1
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
5 6 7 8 9
1
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]
10 11
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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1
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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.
2
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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
3
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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.
84 85
Terminology used in the morphological description of mud crabs follows Keenan et al.3.
86 87
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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4
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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
5
114
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).
128 129
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.
143 144
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.
151 152
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.
232 233
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. ####).
237 238
References
239
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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6. Man de, J. G., The fauna of brackish ponds at Port Canning, Lower Bengal -
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near Calcutta and in the Dacca district, Eastern Bengal, Rec. Ind. Mus., 2 (1909)
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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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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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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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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
319
Table 1. Range, mean and standard deviation of morphometric ratios of S. serrata (N =
320
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
337
Table 2. Results of Student’s t–test indicating differences in morphological ratios
338
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
16
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Figure captions
357
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
366
Fig. 7. Male gonopod (G1) of S. olivacea (a) entire, (b) distal tip, (c) enlarged view of
367
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
17
379
380 381
Fig.1.
382 383 384 385 386 387 388 389 390 391 392 18
393
394 395
Fig. 2.
396 397 398 399 400 401
19
402
403 404
Fig. 3.
405 406 407 408 409 410 411 412 413 414 415 20
416
417 418
Fig. 4.
419 420 421 422 423 424 425 426 427 428 429 430 431
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432
433 434
Fig. 5.
435 436 437 438 439 440 441 442 443 444 445 446 447
22
448
449 450
Fig. 6.
451 452 453
23
454
455 456
Fig. 7.
24