A new record of Scylla olivacea 1 A new record of ...

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Scylla species. Keenan et al. 3 employed twenty three morphological parameters and. 28 twenty seven ratios to identify four distinct species (S. serrata (ForskÃ¥l, ...
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Running head: A new record of Scylla olivacea

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A new record of Scylla olivacea (Decapoda, Brachyura, Portunidae) from Goa, central

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west coast of India – A comparative diagnosis

4 Vinay P. Padate1, Chandrashekher U. Rivonker 1* and A.C. Anil2

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Department of Marine Sciences, Goa University, Taleigao Plateau, Goa 403 206, India 2

National Institute of Oceanography, Dona Paula, Goa 403 004, India Email: *[email protected]

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Taxonomic studies on mud crabs (Scylla) reveal considerable ambiguity in

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species identification owing to overlap of morphological characters. The identification of

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mud crabs from Goa using morphological characters (cheliped armature, frontal lobe

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spines of carapace, polygonal markings on chelipeds, legs, female abdomen, and twenty

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three morphometric ratios) revealed two forms which exhibited significant differences in

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two morphometric ratios, in addition to differences in morphological characters. This

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paper presents a new record of S. olivacea along with a comparative diagnosis with S.

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serrata from the region. Further, minor phenotypic variations in the present S. olivacea

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specimens could be attributed to geographical isolation from existing populations.

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Keywords: New record, Scylla, morphometry, Goa, India

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Introduction

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Mud crabs of the genus Scylla de Haan, 1833 (Decapoda, Brachyura, Portunidae)

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are common inhabitants of mangrove–vegetated estuaries throughout the Indo–Pacific

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region 1. Taxonomic studies2,3 revealed ambiguity in the taxonomy and identification of

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Scylla species. Keenan et al.3 employed twenty three morphological parameters and

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twenty seven ratios to identify four distinct species (S. serrata (Forskål, 1775), S.

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olivacea (Herbst, 1796), S. tranquebarica (Fabricius, 1798), and S. paramamosain

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Estampador, 1949), and provided a key to identify them.

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Taxonomic studies of mud crabs along Indian coasts2,4,5,6,7,8,9,10,11,12,13,14 suggested

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the occurrence of two species, S. serrata and S. tranquebarica and one subspecies, S.

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serrata serrata from Indian waters. Joel & Raj2 reported two species namely S. serrata

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and S. tranquebarica from the Pulicat Lake, East coast of India. Our observations on

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community structure of demersal resources for two years (January, 2006 to April, 2008),

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revealed differences in mud crab specimens and a comparative assessment among various

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congeners further revealed a new record of S. olivacea from the west coast of India.

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Materials and methods

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Study area

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The coastal region of Goa, central west coast of India (Fig. 1) harbours thickly

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vegetated mangrove habitats among narrow inter–tidal mudflats along the banks of

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estuaries and shores of estuarine islands of the Mandovi–Zuari estuarine complex15.

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Mangrove habitats function as nursery grounds for juveniles of mud crabs, and provide

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food and shelter for adult stages of mud crabs16, 17.

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Sampling

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Sixty nine trawls were undertaken in the estuarine and offshore waters (up to 20 m depth)

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of Goa, central west coast of India (Fig. 1) to assess the diversity and total community

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structure of demersal fish fauna. Nine trawls were taken from estuaries in May and

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December, 2005, September and October, 2006, May, September and December, 2007

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along the navigational channel off the Mormugao Port Trust region and the southern

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region of Mormugao Bay (between 15˚24’N, 73˚48’E and 15˚27’N, 73˚51’E) and one

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trawl in the Aguada Bay in February, 2007. The offshore trawls were undertaken from

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January to April, 2006 and from December, 2006 to February, 2008 between latitudes

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15˚29’6”N and 15˚33’15.2”N and between longitudes 73˚40’6”E and 73˚51’11”E. Trawl

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nets with mesh sizes of 15 mm (mouth end) and 9 mm (cod end) were towed at a speed of

