http://france.elsevier.com/direct/GEOBIO/

Geobios 39 (2006) 311–317

* CorrE-ma

0016-69doi:10.1

Polymorphism in recent ostracods of southwestern Spain:

an environmental approach

Le polymorphisme des ostracodes actuels du Sud-Ouest de l’Espagne :une approximation environnementale

Francisco Ruiz a,*, Manuel Abad a, Pierre Carbonel b, Juan Manuel Muñoz c

a Departamento de Geodinámica y Paleontología, Universidad de Huelva, Avenida de las Fuerzas Armadas, s/n. 21071 Huelva, Spainb Département de Géologie et Océanographie, Université de Bordeaux-I, Avenue des Facultés, 33405 Talence cedex, France

c Departamento de Estadística e Investigación Operativa, Facultad de Matemáticas, Universidad de Sevilla, Avenida Reina Mercedes, s/n. Sevilla, Spain

Received 27 January 2003; accepted 20 September 2004Available online 23 January 2006

Abstract

The analysis of the surface ornamentation permits us to differentiate two morphotypes in the recent populations of the ostracod Urocythereisoblonga collected in shallow marine sediments of the Huelva littoral (SW Spain). An environmental analysis was made and its abundances werecompared with different environmental parameters. Morph 1 (reticulated or “normal”) is almost always dominant, characterizing microenviron-ments with low salinity water changes, low to moderate energy levels, poorly sorted bottom sediments with high percentages of fine grain sizes(very fine sands, silts and clays) and moderate to high ostracod diversity. Morph 2 (punctuated or “degraded”) is significantly represented(> 10%) in stressing conditions, i.e., moderate salinity fluctuations, moderate to high hydrodynamic levels, low organic matter contents, moderateto well-sorted sediments with high percentages of medium sands and low to very low ostracod diversity. These differences make clear thepotential of these microcrustaceans as environmental tracers in these transitional areas.© 2006 Elsevier SAS. All rights reserved.

Résumé

Ce travail étudie les populations d’Urocythereis oblonga (Crustacea, Ostracoda) dans les sédiments marins actuels peu profonds du littoral deHuelva (Sud-Ouest de l’Espagne). L’analyse de la réticulation de la carapace de cette espèce permet de distinguer deux groupes de variants (oumorphes). Les pourcentages des deux morphes ont été mis en relation avec certaines variables physico-chimiques du milieu. Le Morphe 1(réticulé ou « normal ») est dominant dans la plupart des environnements de la zone étudiée et caractérise les fonds marins à faibles oscillationsde salinité, à faible hydrodynamisme, dont les sédiments sont mal classés (importants pourcentages de sables très fins et d’argiles), et dont ladiversité est modérée ou haute. Le Morphe 2 (ponctué ou « dégradé ») se rencontre dans les environnements les plus défavorables, dans lesquelsles fluctuations de salinité sont plus importantes, l’hydrodynamisme modéré à fort, la matière organique dégradée, le support édaphique dessables à granulométrie moyenne et où les espèces d’ostracodes sont peu nombreuses.© 2006 Elsevier SAS. All rights reserved.

Keywords: Ostracodes; Polymorphism; Recent; SW Spain

Mots clés : Ostracodes ; Polymorphisme ; Environnements littoraux ; Actuel ; SO Espagne

esponding author.il address: ruizmu@uhu.es (F. Ruiz).

95/$ - see front matter © 2006 Elsevier SAS. All rights reserved.016/j.geobios.2004.09.003

F. Ruiz et al. / Geobios 39 (2006) 311–317312

1. Introduction

In the last decades, numerous investigations have been fo-cused on the application of morphometric techniques to thecarapace variations present in numerous ostracod species. In afirst way, some multivariate methods (covariance matrix, prin-cipal component analysis, cross-validation, discriminant func-tion analysis) have been applied to the variations of some se-lected landmarks located generally in the extern part of thecarapace (i.e. Reyment, 1993). Results are very interesting instudies concerning the phenotypic evolution of a lineage(Reyment, 1985), the determination of the geographical valid-ity of shape patterns (Reyment, 1995a) or the comparison be-tween males and females of the same species (Reyment,1995b).

Other works have analyzed the incidence of the environ-mental variables on the polymorphism. According to the mainresults obtained by this second way, the morphological varia-bility of an ostracod species can vary due to changes in envir-onmental variables such as ionic ratios (Carbonel, 1982; Vanden Bold, 1990), hydrodynamic levels (Dequan, 1990), theseasonal conditions (Carbonel et al., 1990) or the physical-che-mical equilibriums at the water–sediment interface (Carboneland Tölderer-Farmer, 1988). One of the most frequent conse-quences is the presence of different types of reticulation, withdiverse morphotypes that respond to the “agradation–degrada-tion” phenomenon defined by Peypouquet et al. (1987, 1988).The majority of these studies have been centered in a qualita-tive analysis of the data collected, indicating the rank of theenvironmental variables in which the distinct types of reticula-tions appear.

