﻿Two new records and an updated checklist of freshwater crabs (Arthropoda, Malacostraca, Decapoda, Potamidae and Gecarcinucidae) from Bangladesh

﻿Abstract The species diversity of freshwater crabs of Bangladesh is poorly known. In this study, Acanthopotamonfungosum (Alcock, 1909) and Maydelliathelphusaedentula (Alcock, 1909) are reported as new records from Bangladesh. The two species were identified through morphology and molecular phylogeny based on 16S rDNA gene sequences. Herein, diagnostic characters of both species are provided respective to close congeners. There is concern over the conservation status of A.fungosum due to its narrow distributional range. In addition, an updated checklist and a key to the freshwater crabs of Bangladesh are provided.


Introduction
Freshwater crabs are common in the tropical and subtropical regions where they inhabit a wide range of habitats such as rivers, swamps, lakes, and caves (Yeo et al. 2008).Bangladesh is a riverine country and has various bodies of freshwater suitable for crabs.For example, the hilly areas of the districts of Khagrachari, Rangamati, Bandarban, Chattogram, Cox's Bazar, Mymensingh, Netrokona, Sylhet, Moulvibazar, and Habiganj have watersheds consisting of rivers, lakes, streams, and waterfalls that harbour unique aquatic biodiversity, including crabs.The wetlands at Manikganj, Jessore, Narail and Rajshahi, and the Halda and Meghna rivers are also suitable habitats for crabs (Hasan and Rashid 2016).

Material and method
During a field survey in August 2021, two specimens of freshwater crab were collected from a small hilly stream near the Chittagong University Campus, Chattogram and from the Kangsa River, Mymensingh (Fig. 1).Crabs were collected by hand and with a fishing net.Both specimens were photographed, diagnosed based on their morphometric characters, preserved in 95% ethanol, and had tissue sampled for molecular study.These specimens were transferred to the laboratory of Institute of Marine Sciences (IMS), University of Chittagong (CU) for further study.The specimens were identified following the taxonomic keys of Pati et al. (2019) and Das (2021).The two crab specimens were deposited in the IM-SCU (voucher numbers IMSCU/FW-crab2108.01 and IMSCU/FW-crab2108.02) for future reference.The following abbreviations used are: G1 for the male first gonopods; G2 for the male second gonopods.The terminology of morphological characteristics used follows that of Ng (1988) and Davie et al. (2015).Genomic DNA was extracted from the gill tissue using Trelief Animal Genomic DNA kit (Tsingke, China) according to the manufacturer's protocol and sequenced with an Illumina HiSeq X Ten platform (150 bp paired end).Full-length mitochondrial 16S sequences were assembled using MITOZ (Meng et al. 2019).Sequences were uploaded to NCBI GenBank with accession numbers OQ788486 and OQ788487.Together with downloaded sequences, the matrix was aligned with MAFFT (Katoh and Standley 2013) using the default setting.IQ-TREE (Nguyen et al. 2015) was employed to conduct a maximum-likelihood (ML) analysis.Node supports were obtained through 1,000 ultra-fast bootstrap replicates (Minh et al. 2013).The phylogenetic tree was visualized using ITOL (Letunic and Bork 2019).Material examined.1 male, 28.84 × 24.28 mm (Table 1), Chittagong University Campus, Chattogram, Bangladesh, 22°28'07"N, 91°46'48"E; 15 August 2021, collected by Shibly Sadique Shashi.
The IUCN conservation status of A. fungosum was assessed as Data Deficient (DD) (Cumberlidge 2008a).The species is distributed over a small geographical area, i.e., eastern Bangladesh and southern Assam, India.Due to restricted distributional range and increasing threats to freshwater habitats of this region from various human activities, A. fungosum is likely more threatened than M. edentula.Further field surveys are needed to determine population size and threats.
Chelipeds surface smooth, unequal, right cheliped larger; carpus with distinct spine on inner angle; fingers longer than palm, movable finger strongly curved downward, immovable finger smoothly curved upward, wide gap between dactyl and pollex when closed, movable finger comparatively larger than immovable finger, inner margin of fingers lined with numerous round and blunt teeth (Fig. 3d); ambulatory legs stout; second pair of ambulatories longest while the fourth pair shortest (Fig. 3b); dactylus slender, longer than propodus, with 4 rows of spines on the margin (Fig. 3b).
The IUCN conservation status of M. edentula is Near Threatened (NT) because of its limited distribution range and vulnerable habitat (Cumberlidge 2008b).Although the current study has expanded its known geographic distribution, the conservation status of this species is still not optimistic.In western Bangladesh, some local people, especially fishermen, eat this crab on a limited scale.In addition, various types of human activities, like pollution, urbanization and sand mining are impacting freshwater habitats in that area and, consequently, are threats to the population of M. edentula.
Until now, five species belonging to the genus Maydelliathelphusa have been recorded from India, Bhutan, and Nepal (Cumberlidge 2008c;Valarmathi 2017;Das 2021).The morphology of the carapace of M. edentula is superficially similar to that of other four species.However, M. edentula can be easily distinguished externally by its large, distinctively asymmetric chelipeds and its united epigastric and postorbital cristae (Bott 1970;Das 2021).

Check list
Herein, two freshwater crab species, A. fungosum and M. edentula, are documented for the first time from Bangladesh.A molecular phylogeny based on 16S rDNA sequences confirmed their identification (Fig. 5).Specifically, A. fungosum and M. edentula are clustered separately with A. panningi and M. lugubris.With these two species included, there are now seven species of true freshwater crab known to inhabit Bangladesh, namely Acanthopotamon fungosum, A. martensi, Lobothelphusa woodmasoni, Lamella lamellifrons, Maydelliathelphusa edentula, Sartoriana spinigera, and S. trilobata (Table 2).

Figure 1 .
Figure 1.Map showing the collection points of two crab species from Bangladesh.Station 1: Acanthopotamon fungosum collected from Chittagong University Campus, Chattogram.Station 2: Maydelliathelphusa edentula collected from the Kangsa River, Mymensingh.

Figure 2 .
Figure 2. Acanthopotamon fungosum a dorsal view b ventral view c frontal view d major chela e minor chela.Scale bars: 1 cm.

Figure 3 .
Figure 3. Maydelliathelphusa edentula a dorsal view b ventral view c frontal view d major chela and e minor chela.Scale bars: 2 cm.

Figure 5 .
Figure 5. Reconstructed maximum-mikelihood tree based on 16S rDNA sequences.Newly obtained sequences are shown in red.Numbers on branches represent bootstrap values.