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2.16 knots (4 km.h -1). The offshore trawls were operated for 1 to 3 hours, whereas, the

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estuarine trawls were towed for a maximum of 1 hour, due to interference from irregular

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bottom topography. The trawl catch obtained was immediately examined for species

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composition by taking five sub–samples and uncommon (or rare) specimens were sorted

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separately and temporarily preserved in ice. In addition, beach seines were operated for

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sample collection in the estuarine embayment (one along Betim (15˚30’18”N

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73˚49’52”E) in December 2005 and two in the vicinity of Mormugao Port Trust

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(15˚24’16”N 73˚48’56”E; Fig. 1) in December 2005 and September 2006) for one hour

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each, as trawl nets could not be operated. Crab traps were also employed to obtain

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samples from the estuarine embayment. In addition, crab specimens were obtained from

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commercial outlets over an extended duration as per availability of large–sized crabs. All

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biological samples were temporarily preserved in ice and a detailed morphological study

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of these was carried out in the laboratory.

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Morphology and morphometric analyses

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Mud crab samples (N = 31) were primarily identified using phenotypic criteria such as

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morphology, colouration and armature of carapace and appendages. Subsequently,

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morphometric parameters (N = 23) were measured and morphometric ratios (N = 27)

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derived from these following Keenan et al.3. In addition, illustrated descriptions of the

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male gonopod (G1) of these crabs were carried out. Based on the observed differences in

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morphological characters among the mud crabs, they were categorized into two species.

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Abbreviations

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The following abbreviations are used: ICW – Internal carapace width; FMSH – Frontal

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median spine height; FW – Carapace frontal width; ICS – Inner carpus spine; OCS –

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Outer carpus spine.

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Terminology used in the morphological description of mud crabs follows Keenan et al.3.

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Statistical analysis

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Student’s t–test was carried out to test the null differences between S. serrata and S.

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olivacea with respect to three morphometric ratios namely ICS/OCS, FMSH/FW and

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FW/ICW.

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Results

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Preliminary morphological examination of mud crab specimens (N = 31) as

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suggested by Keenan et al.3 revealed two species namely Scylla serrata and S. olivacea

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(Figs. 2 - 7).

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S. serrata (Forskål, 1775) – present study

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Material examined.

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One male, Betim, 13 Dec.2005, coll. K. Venkat; 4 males, Panaji market, 27 Sept.2007,

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coll. V.P. Padate; 2 females, Mandovi estuary, 27 Sept.2007, coll. V.P. Padate; 1 male, 1

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female, Panaji market, 16 Jan.2008, coll. V.P. Padate; 1 male, Panaji market, 12

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Mar.2008, coll. V.P. Padate; 1 male, Panaji market, 15 Mar.2008, coll. V.P. Padate.

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General description.

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Carapace broader than long, with prominent H–shaped groove present on cardiac region

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(Fig. 2a). Frontal margin (excluding inner supra–orbital angles) with four bluntly pointed

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spines with slightly concave margins and separated by inverted–V shaped interspaces,

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their height approximately 0.06 times frontal width measured between orbital sutures

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(Fig. 3a,b). Antero–lateral margins of carapace longer than postero–lateral margins,

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divided into nine sharp spines. Chelipeds massive, smooth, longer than legs; merus with

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three spines on anterior margin and two on posterior margin; carpus with acute spine at

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inner angle and two prominent spines at outer angle (Fig. 4a); propodus with strong spine

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at carpus articulation and two well developed spines on dorsal margin behind insertion of

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the dactyl (Fig. 4a). Four pairs of pereiopods, first three pairs similar, fourth pair

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natatorial.

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Colouration varied, olive green to dark brown with randomly scattered polygonal

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patterning (Fig. 2a), ventral surface of abdomen cream colour (only female abdomen with

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polygonal patterning; Fig. 5a), chelipeds and legs with conspicuous polygonal patterning

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(Fig. 2a,c).

