In the study, we investigated at five sites of Bangladesh. Selected study area is Saint Martin’s Island and adjacent water bodies of Bay of Bengal, Sathkhira, and Sundarban for crab survey (Figure 1). Live coral sites were selected for a survey in Saint Martin’s Island. Site 1: This area was covered by the sandy, rocky, and sandy-rocky beach. Site 2: Freshwater ditch is containing marsh. Site 3: Marine water is a stony area and deep 60 m from the surface. Site 4: Satkhira area (brackish and freshwater), mainly Maloncho river dam of Munshiganj union, Shyamnagar, Site 5: Sundarban (mangrove forest area), mainly Dublarchar, Sundarban, Bangladesh. This study was investigated and collected all samples during December 2013 to October 2014.

Crab samples were collected by hand picking from beach and fish landing centers, and some were obtained from fishermen. Several samples were collected from fishing nets in shallow coastal areas and the deep sea. The samples were washed with pure water after collecting, preserved and photo session. Photographs of the crabs were taken by Nikon USA D3300 digital SLR Camera before

their color changed. Fresh specimens were separated and placed in the ice box for preservation. All specimens were carried out to the fisheries Lab, Department of Zoology, Jagannath University. After observations and measurements, specimens were reserved and preserved with 6% - 10% formalin in the Zoological Museum of Jagannath University, Dhaka.

All morphological and meristic characteristics were measured by the digital calipers scale as millimeter (mm) [13]. Species were identified using pictorial, diagnostic characters as well as morphological and meristic characteristics such as color of body part, external features, shape of the carapace, dorsal surface, cheliped and legs, shape of anterolateral margins, antennule and antenna, eye shape, and pereiopod [14]. The following the references for the species identification [4] [9] [15] - [27]. Few species establish local names according to origin. Then, the identified species were tagged with a zoological certain number. To establish present distribution of the crab species is based on field survey, all available reference, and data from various biodiversity databases in addition to SeaLifeBase (https://www.sealifebase.ca), Marine Species Identification Portal (MSIP) (http://species-identification.org/index.php). All the particular references are mentioned in the above databases. Discrepancies in nomenclature were sorted following the World Register of Marine Species. All measurement data were inputted in the Microsoft excel 2010 data sheet timely and analyzed. Graph figures were plotted by using SigmaPlot 12.0.

Among all crab species maximum wet weight was 181.88 g for the crab species Zosymus aeneus. Carapace length and breadth measured as a major identification key that observed maximum length 68 mm and breadth 88 mm in Z. aeneus. Posterior border length: 18 mm; frontal margin length: 18 mm; antennule length: 13 mm; antenna length: 6 mm; eye stalk length: 3 mm; eye diameter: 2 mm (Table 1).

Each crab species has two (left and right) cheliped (Table 1). Pereiopod is a key character of the leg of the crab species and it includes six parts that are represented in Figure 2 and Figure 3.

Coxa: Its small segment of the leg that connects to the body. This is the first part of cheliped and pereiopod from the body of crab species. In cheliped, maximum size of coxa was 15 mm (left) and 18 mm (right) recorded whereas in pereiopod coxa average size was 9.25 ± 2.98 mm for Z. aeneus.

Basi-ischium: Its second part that situate between coxa and merus in cheliped and pereiopod of crab species. In cheliped, large size of basi-ischium was 18 mm (left) and 16 mm (right) whereas in pereiopod average size was 14.25 ± 2.87 mm present in C. armatum.

Merus: Its third part that situate between basi-ischium and carpus in cheliped and pereiopod of crab species. In cheliped, maximum size of merus is 37 mm (left) for Z. aeneus and 28 mm (right) for C. armatum whereas in pereiopod, maximum average size was 35.00 ± 2.44 mm recorded from C. armatum.

Carpus: As the 4th part of cheliped and pereiopod, its position between merus and propodus in cheliped and pereiopod of crab species. In cheliped, maximum size was 36 mm (left) for Z. aeneus and 22 mm (right) for C. armatum but in pereiopod average maximum size of carpus 20.25 ± 0.50 mm recorded from Lophozozymus pictor.

Propodus: Its 5th part of cheliped and pereiopod that position between carpus and dactylus in cheliped and pereiopod of crab species. In cheliped, height size of propodus was 67 mm (left) and 73 mm (right) for C. carnifex whereas average highest size of propodus was 18.50 ± 3.03 mm in pereiopod of C. armatum.

Dactylus: Its last part of cheliped and pereiopod that join with only propodus in cheliped and pereiopod of crab species. In cheliped, maximum size of dactylus was 35 mm (left) and 32 mm (right) for C. armatum whereas average maximum size was 19.75 ± 3.20 mm in pereiopod of Lophozozymus pictor.

