Six iso-nitrogenous (350 g protein kg )1 ) and iso-caloric (4100 kcal kg )1 ) diets with or without probiotics supplementation namely T 1 (Basal feed (BF) without probiotics; control), T 2 (BF + Bacillus subtilis and Lactococcus lactis), T 3 (BF + L. lactis and Saccharomyces cerevisiae), T 4 (BF + B. subtilis and S. cerevisiae), T 5 (BF + B. subtilis, L. lactis and S. cerevisiae) and T 6 (BF + heat-killed bacteria of B. subtilis, L. lactis and S. cerevisiae) were fed to Labeo rohita fingerlings (6.0 ± 0.06 g) for 60 days in triplicate tanks (30 fish per tank). In all probiotic-supplemented diets, the probiotic concentration was maintained at 10 11 cfu kg )1 feed. After 60 days of culture, the fish fed combination of three probiotics at equal proportion (T 5 ) had higher (P < 0.05) growth, protein efficiency ratio, nutrient retention and digestibility and lower (P > 0.05) feed conversion ratio over other treatment groups. Total heterotrophic bacterial population in intestine was drastically reduced on 15th and 30th days of sampling than the initial value (0 day of sampling) for T 3 , T 4 and T 5 groups. Except T 6 , the gut colonization of respective probiotics, which were supplemented through the diets, was also increased up to 30 days of culture of fish and thereafter remained constant. KEY WORDS

A diverse array of sex determination (SD) mechanisms, encompassing environmental to genetic, have been found to exist among vertebrates, covering a spectrum from fixed SD mechanisms (mammals) to functional sex change in fishes (sequential hermaphroditic fishes). A major landmark in vertebrate SD was the discovery of the SRY gene in 1990. Since that time, many attempts to clone an SRY ortholog from non-mammalian vertebrates remained unsuccessful, until 2002, when DMY/DMRT1BY was discovered as the SD gene of a small fish, medaka. Surprisingly, however, DMY/DMRT1BYwas found in only two species among more than 20 species of medaka, suggesting a large diversity of SD genes among vertebrates. Considerable progress has been made over the last 3 decades, such that it is now possible to formulate reasonable paradigms of how SD and gonadal sex differentiation may work in some model vertebrate species. This review outlines our current understanding of vertebrate SD and gonadal sex differentiation, with a focus on the molecular and cellular mechanisms involved. An impressive number of genes and factors have been discovered that play important roles in testicular and ovarian differentiation. An antagonism between the male and female pathway genes exists in gonads during both sex differentiation and, surprisingly, even as adults, suggesting that, in addition to sex-changing fishes, gonochoristic vertebrates including mice maintain some degree of gonadal sexual plasticity into adulthood. Importantly, reviewing various SD mechanisms among vertebrates suggest that this is the ideal biological event that can make us understand the evolutionary conundrums underlying speciation and species diversity.

Summary To meet the ever‐increasing demand for animal protein, aquaculture continuously requires new techniques to increase the production yield. However, with every step towards intensification of aquaculture practices, there is an increase in stress level on the animal as well as on the environment. Feeding practices in aqua farming usually plays an important role, and the addition of various additives to a balanced feed formula to achieve better growth is a common practice among the fish and shrimp culturists. Probiotics, also known as ‘bio‐friendly agents’, such as LAB (Lactobacillus), yeasts and Bacillus sp., can be introduced into the culture environment to control and compete with pathogenic bacteria as well as to promote the growth of the cultured organisms. In addition, probiotics are non‐pathogenic and non‐toxic micro‐organisms, having no undesirable side effects when administered to aquatic organisms. Probiotics are also known to play an important role in developing innate immunity among the fishes, and hence help them to fight against any pathogenic bacterias as well as against environmental stressors. The present review is a brief but informative compilation of the different essential and desirable traits of probiotics, their mode of action and their useful effects on fishes. The review also highlights the role of probiotics in helping the fishes to combat against the different physical, chemical and biological stress.