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G1 with long distal portion, comparatively broader than in S. olivacea (Fig. 6a–b),

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ends in short, sharply pointed tip; outer margins of apex slightly convex (Fig. 6b–c);

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chromatophores absent. Details of morphometric measurements (range, mean and

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standard deviation) are provided in Table 1.

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It is pertinent to note that these crabs share the character “Palm of chelipeds with

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pair of distinct spines on dorsal margin behind insertion of the dactyl” (Fig. 4a) with three

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species (S. serrata, S. tranquebarica and S. paramamosain) described by Keenan et al.

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(1998).

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S. olivacea (Herbst, 1796) as described by Keenan et al.3

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“Frontal lobe spines low (mean height approximately 0.03 times frontal width measured

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between medial orbital sutures), rounded with shallow interspaces. Antero–lateral

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carapace spines broad, with outer margin convex. Carpus of chelipeds usually with one

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small blunt prominence (may be spinous in juveniles) ventro–medially on outer margin;

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reduced second spine may be present dorso–distally in juveniles and young adults. Palm

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of cheliped usually with a pair of blunt prominences on dorsal margin behind insertion of

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the dactyl, inner larger than outer; may be spinous in juveniles and young adults.

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Chelipeds, legs and abdomen all without obvious polygonal patterning for both sexes.

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Colour varies from red through brown to browny / black depending on habitat.” In

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addition to morphological characters, Keenan et al. (1998) used three morphometric

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ratios namely, ICS/OCS, FMSH/FW and FW/ICW to distinguish S. olivacea from its

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congeners. The ratios assigned to S. olivacea are 0.006±0.035, 0.029±0.005, and

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0.415±0.017, respectively.

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S. olivacea – present study

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Material examined.

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One male, Mormugao Port Trust, 8 May.2005, coll. K. Venkat; 1 female, Potential

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fishing ground off Goa, 25 Feb.2006, coll. V.P. Padate; 1 female, Potential fishing

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ground off Goa, 27 Mar.2006, coll. V.P. Padate; 1 male, 1 female, Potential fishing

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ground off Goa, 26 Sept.2006, coll. V.P. Padate; 4 males, Panaji market, 16 Jan.2008,

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coll. V.P. Padate; 7 males, 4 females, Panaji market, 15 Mar.2008, coll. V.P. Padate.

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General description.

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The specimens obtained during the present study and those resembling S. olivacea are

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described as “Carapace broader than long, with prominent H–shaped groove present on

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cardiac region (Fig. 2b). Frontal margin (excluding inner supra–orbital angles) with four

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rounded spines separated by rounded interspaces, their height approximately 0.026–0.066

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times frontal width measured between orbital sutures (Fig. 3c,d). Antero–lateral margins

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of carapace longer than postero–lateral margins, divided into nine sharp spines. Chelipeds

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massive, smooth, longer than legs; merus with three spines on anterior margin and two on

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posterior margin; carpus with acute spine at inner angle, outer angle either lacks or may

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possess rudimentary spines (Fig. 4b); propodus with strong spine at carpus articulation,

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two reduced spines may be present on dorsal margin behind insertion of the dactyl (Fig.

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4b). Four pairs of pereiopods, first three pairs similar, fourth pair natatorial.

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Colouration varied from greenish brown to dark brown and generally devoid of

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polygonal patterning (Fig. 2b). Some specimens display light yellow polygonal marking

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on epibranchial region of carapace and chelipeds. Ventral surface of male abdomen

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cream coloured. Female abdomen characterized by alternating transverse dark and light

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green or brownish bands, and lack of polygonal patterning (Fig. 5b). Chelipeds generally

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devoid of polygonal patterning (Fig. 2b,d), however some specimens display indistinct

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patterning. Legs devoid of polygonal patterning (Fig. 2b,d).

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G1 with long and slender distal portion comparatively narrower than in S. serrata

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(Fig. 7a–b), ends in long, bluntly pointed tip; outer margins of apex convex (Fig. 7b–c).