We measured the meristic characters of 18 crab species in Table 2. Large species C. armatum, counted anterolateral spine: i pair; number of teeth: xv + xiv; merus of cheliped contains i spine in dorsa side and i spine in the ventral side; each propodus of the last four periopods contains ii spines in the dorsal side; dactylus of the 2nd periopod contains xviii spines in the dorsal side and iv spines in the ventral side; dactylus of 3rd periopod contains xviii spines in the dorsal side and ii spines in the ventral side; dactylus of the 4th periopod contains xiv spines in the dorsal side and iv spines in the ventral side; dactylus of 5th periopod contains xx spines in the dorsal side.

In the study crab species were remark with sex and color through observation. Species Calappa lophos (Herbst, 1782) is male [♂]. (Figure 4(a)), ground yellowish beige yellow; purple spots on posterior carapace; transverse red stripes on postero lateral part. Species Criocarcinus superciliosus (Linnaeus, 1758) is unknown (Figure 4(b)), dorsal and ventral surface of carapace ash; cheliped ash and brown in color, but other legs are ash color only. Species Cardisoma armatum (Herklots, 1851) is male [♂] (Figure 4(c)), dorsal surface of carapace is chocolate and ventral surface is reddish-yellow in color. coxa is yellow in color in the dorsal side and brown in the ventral side; basi-ischium is yellow color in the dorsal and brown in the ventral side; merus is violet color in the dorsal and yellow in the ventral side; carpus is violet color in the dorsal and yellow in the ventral side; propodus and dactylus are yellowish in the dorsal side and in the ventral side of cheliped; coxa of last four periopods are reddish color in the dorsal and ventral side; basi-ischium, merus, carpus and propodus of last four periopods are chocolate in color in both dorsal and ventral side; dactylus of last four periopods are reddish in both dorsal and ventral sides.

Species Cardisoma carnifex (Herbst, 1796) is female [♀] (Figure 4(d)), dorsal surface of carapace blackish-grey and ventral surface yellowish-white in color; coxa is yellowish-white in the dorsal side and yellowish-white in the ventral side.

Basi-ischium is yellowish-white color in the dorsal side and pale white in the ventral side, merus is purple color in the dorsal side and whitish in the ventral side, carpus is purple color in the dorsal side and whitish in the ventral side, propodus is greenish-yellow color in the dorsal side and yellow in the ventral side, dactylus is yellow color in the dorsal side and yellow in the ventral side of cheliped; coxa is yellowish color in the dorsal side and yellowish in the ventral side. Species Grapsus albolineatus (Lamarck, 1818) is female [♀] (Figure 4(e)), dorsal surface of carapace yellow and black; ventral surface off-white in color; all legs are brown black and off-white. Species Matuta lunaris (Forskal, 1775) is male [♂] (Figure 4(f)), dorsal surface of the carapace is gray and yellow; and ventral surface is whitish; legs are cream in color. Species Ocypode pallidula (Jacquinot, 1846) (Linnaeus, 1758) is male [♂] (Figure 4(g)), dorsal surface of carapace ash and ventral surface off-white in color; legs are deep grayish and light grayish. Species Ocypode macrocera (Milne Edwards, 1852) is Male [♂] (Figure 4(h)), dorsal and ventral surfaces of carapace red in color. Species Uca rosea (Tweedie, 1937) is unknown (Figure 4(i)), dorsal surface of carapace red and ventral surface grayish; cheliped is combined of reddish and brownish, other legs are deep grayish and reddish. Species Panopeus lacustris (Desbonne, 1867) is Male [♂] (Figure 4(J)), dorsal surface of carapace coffee color and ventral surface brownish; cheliped coffee color, other legs are coffee and ash in color. Species Pseudosesarma bocourti (A. Milne-Edwards, 1869) is female [♀] (Figure 5(a)), the dorsal surface of carapace is chocolate, dark, and yellowish. The ventral surface is black and yellowish in color; coxa and basi-ischium is reddish color in the dorsal side and yellowish in the ventral side, but merus is creamy in the ventral side, and carpus is whitish in the ventral side. Species Episesarma sp (Latreille, 1803) is Male [♂] (Figure 5(b)), the dorsal surface of carapace is deep grey; the ventral surface is whitish in color. Coxa, basi-ischium, merus, and carpus are grayish color in the dorsal side and yellowish in the ventral side. Propodus is reddish color in the dorsal side and off-white in the ventral side. Dactylus is white in color in the dorsal side, and white in the ventral side of the cheliped. The last four periopods are grayish in color in the dorsal side and yellowish in color in the ventral side.

