Sample collection: About two hundred fifty wild mud crab samples were collected from each of the three coastal districts along the eastern (Cox's Bazar and Chittagong) and western (Bagerhat) regions of the coast in the Bay of Bengal by the help of local fishermen from June 2017 to April 2018. For Bagerhat region (adjacent to the Sundarbans mangrove forest), crabs were collected from the Mongla (22.48°N 89.61°E), while crabs were collected from the Maheshkahli Channel (21.55°N 91.95°E)for Cox's Bazar region's samples and from the Sandip (22.47°N 91.54°E) for Chittagong region's crab samples. After sample collection, the in-tensive morphological and morphometric studies were carried out at the Fisheries Oceanography Laboratory, Department of Marine Bior-esource Science, Chattogram Veterinary and Animal Sciences University, Bangladesh. Before proceeding for intensive morphometric analysis, specimens were broadly assigned to S. olivacea and S. serrata based on the morphological characters provided by Estampador (1949), Kathirval andSrinivasagam (1992), Keenan et al. (1998) and Jirapunpipat et al.(2008). The shape and number of carpus spine was considered for primary identification between S. serrata and S. olivacea. Primarily, S.olivacea was identified by blunted outer and absent inner carpus spine and S. serrata by prominent outer and blunted inner campus spine. Subsequently, S. olivacea were more precisely identifiedby distinct morphological features of blunted frontal lobe, thick and slightly curved dactylus, blunted outer and inner propodus spine, absence of polygonal patterns on body and legs. In contrast, S. serrata were identified by distinct morphological features of bluntly pointed frontal lobe, elongated dactylus, prominent outer and inner propodus spine, presence of polygonal patterns on body and legs. For further taxonomic identification of crabs, the 1st and 2nd male gonopod were removed carefully using clean forceps, and studied under a microscope(Optika microscope Italy, B-190 Series) to observe their structure. Afterward, S. olivacea was further confirmed by a narrow mouth tip of 1st male gonopod and wider bilobed structure of 2nd male gonopod and S. serrata was confirmed by wide and thick mouth tip of 1st male gonopod, and less wide bilobed structure of 2nd male gonopod. 2h). About hundred similar-sized (112.54 ± 5.26) default specimen with no missing appendages or broken parts for each of S. olivacaea (Cox's Bazar = 30, Chittagong = 35; Bagerhat = 35) and S. serrata (Cox's Bazar = 30, Chittagong = 30; Bagerhat = 40) were selected for detail morphometric analysis by using a multivariate approach. The male and female ratio were kept equal (1:1) to reduce the sexual dimorphism effects on the morphometric analysis. Twenty four morphometric characteristics include carapace width (CW), internal carapace width(ICW), carapace width at spine 8 (8CW), carapace length (CL), posterior width of carapace (PWC), 9th lateral spine height (LSH), frontal width(FW), frontal median spine height (FMSH), the distance between frontal median spines (DFMS), distance between frontal lateral spines (DFLS), sternum width (SW), abdomen width (AW), dactylus length (DL), propodus length (PL), propodus width (PW), propodus depth (PD), inner propodus spine (ICS), outer propodus spine (IPS), inner carpus spine(ICS), outer carpus spine (OCS), merus length (ML), 5th pereiopod dactyl length (5 PL), 5th pereiopod dactyl width (5 PW), 3rd pereiopod merus length (3PML) were measured by using digital calipers to the nearest 0.01 mm. Prior to the analysis, size effects were eliminated from the dataset for ensuring the morphological variations were attributed to body shape differences, and not to the relative sizes of the mud crabs (Hoqand Alam, 2018). Among the 24 morphometric measurements, 11morphometric lengths (ICW, 8CW, CL, PWC, SW, DL, PL, PW, ML, 5PLand 5 PW) were found to have significant linear correlations with the CW of the collected mud crabs. Therefore, size-dependent variations from these 11 morphological measurements were eliminated by adapting an algometric method as suggested by Elliot, Haskard, and Koslow (1995).