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Chromatophores just below tip of the first pair of abdominal appendages of male give a

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brownish red appearance in fresh specimens. The brownish red colouration fades in

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preserved specimens.” Details of morphometric measurements (range, mean and standard

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deviation) of the new variety of S. olivacea are provided in Table 1.

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Based on the above description, it is evident that the present specimens resemble

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S. olivacea in “lacking two well developed spines on distal half of outer margin of carpus

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of cheliped”, “frontal lobe spines rounded with shallow interspaces”, “palm of cheliped

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usually with a pair of blunt prominences on dorsal margin behind insertion of the dactyl”,

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and “chelipeds, legs and abdomen without obvious polygonal patterning”.

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However, they differ from S. olivacea in the following:

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1. Frontal lobe spine height “approximately 0.026–0.066 (0.045±0.012) times frontal

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width measured between orbital sutures (FMSH/FW)”, as compared to “0.018–

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0.037 (0.029±0.005)” in S. olivacea.

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2. “May possess rudimentary inner carpus spine or tubercle (range 0.000–0.500;

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0.101±0.177)”,

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0.006±0.035)”.

whereas

S.

olivacea



lacks

any

(range

0.000–0.250;

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3. “Chromatophores just below tip of the first pair of abdominal appendages of male

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give a brownish red appearance in fresh specimens (faded in preserved

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specimens).”

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Statistical analysis

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The Student’s t–test (Table 2) suggests significant differences (P ≤ 0.001) between S.

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serrata and S. olivacea with respect to two morphometric ratios namely FMSH/FW and

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ICS/OCS.

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Discussion

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The observations made in the present study along with a comparative diagnosis of the

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morphological characters of two closely related and ambiguous congeners imply a new

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record of S. olivacea from the west coast of India and suggest that the population of

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Scylla is represented by two species. These congeners with distinct ecological

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backgrounds3 were observed to co – exist in the study area, which emphasizes the

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complexity of habitats and dynamic nature of the coastal processes within a small area.

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Further, minor phenotypic variations were observed in S. olivacea, which probably reflect

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regional and habitat – specific differences18 arising from the prevailing physical and

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ecological barriers. The physical barriers include the bathymetric setting of the habitats

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and the prevailing hydrological processes therein. The estuaries of Goa are generally

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short, narrow, fast flowing19, lined with thin patches of mangrove vegetation 15 and

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separated from adjacent estuaries by pocket beaches and rock promontories20. Their small

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lengths (approximately 50 km each) and wide mouths render these estuaries exposed to

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coastal processes19. These estuaries have high discharge rates21, semi – diurnal tidal

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duration and marked seasonal variability in the flushing times which probably exert

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strong influence on the settling time of megalopa larvae recruited to the estuarine

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environment. Secondly, these natural forces generate turbulence, advective and

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longitudinal mixing, trap coastal water thereby influencing the rate of sedimentation and

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erosion of mud flats on which the mangroves grow22. This probably results in patchiness

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in formation of mud crab habitats. The ecological barriers constitute the tolerance of the

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species towards fluctuations in environmental variables. The current – enabled vertical

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stratification of the Mandovi – Zuari estuaries lead to frequent oscillations in the salinity

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regime within these estuaries19, and S. olivacea being a stenohaline species inhabiting

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relatively lower saline embayments3 may be forced to remain buried within the

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substratum or frequently change habitats. Further, such fluctuations may also affect

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biological processes such as spawning and larval dispersal. Estampador18 attributed

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colour variations among its congener, S. serrata populations within similar geographical

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limits to niche specificity, whereas Overton et al.23 stressed the need to study the role of

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environmental factors in determining phenotypic variations within a mud crab species.

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The present observations demand further investigation into the population

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genetics of these crabs to provide better understanding of their population structure in the

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region and phylogenetic relations with similar populations distributed across the known

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geographical range of this species.