Species Varuna litterata (Fabricius, 1798) is unknown (Figure 5(c)), the dorsal surface of the carapace is grayish, and the ventral surface is whitish. The cheliped is grayish and orange-grayish. The other legs are deep grayish and light grayish. Species Atergatis floridus (Linnaeus, 1767) is Male [♂] (Figure 5(d)), the dorsal surface of carapace is chocolate in color, and the ventral surface is yellowish. The cheliped is a combination of yellow, off-white, and black. The other legs are a combination of yellow and chocolate colors. Species Atergatis integerrimus (Lamarck, 1818) is Male [♂] (Figure 5(e)), the dorsal and ventral surfaces of the carapace are brown. The dactylus and propodus tip of the cheliped are black. The other legs are brown in color. Species Lophozozymus pictor (Fabricius, 1798) is Female [♀] (Figure 5(f)), the dorsal surface of carapace is gray spotted red, and the ventral surface is also gray spotted red. The cheliped is

a combination of white, red-white, chocolate, off-white, red-black, and black. The other legs are a combination of red-white, white, black-red, chocolate, and black. Species Xantho poressa (Olivi, 1792) is Male [♂] (Figure 5(g)), the dorsal surface of carapace is grayish. The ventral surface is whitish. The cheliped is off-white and black, other legs are ash and off-white in color. Species Zosymus aeneus (Linnaeus, 1758) is Male [♂] (Figure 5(h)), dorsal surface of carapace is reddish, and the ventral surface is off-white. The legs are off-white and reddish in color.

In this study, Calappidae, Epialtidae, Gecarcinidae, Grapsidae, Matutidae, Ocypodidae, Panopeidae, Sesarmidae, Varunidae, and Xanthidae families included 18 species whereas ten species (C. superciliosus, C. armatum, C. carnifex, O. pallidula, P. lacustris, P. bocourti, A. floridus, L. pictor, X. poressa, Z. aeneus) newly found in this south east region as distribution and habitat. Seven species (C. lophos, G. albolineatus, M. lunaris, O. pallidula, O. macrocera, P. lacustris, X. poressa) were recorded from marine water habitat at site 1 (20˚39'N and 92˚18'E); Five Species (C. armatum, C. carnifex, P. bocourti, Episesarma sp., V. litterata) were recorded from freshwater ditch is containing marsh habitat at site 2 (20˚38’N and 92˚19'E) and Five species were (C. superciliosus, A. floridus, A. integerrimus, L. pictor, Z. aeneus) recorded from marine water habitat at site 3 (20˚36'N and 92˚18'E) in Saint Martin’s Island. Species Uca rosea recorded from brackishand freshwater habitat at site 4 (22.3306˚N and 89.1028˚E) in Satkhira area, Bangladesh.

In the world, Bangladesh is the climatically vulnerable country for fisheries [28]. Researcher, twenty eight species reported from marine and freshwater [9] and most of the species used for human consumption [25] but still incomplete study with distinguishing morphometric and meristic character that has impact in the environment for long term sustain with other species. In crab culture have emerged employment and income opportunities [29] [30], but species introducer as distribution and character evolution is very few study happened in South East Asia. Species Criocarcinus superciliosus (Linnaeus, 1758) from china seas [31], Cardisoma armatum from Cape Verde and Senegal to Angola [32], Cardisoma carnifex (Herbst, 1796) from Kenyan mangrove swamp [33], Ocypode pallidula from Leigh Marine Reserve, New Zealand [34] but as distribution first time we find in marine water of Saint Martin’s Island. Panopeus lacustris has been introduced in Hawaii in the 1950s [35], Indian River Lagoon [36], Brazil [37] but as distribution for the first time we found in marine water of Saint Martin’s Island. Species Pseudosesarma bocourti under Sesarmidae family recorded from Japan [38], Kagoshima [39]; Thailand - Bangkok [40] but first time we found in marine water of Saint Martin’s Island that is very extensive and important for ecology and environment to develop history and sustainable cultural in the coastal region.

Xanthidae family Species Atergatis floridus recorded from Sri Lanka [41], Hainan Island and Paracel Islands in China [42], Indian Pacific Ocean [43], Lophozozymus pictor fromnorth-eastern Taiwan, Thailand [44] Hainan Island in Singapore [41], China [42]; Xantho poressa reported from Bulgaria and Turkey coasts. [45], and Zosymus aeneus reported from Okinawa in Japan [46], Hainan Island and Paracel Islands in China [42], Durban in South Africa [47] but as distribution first time introduced with distinguishing character in this South East Asia region of Bangladesh. Habitat modification and introduction of crab species combined to cause loss of biodiversity occurred through anthropogenic activities. However, due to habitat loss, climatic conditions, poor transportation, limitation and lack of funds and resources many species unidentified in marine and freshwater of Bangladesh. Lack of studies on crustacean species identity and importance in South East Asia these results could be significant for next generation researchers around the world with Bangladesh. Therefore, biological investigation and an improved understanding of crab species by local people, fishermen regarding the importance of marine ecosystem of crabs in the South East Asia.