Some Aspects of Biology and Nutrition of Scylla serrata (Forskal, 1755) From the South Western Region of Bangladesh

Abstract

Some aspects of biology, viz. size-frequency distribution, sex ratio, length-weight relationship and relative condition factor, and the proximate biochemical composition of different body parts, viz. gill, shell, meat, hepatopancreas and egg tissue of the mud crab (Scylla serrata, Forskal 1755), were studied from the SW region of Bangladesh for one year at monthly intervals in 2001. The maximum number of the mud crab (Scylla serrata) harvested from the SW region was between 60 and 100 mm external carapace width in size. About 82.15% (1,284 individuals out of 1,563) of the crabs caught were within this size range. Again, 336 female crabs out of 697 samples were within the 70–80 mm size class. The highest number of males, 216 individuals out of 866 samples, were within the 80–90 mm size class. Crabs caught from the high-salinity zone were larger than those from the low-salinity zone. The maximum and minimum sizes of mud crab in the samples were 131 mm and 31 mm, respectively. The overall sex ratio of mud crab in the sample was 1:0.81 (M:F). The sex ratio did not vary with geographic location; in the low- and high-salinity zones, the ratios were 1:0.81 and 1:0.79, respectively. The highest and lowest sex ratios were found in May (1:1.561) and July (1:0.408), respectively. Male crabs dominated over females throughout the year except in May and December. In the low- and high-salinity zones, females dominated over males in May and November, and in March and May, respectively. The ECW-weight relationship for males and females in the SW region was W = 0.3327 L²•⁷⁸⁷⁷ (r = 0.89; n = 866) and W = 1.1138 L¹•³¹⁰⁴ (r = 0.89; n = 697), respectively. The combined ECW-weight relationship for males and females was W = 1.2491 L¹•⁴⁶⁷ (r = 0.94; n = 1,563). The condition of the crab was better in all size classes except the 70–100 mm size class. This may be due to mating and spawning of crabs. The proximate composition of different body parts of male and female mud crabs from two different geographical locations showed variation among body parts, sexes and geographical locations. The protein content was found to be highest in the ovary (27.57%) of females and lowest (7.86%) in the shell. Male body meat contained 16.34% protein, followed by Hepatopancreas (9.78%), gill tissue (9.17%) and carapace shell (7.86%). The fat content was found to be the highest in hepatopancreas (13.67%), followed by ovary (3.90%), carapace shell (1.34%), gill tissue (0.64%) and body meat (0.38%). The maximum ash content was found in carapace shell (33.83%), which was followed by gill tissue (3.91%), ovary (2.35%), hepatopancreas (3.34%) and body meat (2.18%). The highest moisture content was noticed in gill tissue (84.02%), followed by body meat (79.83%), hepatopancreas (68.83%), ovary (54.23%) and carapace shell (47.71%). Moisture content in all body parts was higher during the post-monsoon season than during the monsoon season. Protein content in the gill, meat, hepatopancreas and egg was higher during the monsoon than during the post-monsoon season. Ash content in gill, shell, meat and egg was higher during the monsoon, whereas in hepatopancreas it was higher during the post-monsoon season. Lipid content in shell and hepatopancreas was higher during the monsoon, while in gill, meat and egg it was higher during the post-monsoon season. Carbohydrate content in all body parts was higher during the monsoon than during the post-monsoon season. Research on mud crab (Scylla serrata) in Bangladesh has not yet been conducted sufficiently. Therefore, people have limited information about its biology and nutritional value. As a result, the domestic market for mud crab has not yet developed in this region. Commercial culture has also not yet been developed in the southwest region. If detailed biological and biochemical research can be conducted on this species, this commercially important species will become more desirable in Bangladesh as well as in foreign markets.