In this work, we analyze the variation of the ostracod reticu-lation on Urocythereis oblonga, one of the most representativespecies of the Atlantic coasts of Europe from Skagerrak toSpain (Wagner, 1957; Yassini, 1969; Carbonel, 1973; Ather-such et al., 1989; Pascual, 1990; Ruiz et al., 1997). The distri-bution and abundance of its morphotypes in shallow marine se-diments of southwestern Spain are related to someenvironmental parameters (ostracod diversity and density, grainsize, depth).

2. The Huelva littoral

The Huelva littoral is formed by large sandy beaches(145 km long), only interrupted by the presence of estuarinemouths (Fig. 1A: Guadiana, Piedras, Tinto-Odiel and Guadal-quivir). In this zone of the Cádiz Gulf, the shelf is relativelywide (50 km average width), with bottom sediments consti-tuted by fine to very fine sands in the innermost zone(< 20 m depth) and silty clays in the deeper areas (IGME,1974; Ruiz et al., 1997). The littoral morphology of this areais mainly linked to five morphodynamic factors: tidal regime,wave action, coastal drift currents, fluvial dynamics and artifi-cial groynes.

The tidal regime is mesotidal (Davies, 1964) or high meso-tidal (Hayes, 1979), with a mean range of approximately 2 m

(Borrego and Pendón, 1989). Wave energy is medium, because75% of the waves do not exceed 0.5 m in height (CEDEX,1991). The main transport of sediments is oriented toward theeast, although it is interrupted by modern protective groynes,causing either total (Ayamonte, Huelva) or partial (Isla Cristi-na, Punta Umbría) obstacles to the sedimentary fluxes.

The Guadiana river is the main sediment source of the Huel-va littoral, with a mean water discharge of 144 m3 s–1. Theremainder rivers (Piedras, Tinto and Odiel) have very limitedflows and a scarce importance in the sedimentary dynamics ofthe southwestern Spanish coast.

Tidal fluxes have important differences between the opensea and the estuarine domains. In the inner shelf, these cyclicalcurrents come from the southeast, with a mean velocity up to0.65 m s–1. In the estuarine mouths, there is a considerablereduction (mean velocity: 0.26 m s–1 in the Guadiana river)due to the bottom friction (Morales, 1993).

In this area, several studies have delimitated the main ostra-cod assemblages of the estuarine and shallow marine areas. Inthese later environments, Urocythereis oblonga is one of themost important species, constituting more than the 25% ofthe total ostracod fauna (Ruiz et al., 1997, 2000).

3. Methodology

The total ostracod population was examined from samplescollected at 54 stations located in the Huelva littoral, at depthsdown to 20 m (Fig. 1B). The surface sediments(500–1000 cm3) were obtained with a modified Van Veengrab, determining the grain size by standard sieve analysis(Table 1). A fixed quantity of dried sediments (200 g) waswet sieved (500, 250 and 125 μm mesh) and dried in an ovenat 70 °C for the ostracod analysis. If possible, 100 individualsof each fraction were picked from each sample and the countwas then recalculated to yield the total number of ostracods inthe whole sample (cf. Whatley and Watson, 1990). Additionalinformation about the ostracod associations of this area may beobtained from Ruiz et al. (1997, 2000).

The total population of Urocythereis oblonga was separatedand counted. In a first step, the fossae of the reticulate carapacewere disposed in rows and enumerated (Fig. 2A; cf. Liebau,1977). The abundance of the different morphotypes observedwas determined and compared with: (a) the ostracod diversity,expressed as number of species in each sample, NS, and theShannon–Weaver index, H, expressed as H = – piLnpi wherepi is the proportion of the total ostracod fauna contributed bythe it species; (c) the percentages of each grain size; and (b) thedepth.

4. Results

4.1. Morphotypes: abundance and distribution

Two morphotypes may be differentiated:

Fig. 1. A. Main features of the southwestern Spanish coast. B. Location of the samples collected.Fig. 1. A. Données principales du littoral du Sud-Ouest de l’Espagne. B. Localisation des échantillons.