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Acknowledgements

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The authors are grateful to the Ballast Water Management Programme, India executed by

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the National Institute of Oceanography, Dona Paula, Goa for Directorate General of

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Shipping, Ministry of Shipping, Government of India. (NIO Contribution No. ####).

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References

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1. Stephenson, W. H., Evolution and ecology of portunid crabs with special

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reference to the Australian species, In: Evolution of organisms edited by G.W.

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Melbourne, (Melbourne University Press, Melbourne) (1962), pp. 311-324.

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2. Joel, D. R. & Raj, P. J. S., Taxonomic remarks on two species of the genus Scylla

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de Haan (Portunidae: Brachyura) from Pulicat lake, J. Inland Fish. Soc. India, 12

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(1980) 39-50.

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3. Keenan, C. P., Davie, P. J. F. & Mann, D. L., A revision of the genus Scylla de

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Haan, 1833 (Crustacea: Decapoda: Brachyura: Portunidae), Raff. Bull. Zool., 46

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(1998) 217-245.

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4. Fabricius, J. C., Supplementatione Entomologiae Systematicae, (Proft et Storch, Hafniae) (1798), pp. 572.

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5. Alcock, A., Materials for a Carcinological Fauna of India, no. 4. The Brachyura

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Cyclometopa, Part 2. A revision of the Cyclometopa with an account of the

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families Portunidae, Cancridae and Corystidae, J. Asiat. Soc. Bengal, 68 (1899) 1-

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104.

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6. Man de, J. G., The fauna of brackish ponds at Port Canning, Lower Bengal -

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Decapoda Crustacea, with an account of a small collection from brackish-water

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near Calcutta and in the Dacca district, Eastern Bengal, Rec. Ind. Mus., 2 (1909)

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211-231.

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7. Kemp, S., Fauna of the Chilka Lake, Mem. Ind. Mus., 5 (1915) 199-325.

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8. Gravely, F. H., Crustacea, In: The littoral fauna of Krusadai islands in the Gulf of

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Mannar edited by Director, Madras Government Museum, (Madras Government

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Museum, Madras, India) (1927), pp. 141-155.

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9. Pearse, A. S., Observations on the ecology of certain fishes and crustaceans along the bank of the Malta River of Port Canning, Rec. Ind. Mus., 34 (1932) 289-298.

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10. Chopra, B. & Das, K. N., Further notes on Crustacea Decapoda in the Indian

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Museum. IX. On three collections of crabs from Tavoy and Mergui Archipelago,

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Rec. Ind. Mus., 39 (1937) 377-434.

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11. Pannikar, N. K. & Aiyar, R. G., The brackish-water fauna of Madras, Proc. Ind. Acad. Sci., 6 (1937) 284-337. 12. Chhapgar, B. F., On the marine crabs (Decapoda: Brachyura) of Bombay State, Part I, J. Bombay Nat. Hist. Soc., 54 (1957) 399-439.

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13. Radhakrishnan, C. & Samuel, C. T., Report on the occurrence of one subspecies

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of Scylla serrata (Forskal) in Cochin backwater, Fish. Technol., 19 (1982) 5-7.

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14. Kathirvel, M. & Srinivasagam, S., Taxonomy of the mud crab, Scylla serrata

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(Forskal) from India, In: The Mud Crab, a Report of the Seminar on the Mud

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Crab Culture and Trade held at Surat Thani, Thailand during November 5-8,

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1991, edited by C.A. Angell, (Bay of Bengal Program, Madras, India) (1992), pp.

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127-132.

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15. Qasim, S. Z. & Sen Gupta, R., Environmental Characteristics of the MandoviZuari Estuarine System of Goa, Estuar. Coast. Shelf Sci., 13 (1981) 557-578.

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16. Walton, M. E., Le Vay, L., Truong, L. M. & Ut, V. N., Significance of

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mangrove–mudflat boundaries as nursery grounds for the mud crab, Scylla

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paramamosain, Mar. Biol., 149 (2006) 1199-1207.

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17. Nagelkerken, I., Blaber, S. J. M., Bouillon, S., Green, P., Haywood, M., Kirton, L.

