5 5–2–220108520 5    F  u  n  c   t   i  o  n   2 ANF ASP KAP CRT KTR KTN DIA NAX PXM IKR Function 1 Function 3 Function 5    F  u  n  c   t   i  o  n   4   F  u  n  c   t   i  o  n   6 –5–10–9 –6 –3 0 3 6 9 –6.0–5.05.0––3.0 0.0 3.0 6.0 010 Fig. 4. First six GDA functions of 77 Fourier coefficients of malepleopod exopodite I, with the ex-tremes of exopodite shape vari-ation along each function. Thepercentage of variance explainedby each function is also shownFig. 5. Cluster analysis (UPGMA) of the 10 populations based on thesquared Mahalanobis distances ( D 2 ) for the shape of first male pleopodexopodite Morphometrics in an isopod species 223J Zool Syst Evol Res (2009)  47 (3), 219–226   2009 Blackwell Verlag GmbH
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