F. Ruiz et al. / Geobios 39 (2006) 311–317 313

● Morph 1 (Fig. 2B), “reticulated or normal”, correspondingto the initial description of this species. The fossae are ar-ranged in 15 groups or files (designated A to V in Fig. 2)separated by fine ribs. In this morphotype, these depressionsare relatively deep and have an irregular shape (polygonal,surrounded and elongated).Except in sample 44, these reticulated forms are alwaysmore abundant than morph 2, with percentages up to 80%in most cases. The ostracod faunas are normally rich (> 150individuals/100 g) and diversified (NS > 20; H > 2) in thesesamples, where Urocythereis oblonga varies in abundance(2–76%).

● Morph 2 (Fig. 2C), punctuated or “degraded”, characterizedby strong ribs delimiting a great number of rounded, smallerdepressions in relation to the morph 1 (i.e. N3, E3, P3, V1or V11). In some cases, the fossae of the normal reticulationare divided in two to three new, smaller fossae (i.e. F3, M1,M4 or N2).

These forms present significant percentages (up to 10%)near the mouth of the rivers. In these areas, Urocythereis ob-longa is the main ostracod species, comprising about 20–80%of the whole ostracod fauna. Both the ostracod density (< 100individual/100 g) and diversity (NS < 18; H < 1.8) are usuallylow to very low in these samples.

5. Discussion

5.1. Morphotypes and environment

The reticulated individuals of U. oblonga are found in veryhigh percentages in almost all samples collected in the areastudied and may be considered as indicative of normal marineconditions. These “normal” specimens live at or near thewater–sediment interface, with low salinity changes (< 2‰),alkaline to neutral pH, medium to high organic matter contents

Table 1Database of the environmental study carried out. NS: number of species; H: Shannon–Weaver index; N: number of individuals of Urocythereis oblonga; P:percentages of Urocythereis oblonga; RT: percentages of Morph 1; PT: percentages of Morph 2; GR: gravel; VCS: very coarse sands; CS: coarse sands; MS:medium sands; FS: fine sands; VFS: very fine sands; SC: silts and claysBase de données de l’étude environnementale effectuée. NS : nombre d’espèces ; H : indice de Shannon-Weaver ; N : nombre d’individus d’Urocythereis oblonga ;P : pourcentages d’Urocythereis oblonga ; RT : pourcentages du morphe 1 ; PT : pourcentages du morphe 2 ; GR : graviers ; VCS : sables très lourds ; CS : sableslourds ; MS : sables moyens ; FS : sables fins ; VFS : sables très fins ; SC : boues et argiles

Sample NS H N P RT PT GR VCS CS MS FS VFS SC Depth1 16 1.75 43 56.6 83.7 16.3 1.4 5.2 36.9 40.8 13.7 2 0 3.5