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G., Meynecke, J. –O., Pawlik, J., Penrose, H. M., Sasekumar, A. & Somerfield, P.

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J., The habitat function of mangroves for terrestrial and marine fauna: A review,

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Aquat. Bot., 89 (2008) 155-185.

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18. Estampador, E. P., Studies on Scylla (Crustacea: Portunidae), ‘I’. Revision of the genus, Philippine J. Sci., 78 (1949) 95-108.

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19. Shetye, S. R., Shankar, D., Neetu, S., Suprit, K., Michael, G. S. & Chandramohan,

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P., The environment that conditions the Mandovi and Zuari estuaries, In: The

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Mandovi and Zuari estuaries, edited by S.R. Shetye, M. Dileepkumar & D.

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Shankar, (National Institute of Oceanography, Goa, India) (2007), pp. 3-27.

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20. Wagle, B. G., Geomorphology of Goa and Goa coast - A review, G. Geol. 55 (1993) 19-24.

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21. Sundar, D. & Shetye, S. R., Tides in the Mandovi and Zuari estuaries, Goa, west coast of India, J. Earth Syst. Sci. 114 (2005) 493-503. 22. Alongi, D. M., Present state and future of the world’s mangrove forests, Environ. Conserv., 29 (2002) 331-349.

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23. Overton, J. L., Macintosh, D. J. & Thorpe, R. S., Multivariate analysis of the mud

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crab Scylla serrata (Brachyura: Portunidae) from four locations in South-east

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Asia, Mar. Biol., 128 (1997) 55-62.

302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 14

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Table 1. Range, mean and standard deviation of morphometric ratios of S. serrata (N =

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11) and S. olivacea (N = 20) obtained during the present study

321 S. No.

Morphometric

S. serrata (N = 11)

S. olivacea (N = 20)

Ratio

Range

Range

1.

FMSH/FW

0.049–0.077 0.062±0.009 0.026–0.066 0.045±0.012

2.

FW/ICW

0.383–0.463 0.424±0.024 0.397–0.472 0.431±0.019

3.

ICS/OCS

0.000–5.636 2.155±1.844 0.000–0.500 0.101±0.177

Mean ± SD

322 323 324 325 326 327 328 329 330 331 332 333 334 335 336

15

Mean ± SD

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Table 2. Results of Student’s t–test indicating differences in morphological ratios

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between S. serrata (N = 11) and S. olivacea (N = 20)

339 S. No.

Morphometric t-value

df

Critical value

Ratio

(P = 0.001)

1.

FMSH/FW

4.661

29

3.659

2.

ICS/OCS

3.684

29

3.659

340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355

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Figure captions

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Fig. 1. Map of study area indicating sampling sites

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Fig. 2. Dorsal view of carapace (a) S. serrata (b) S. olivacea; Frontal view of carapace (c)

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S. serrata, (d) S. olivacea

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Fig. 3. Frontal margin of carapace of S. serrata (a) coloured photograph, (b) line diagram

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and S. olivacea (c) coloured photograph, (d) line diagram

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Fig. 4. Right cheliped (a) S. serrata, (b) S. olivacea

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Fig. 5. Female abdomen (a) S. serrata, (b) S. olivacea

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Fig. 6. Male gonopod (G1) of S. serrata (a) entire, (b) distal tip, (c) enlarged view of

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distal tip

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Fig. 7. Male gonopod (G1) of S. olivacea (a) entire, (b) distal tip, (c) enlarged view of

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distal tip; inset on top left margin, G1 of S. olivacea – Keenan et al., 1998)

368 369 370 371 372 373 374 375 376 377 378

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380 381

Fig.1.

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Fig. 2.

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Fig. 3.

405 406 407 408 409 410 411 412 413 414 415 20

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Fig. 4.

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Fig. 5.

435 436 437 438 439 440 441 442 443 444 445 446 447

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449 450

Fig. 6.

451 452 453

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Fig. 7.

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