With the objective to develop an indoor tank seed rearing system for pearlspot (Etroplus suratensis), effects of three stocking densities in presence or absence of soil base were evaluated on growth, weight variation, survival and body composition in a 60‐day trial. The experiment had a 3 × 2 factorial design with three levels of stocking density (150, 300 and 450 fish m−3) and two levels of soil base (with and without) in triplicates. Pearlspot fry (27.5 ± 0.5 mm/0.39 ± 0.02 g) were stocked in experimental tanks (Length×Breadth×Height: 0.65 × 0.50 × 0.48 m) and fed with a formulated diet containing 29.85% crude protein. Increasing the stocking density from 150 to 450 fish m−3 significantly decreased the growth (average body weight and total length), daily weight gain, specific growth rate and survival (P < 0.001) and increased the feed conversion ratio (P < 0.01). Significantly lower growth and survival were observed in treatment having soil base compared with that of without soil base (P < 0.001). However, coefficients of variation at harvest weight and body composition of fingerlings were independent to stocking density and presence or absence of soil base (P > 0.05). A significant positive interaction effect of stocking density and presence or absence of soil base was observed on average body weight (P < 0.05) and survival (P < 0.001) of pearlspot fingerlings. This study suggests that stocking density of 150 fish m−3 without soil base in tanks would be appropriate for raising pearlspot fingerlings in brackishwater indoor seed rearing system.

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Reduced sulfur compounds produced by the metabolism are the one of the major problems in aquaculture. In the present study, herbivorous fishes have been cultured as biomanipulators for secretions of slime, which enhanced the production of greenwater containing beneficial bacteria. The genes encoding soxB which is largely unique to sulfur-oxidizing bacteria (SOB) due to its hydrolytic function has been targeted for examining the diversity of SOB in the green water system of coastal aquaculture. Novel sequences obtained based on the sequencing of metagenomic clone libraries for soxB genes revealed the abundance of SOB in green water system. Phylogenetic tree constructed from aligned amino acid sequences demonstrated that different clusters have only 82-93% match with Roseobacter sp., Phaeobacter sp., Roseovarius sp., Sulfitobacter sp., Ruegeria sp., and Oceanibulbus sp. The level of conservation of the soxB amino acid sequences ranged from 42% to 71%. 16S rRNA gene analyses of enrichment culture from green water system revealed the presence of Pseudoxanthomonas sp., which has 97% similarity with nutritionally fastidious Indian strain of Pseudoxanthomonas mexicana-a sulfur chemolithotrophic gamma-proteobacterium. Our results illustrate the relevance of SOB in the functioning of the green water system of coastal shrimp aquaculture for oxidation of reduced sulfur compounds, which in turn maintain the sulfide concentration well within the prescribed safe levels.

The grey mullet Mugil cephalus is one of the popular and fast growing fishes being cultured in tropical and subtropical regions. In the present study, histological observation of gonadal development and corresponding changes in sex steroid levels from different maturity stages of wild caught male and female were studied. In female, testosterone and 17β-estradiol increased with the advancement of maturation and reached peak (17β-estradiol, 323 ± 13 pg/ml; testosterone, 938 ± 7.87 pg/ml) in mature stage, whereas the level of progesterone was maximum (488 ± 4.9 pg/ml) during ripe stage. Vitellogenin level in serum showed a similar trend as 17β-estradiol. In case of male, the testosterone level in serum increased gradually with advancement of maturation and was maximum (1820 ± 40.25 pg/ml) during ripe stage, whereas significant decrease in 17β-estradiol and progesterone was noticed with advancement of maturation. The fundamental information from this investigation would be useful for developing protocol for accelerating maturation and spawning under captive condition.

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Spermatozoa morphology and fine structure were studied in the grey mullet, Mugil cephalus using light, scanning and transmission electron microscopy. Light microscopy observations indicate a semi-cystic type of spermatogenesis in the testis. The electron microscopy micrograph showed that the spermatozoon of M. cephalus is uniflagellated (total length 5.78 AE 1.26 lm), differentiated into an ovoid-shaped head without acrosome (1.80 AE 0.35 lm in length and 1.91 AE 0.30 lm in diameter), with a short midpiece and a long cylindrical flagellar tail (length 3.60 AE 0.50 lm). The midpiece is characterized by the presence of four to five vacuoles, a cytoplasmic canal, two centriole and two spherical mitochondria having a flat type of cristae. Chromatin granules of the nucleus form an electron-dense homogeneous mass. The flagellum consists of nine peripheral microtubules and a central pair (9 + 2) surrounded by the plasma membrane with side fins. The results confirm that spermatozoa of M. cephalus are perciform or teleostean type II. Information generated from the present study will be useful in taxonomic classification, cryopreservation and breeding work.