2 9 0.88 53 79.4 87 13 0 0.1 2.6 85.9 11 0.3 0 4.5

3 3 0.8 7 70 100 0 1.3 3.4 39.7 51.9 3.5 0.2 0 5.5

4 24 2 56 7.2 100 0 1 0.3 0.5 1.5 5.5 80.2 10.6 11

5 14 1.44 66 66 87.9 12.1 0.4 1.2 6.8 58.6 30.1 2.8 0.1 4.5

6 5 0.87 16 76.2 100 0 0.5 1 18 77.5 3 0.1 0 5.5

7 15 1.42 82 63.1 81.7 18.3 0.2 0.8 4.8 43.6 44.8 5.7 0.1 3.5

8 27 2.6 1218 9 100 0 5.6 4.1 2.9 4.7 9.4 71.5 1.7 14.6

9 18 1.65 76 63.3 97.4 2.6 0.1 0.3 3.4 35.4 54.9 6.2 0.1 6

10 24 1.74 482 58.2 98.1 1.9 0 0.3 1 3.1 26.6 65.7 3.4 5.5

11 27 2.16 130 44.4 95.4 4.6 0.5 0.5 1.1 4.3 44.8 46.5 2.2 6

12 30 2.4 791 24.2 100 0 2.1 3.8 11.9 40.7 26.5 9.3 5.6 14.6

13 26 2.42 188 31.8 100 0 58.8 8.1 8.2 6.3 8.9 8.6 1.1 12.8

14 22 2.5 146 13.5 96.6 3.4 0.1 0.4 1.5 6.2 27 61.6 3.2 5.5

15 20 1.85 143 51.3 95.8 4.2 0.2 0.3 1.7 7.7 30.7 58 1.4 6

16 29 2.66 788 23.7 100 0 23.1 5.5 4.9 5.3 14.6 41.9 4.7 10.5

17 37 2.6 160 25.7 89.4 10.6 0 0.1 0.1 1.4 34 63.5 1.6 5.5

18 28 1.75 634 59.8 98.1 1.9 10.3 1.7 4.3 10.7 11.8 59.2 2.1 7.5

19 33 2.44 1816 27.7 99.4 0.6 0.1 0.1 0.2 0.9 39.1 56.2 3.4 5.5

20 27 2 10093 51.4 100 0 0 0.2 0.3 5.4 70.6 24.3 0.8 8.2

21 9 1.22 35 64.8 80 20 3.9 6.1 36.2 49.4 3.9 0.6 0 4.5

22 29 1.92 541 56.8 99.1 1.9 0.6 0.2 1.5 7.1 6 80.5 4.1 5.5

23 18 1.27 310 65 83.9 16.1 0.2 0.2 0.8 5.6 76.6 14.3 2.3 5.5

24 13 1.9 14 46.3 100 0 2.8 3 26.1 57.8 8.4 1.9 0 5.5

25 15 1.15 177 70.8 100 0 16.2 16.1 20.3 40.4 5.8 0.8 0.4 15

26 34 2.23 122 37.8 93 7 2.7 1.2 2.6 13.5 55 23.9 1 8.5

27 18 1.37 458 65.3 84.7 15.3 4.1 5.1 12 38.7 39.5 0.6 0.1 7.3

28 32 2.25 127 33.5 97.7 2.3 0.6 0.4 1.3 2.7 45.1 42.7 7.1 11.5

29 30 2.35 2003 26.3 98.1 1.9 1.6 1.3 2.2 13.4 40 26 16 12.8

30 20 1.82 90 55.6 96.7 3.3 0.2 0.5 2.2 20.2 33.8 39.6 3.6 7.3

31 18 2 50 46.8 72 28 0.5 0.7 2.2 67.4 25.2 1.8 2.3 13

32 26 1.9 475 54.4 95.4 4.6 0.9 0.5 2.4 15.5 25.4 52.1 3.3 7.3

33 27 2.06 695 12.1 99.3 0.7 1 1.1 3.1 15.9 37.8 34.9 6.3 16.4

34 35 2.76 341 14.3 99 1 2.6 1.9 2.7 11.4 21.4 56.9 3.1 7.3

35 29 2.37 191 21.5 73.3 26.7 1.7 1 3.5 12.6 31.4 41.6 8 12.5

36 29 2.45 387 4.6 100 0 2.7 2.2 1.4 4.6 37.6 18.8 32.8 16.4

37 34 2.74 219 12.5 98.6 1.4 7.8 4.7 4.2 10.2 19.2 50.5 3.4 7.3

38 20 2.34 40 30.3 97.5 2.5 0 0.2 1.9 8.5 47.9 40.6 1 5

39 29 2.5 120 7.4 97 3 3.8 2.6 3.5 9.7 34.8 35.8 9.9 15

40 30 2.63 365 15.7 97.5 2.5 0.6 0.5 1.9 4.5 19.1 70.5 2.9 9.1

41 28 2.46 420 25.7 95 5 0.2 0.8 1.4 5.9 26.5 62.8 2.2 9.1

42 18 1.97 36 27.5 97.2 2.8 0.5 1.5 5.6 63.2 27 1 1.1 16.4

43 30 2.4 84 24.5 91.7 8.3 0.1 0.7 2.3 36.3 57.4 3 0.1 16.4

44 19 1.7 2 1.9 0 100 0.1 0.1 0.1 27 67.8 3 2 5.5

45 21 2.47 33 15.4 87.8 12.1 0 0 0.2 0.7 37.2 60.4 1.3 5

46 22 2.34 140 7.6 96.4 3.6 0.1 0.2 1.2 4 13.7 79.1 1.8 5.4

47 23 2.35 83 3.5 98.9 1.1 3.2 1.3 2 18.2 43.4 30.4 1.5 13

48 21 2.52 284 6.8 100 0 0 0 0.1 0.3 9.5 43.6 46.6 14.6

(continued)

F. Ruiz et al. / Geobios 39 (2006) 311–317314

Fig. 2. Urocythereis oblonga. A. Meshes of the lateral reticulation. B. Morph 1 (reticulated). C. Morph 2 (punctuate).Fig. 2. Urocythereis oblonga. A. Réticulation latérale. B. Morphe 1 (réticulé). C. Morphe 2 (ponctué).

