A Study and Availability Assessment of Freshwater Crabs in the Hill Streams of Bangladesh

A study of crabs was conducted at eight hill streams, located in three different districts of Bangladesh, during January to December, 2012 with a view to assess the availability and to study the taxonomy of freshwater crabs. The study investigated species diversity which includes their taxonomic description, biometric data, habitat, ecological note, distribution, and ecological role. The specimens were collected in different times covering the major seasons rounded the year by using various fishing gears such as cast net, push net, dip net, seine net and trap. A total four species of freshwater crabs under two families (Potamidae and Grapsidae) were recorded. The identified four crabs were Sartoriana spinigera, Sartoriana trilobata, Labothelphusa wood-masoni and Pyxidognathus fluviatilis. Among these the first three were under the family Potamidae and the last one under the family Garapsidae. Sartoriana trilobata was the dominant species in the hill streams of Bangladesh.


Introduction
Bangladesh is a country which is mostly low in elevation and the hill areas occur in the Khagrachari, Rangamati, Bandarban, Chittagong, Cox's Bazar, Mymensingh, Netrokona, Sylhet, Moulavibazar and Habiganj districts. These regions include a remarkable area of hilly rivers, lakes and streams and waterfalls that are identified to be an excellent playground of biodiversity. Various small and moderately large streams have been found all over the country which are very rich of ecologically important species. So streams of hilly areas are predicted to be the biodiversity hot spots in the country containing a high diversity of crab species that have yet to discover.
Freshwater crabs are found throughout the tropical and sub-tropical regions of the world. They live in a wide range of water bodies, from fast-flowing rivers to swamps, as well as in tree boles or caves. The majority of species are narrow endemics, occurring in only a small geographical area. This is at least partly attributable to their poor dispersal abilities and low fecundity [1] and to habitat fragmentation caused by the world's human population [2]. Freshwater crabs of Bangladesh can distinguished from false crabs by not having 5th pair of periopods totally or partly having concealed beneath the carapace, the antennae were always placed between the inner margin of orbit and fused pterygostomial region with endestome [3]. In most decapods, the gonopores (sexual openings) are found on the legs. However, since crabs use the first two pairs of pleopods (abdominal appendages) for sperm transfer, this arrangement has changed. As the male abdomen evolved into a narrower shape, the gonopores have moved towards the midline, away from the legs, and onto the sternum [4]. The hilly crab faunas of Bangladesh have never been broadly surveyed by researchers. It is necessary to know the biodiversity of freshwater crabs in hilly region of Bangladesh for conservation purpose. So the present study was focused to identify crab species in the selected waterfalls, rivers and      slide calipers and scale. The units of the measurement were taken in cm. The selected morphometric or morphological characteristics for identifications of specimens were taken from the fresh and preserved specimens.

Statistical analysis
To estimate the mean value and standard deviation as well as to determine the correlation among carapace length (CL), carapace width (CW), data were subsequently analyzed. Correlation was calculated by using Pearson correlation equation the positive values of one will match up with positive and negative values of the other randomly, and the same with negative values of the first variable. Therefore when we take the sum of ( )( ) , all these positive and negative results will tend to cancel each other out, making r close to 0.
The greatest percentage of crabs found in hill streams was Sartoriana trilobata and which was 43.33% and dominant species in the hill streams of Bangladesh. The lowest abundance recorded was 6.67% for Pyxidognathus fluviatilis of the total specimens. Also the figure shows that Labothelphusa wood-masoni was 30% and Sartoriana spinigera was 20% of the studied specimens. (Figure 2). (Figure 4), and S. trilobata ( Figure 5), are closely related species under same genus. There are some dissimilarities present in their morphology; S. trilobata is more robust than Where Carapace length X represents the values of independent variable and Carapace width Y represents the values of dependent variables. X And Y denotes the average values.

S. spinigera
The value of r ranges between +1 and -1: • r > 0 indicates a positive relationship of X and Y: as one gets larger, the other gets larger.
• r < 0 indicates a negative relationship: as one gets larger, the other gets smaller. x X y Y − − , and r, will be negative. If we calculate the Pearson correlation of X with itself, the result will be 1.
If there is no association between X and Y, there will be no "+" is denoted for the status found. "-" is denoted for the status not found Sartoriana spinigera and carapace of S. spinigera having only one antero-lateral spine which is absent in S. trilobata. Labothelphusa wood-masoni (Figure 3), differs from congeners by a combination of following characters: having 4 spines on antero-lateral side, cheliped equal in both sexes. Pyxidognathus fluviatilis (Figure 6), is clearly remarked by having some characteristics: antero-lateral margin having two spines (3 when eye groove consider as spine), upper and lower margin of propodus and dactylus having setae. They are found in freshwater ecosystem of Rijuk Waterfall and Sangu River.
The habitat of the crabs in the study sides are greatly varied from transparent clear water to turbid flowing water. The water qualities were measured and it was fine according to the accepted standard. The basic water qualities of seven sites such water temperature 20-28 o C, pH was 6.5-6.7, Total dissolved solids (TDS) was 225-227 (µs) and dissolved oxygen (DO) 10-12.4 mg/L. Water was slightly turbid and acidic and pH near 4 at the Kudung cave. In winter clear water flows and in rainy season turbid water flows in Sangu River. Rock stones dominated the core gene of the soil at Boga Lake which is composed of small and hard rocks. The water color of this lake is blue-green and it is important habitat of freshwater organisms. The streams of Barachara were mostly shallow but deep in some areas and the water was crystal clear (Tables 1,2).
Biometrical study such as Carapace length and Width,Abdominal length, Telson length, Merus length, Carpal length and Palm length of 4 studied crabs were measured (Tables 3-6). Correlation of Carapace length and Carapace width were measured. The value of correlation coefficient ( r ) is 0.848 for Sartoriana spinigera, 0.996 for Sartoriana trilobata,, 0.989 for Labothelphusa wood-masoni and 1.00 for Pyxidognathus fluviatilis which implies that there is a strong positive linear association between the variables, carapace length     and carapace width (Figures 7-10). Correlation is significant at the 0.05 level (2-tailed) for S.spinigera while 0.01 level (2-tailed) for S. trilobata, Labothelphusa wood-masoni and Pyxidognathus fluviatilis.

Conclusion
There was no sufficient previous information of crabs in hill streams and thus the comparison of the present findings with previous one was not possible. Total 4 species of crabs were found in this study. Among the 4 species, Sartoriana trilobata (Alcock, 1909) is an IUCN [9] red listed species and this species is under data deficiency conservation rank. Moreover the present study mainly focused on morphological characteristics and habitats of crab species. The hill streams of Bangladesh are regarded as hot spot of biodiversity Carapace width (cm) Carapace length (cm) Figure 9: Labothelphusa wood-masoni.  and Crabs have enhanced this biodiversity. Moreover, tribal people of Bangladesh depend on crabs for food. The present study signals that the hilly area contains a rich biodiversity which are usually less known than other parts of the country and it strongly suggests the further extensive survey of all hilly districts of Bangladesh by long term basis.