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BackgroundMud crabs, Scylla spp., are commercially important large-size marine crustaceans in the Indo-West Pacific region. As females have the higher growth rate and economic value, the production of all female stocks is extremely essential in aquaculture. However, the sex determination mechanism is still unclear. Development of sex-specific genetic markers based on next-generation sequencing proved to be an effective tool for discovering sex determination system in various animals.ResultsRestriction-site associated DNA sequencing (RAD-seq) was employed to isolate sex-specific SNP markers for S. paramamosain. A total of 335.6 million raw reads were obtained from 20 individuals, of which 204.7 million were from 10 females and 130.9 million from 10 males. After sequence assembly and female-male comparison, 20 SNP markers were identified to be sex-specific. Furthermore, ten SNPs in a short sequence (285 bp) were confirmed heterozygous in females and homozygous in males in a large population by PCR amplification and sequencing. Subsequently, a female-specific primer was successfully designed according to the female-specific nucleotide which could amplify an expected band from females but not from males. Thus, a rapid and effective method for molecular sexing in S. paramamosain was developed, meanwhile, this method could successfully identify the sex of S. tranquebarica and S. serrata. Finally, nine and four female-specific SNP markers were detected in S. tranquebarica and S. serrata, respectively.ConclusionsSex-specific SNP markers were firstly identified in crab species and showed female heterogamety and male homogamety, which provided strong genetic evidence for a WZ/ZZ sex determination system in mud crabs S. paramamosain, S. tranquebarica and S. serrata. These findings will lay a solid foundation for the study of sex determination mechanism, sex chromosome evolution, and the development of mono-sex population in crustaceans.Electronic supplementary materialThe online version of this article (10.1186/s12864-018-5380-8) contains supplementary material, which is available to authorized users.

Two feeding experiments were conducted to determine if Brachionus plicatilis and Artemia salina nauplii were ingested by mud crab Scylla serrata larvae. In the first experiment, larvae were fed with increasing densities of Artemia nauplii with or without Brachionus to determine consumption with increasing densities of Artemia and with increasing zoeal stage. This experiment also aimed to determine if the presence of Brachionus as an alternative prey influenced the intake of Artemia by the crab larvae. There was generally an increase in intake with increasing densities of Artemia and increased consumption of Artemia as the larvae grew. Consumption of Brachionus was consistently high in all zoeal stages. There was a significant reduction in the intake of Brachionus with increasing consumption of Artemia in the early zoeal stages (Z1, Z2, Z3), but at later stages (Z4, Z5) the intake of Artemia was no longer affected by the presence of Brachionus. In the second experiment, daily ingestion within instar of zoeal stages and megalopa were compared. There was an increased consumption of Artemia nauplii on the day before molting and increased ingestion of Brachionus on the day after larvae had molted, except at Z3. Megalopae exhibited a decline in Artemia nauplii intake on the days before metamorphosis to crablet.

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An investigation was conducted to determine salinity and temperature effects on the survival and development of larvae and juveniles of the mud crab, Scylla serrata. Zoea, megalopae, and juveniles were reared at various salinity and temperature combinations: 15, 25, 35, and 45 ppt for zoea; 5, 15, 25, 35, and 45 ppt for megalopae; and 0, 5, 15, 25, 35, and 45 ppt for juveniles, at 20, 26, and 32 C. Salinity and temperature affected the survival of the zoea larvae, and there was an interaction between salinity and temperature. Salinity and temperature also affected the survival of megalopa and juveniles, but there was no interaction between salinity and temperature. There was high survival and metamorphosis on zoea reared at 25-35 ppt at 26 and 32 C. Megalopae developed to juveniles at 15-45 ppt at 20, 26, and 32 C. Juveniles survived and developed at 5-45 ppt at 20, 26, and 32 C. Salinity and temperature also affected the development and molt duration. Development was rapid at 25 and 35 ppt, at 32 C, but delayed at 15 and 45 ppt, at 20 and 26 C. Zoea benefited gradual transfer to 15 and 45 ppt but megalopa and juveniles did not. As the larvae grew, there was tolerance to wider salinity and low temperature.