Table 1 (continued)Sample NS H N P RT PT GR VCS CS MS FS VFS SC Depth49 15 2.04 33 28 69.7 30.3 0 0.1 1.2 51.5 38.5 8.7 0.1 4

50 27 2.7 190 13.5 100 0 0 0 0.1 1.2 24.4 72 2.5 5.5

51 25 2.47 135 14.2 98.5 1.5 0.3 0.3 0.4 2 12.2 82.5 2.3 5.4

52 23 2.06 92 7.6 100 0 1.5 0.9 2.6 5.8 48.1 39.9 1.1 10

53 31 2.82 1114 12.3 100 0 0.2 0.1 0.1 0.6 58.5 38.4 2.2 14.6

54 28 2.54 1960 17.9 93.5 6.5 0 0.2 0.8 5.6 21.4 70.3 1.8 14.6

F. Ruiz et al. / Geobios 39 (2006) 311–317 315

(> 1.2% in most cases) and low to medium hydrodynamic le-vels (v < 0.26 m s–1) in most cases (Junta de Andalucía, 1993;Ruiz et al., 1997). These conditions are favorable for the ostra-cod development in shallow marine to transitional environ-ments and are expressed in the moderate to high ostracod di-versities observed (Carbonel and Tölderer-Farmer, 1988).

These individuals are well developed in sediments com-posed by fine sediments with minor inputs of gravels and verycoarse sands. This poor to moderate sorting is necessary topermit the vertical climbing of the nutrients for these micro-crustaceans (Carbonel, 1980).

On the contrary, significant quantities and percentages ofpunctuated forms were only found in special conditions. Theseforms are mainly found near the river mouths, presenting high-er salinity fluctuations and low organic matter contents (Pie-dras: 0.56%; Tinto-Odiel: 0.37–0.93%; Junta de Andalucía,1993). Peypouquet et al. (1987) indicate a degradation of theostracod reticulation in environments with low organic matterdue to dissolvent conditions.

These environments present bottom sediments characterizedby moderate to well-sorted sediments composed of high per-centages of coarse, medium and fine sands. The hydrodynamic

F. Ruiz et al. / Geobios 39 (2006) 311–317316

levels are moderate to high, near 0.6 m s–1, with instable con-ditions of the surface sediments due to bypassing phenomenain the adjacent areas of the fluvial channels (Borrego, 1992;Morales, 1993). A consequence may is the diminution of boththe ostracod density and diversity, which may even disappearowing to this factor (Ruiz et al., 2000).

5.2. Morphotypes and design

The apparition of polymorphism in the ostracod carapace ofUrocythereis oblonga seems to respond to adverse conditionsof the surrounding environment. Near the river mouths, thepopulations of this species are mainly formed by adult females(Ruiz et al., 1998), which they need to distribute their energiesamong the daily subsistence, the growth and maintenance ofthe shell and the reproduction. Consequently, when the avail-ability of calcium carbonate is limited by dissolvent microen-vironments, some compromise in design is required (Benson,1975).

Under these conditions, one of the most representative con-sequences of the economy in design is the presence of de-graded forms in the ostracod populations. This phenomenonis frequent in littoral species, such as Palmoconcha guttata inMorocco (Hamoumi, 1988) or Neomonoceratina reticulata inthe delta of Mahakam, Indonesia (Carbonel and Moyes, 1987).In all cases, a secondary proportion of the total populationchanges its surface ornamentation of reticulated to punctuatedistributions, as it has been observed in Urocythereis oblonga.In this species, the punctuate forms have a thinner carapace anda diminution of the reticulation (~ 15–20%).

These differences indicate the possibilities of these micro-crustaceans in the recognition of stress conditions in the bottomsediments of shallow marine environments. Moreover, thesedistinctions may be applied to the fossil record (i.e., Carboneland Hamoumi, 1990).

6. Conclusions

The study of the recent populations of Urocythereis oblongaallows us to evaluate the potential of this species as an envir-onmental tracer in the littoral areas of southwestern Spain. Instress conditions, the reticulate ornamentation of the carapacesurface may change to punctuated forms in a significant pro-portion of the total population (5–20% in most cases). Accord-ing to the environmental analysis realized, some of them are amoderate to high hydrodynamics, low organic matter contents,bypassing of the bottom sediments, the presence of high per-centages of medium sands with moderate to high sorting andmoderate salinity fluctuations. All they are given near the rivermouths, coinciding with a low density and diversity of the os-tracod associations. In the remaining zones with normal marineconditions, these forms are very secondary and may even dis-appear.

Acknowledgements

Special thanks go to Dr. Richard R. Reyment and Dr. Anne-Marie Bodergat who, as referees for this paper, made severalvaluable suggestions which greatly improved the presentationof the results. In addition, the financial support of the DGYCIT(Projects MAR98-0209 and BTE-2000-1153) and the Andalu-sia Board (PAI RNM-238) is gratefully acknowledged. This isa contribution to the PICG 396 “Continental Shelves in theQuaternary”.

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