Influence of salinity on population growth and reproductive performance of the rotifer, Brachionus plicatilis

Introduction

Rotifers are widely used as live feed organisms in commercial fish and crustacean hatcheries throughout the world. Their suitability as a culture organism is mainly due to their planktonic nature, small size, slow swimming ability, high reproductive rate, tolerance to a wide range of environmental conditions and ability to be cultured at high densities. Several rotifer species such as Brachionus plicatilis, Brachionus rotundiformis, Brachionus calyciflorus, Brachionus angularis, Brachionus rubens and Brachionus patulus are commonly used as live food for marine, brackishwater and freshwater fish larvae and crustacean culture (Anitha et al., 2015) [5] . Among these species, Brachionus rotundiformis is widely preferred in marine larviculture due to its smaller body size, which makes it suitable for feeding fish larvae with small mouth openings. The ability of rotifers to tolerate different salinity conditions makes them highly valuable for aquaculture production. Brachionus species can survive over a wide salinity range, although optimum growth is generally observed at lower salinity levels below 35 ppt (Lavens & Sorgeloos, 1996) [13] . Salinity is one of the most important environmental factors influencing the biological characteristics and population dynamics of rotifers. It affects important physiological processes including osmoregulation, reproduction, growth and survival. Rotifers maintain ionic balance between their body fluids and surrounding environment, and changes in salinity influence energy allocation between reproduction and survival. Previous studies have shown that salinity significantly affects fecundity, developmental time and lifespan of Brachionus species (Snell, 1986; Miracle & Serra, 1989) [19,17] . The salinity requirement of B. rotundiformis varies among strains and geographical populations. Different studies have reported optimum salinity ranges from 11 ppt to 25 ppt for better growth and reproduction (Hagiwara et al., 1995) [11] . Therefore, understanding the effect of salinity on population growth is essential for optimizing mass culture techniques and ensuring a continuous supply of high-quality live feed for aquaculture systems.

Materials and Methods

The experiment was conducted at the Department of Aquaculture, College of Fisheries Science, Kamdhenu University, Veraval. Rotifer samples were collected during April and May from three coastal locations of Veraval, Gujarat: Triveni Sangam (20.884°N, 70.412°E), Devka River (20.927°N, 70.353°E) and Jaleshwer (20.9169°N, 70.344896°E) using fine plankton nets of 45 µm and 60 µm mesh size (Thomas et al., 2016) [20] . The collected samples were examined under a compound microscope for species identification and preserved in 4% formalin (Anitha & George, 2006) [3] . Brachionus plicatilis was selected and stock cultures were maintained under laboratory conditions following standard rotifer culture techniques (Lubzens et al., 2001) [15] . Rotifers were fed with Nannochloropsis oculata, and algal density was maintained using a Neubauer haemocytometer (Brown et al., 1993; Andersen, 2005) [7,1] . Seawater used for culture was pretreated by mechanical filtration and chlorination to remove unwanted microorganisms and contaminants. Filtered seawater was disinfected with sodium hypochlorite and residual chlorine was neutralized using sodium thiosulphate before use (Creswell, 2010; Madhu, 2009) [9,16] . Five salinity treatments were prepared using treated seawater diluted with distilled water as S₁ (10 ppt), S₂ (15 ppt), S₃ (20 ppt), S₄ (25 ppt) and S₅ (30 ppt) to evaluate the effect of different salinity levels on rotifer population growth. The required salinity levels were prepared using the formula V₁S₁ = V₂S₂ (Boyd, 1998) [6] . The experiment was conducted in a Completely Randomized Design (CRD) with five salinity treatments and three replications. Rotifers were stocked at an initial density of 10 individuals mL⁻¹ and cultured for 10 days. Daily sampling was carried out, and population density was estimated using a Sedgewick-Rafter counting chamber. Specific population growth rate was calculated according to Hoff and Snell (1987). The obtained data were analysed using two-way ANOVA under CRD, and treatment differences were tested following Snedecor and Cochran (2014) [18] .

Results

Effect of Salinity on Population Density of Brachionus plicatilis

The population density of Brachionus plicatilis showed a gradual increase from day 1 to day 10 under all salinity treatments (S₁-S₅). The overall mean population density varied significantly among the treatments. The highest population density was recorded in S₂ (

Effect of Salinity on Population Growth Rate of

Brachionus plicatilis

The specific population growth rate of Brachionus plicatilis varied significantly under different salinity treatments. The highest growth rate was recorded in S₂ (15 ppt) (0.34 day⁻¹) followed by S₃ (20 ppt) (0.32 day⁻¹), while the lowest growth rate was observed in S₅ (30 ppt) (0.21 day⁻¹). The treatments S₁ (10 ppt) and S₄ (25 ppt) showed intermediate growth rates. Statistical analysis showed significant differences (p<0.05) among salinity treatments.

During the culture period, the growth rate increased up to the peak growth phase and declined slightly towards the end of the experiment. The maximum population growth rate in 15 ppt salinity indicated that this salinity level was more suitable for reproduction and multiplication The lower population density at higher salinity levels (25 and 30 ppt) may be attributed to increased osmotic stress, which affects feeding activity, reproduction and physiological balance of rotifers. Similar results were reported by Lee et al. (2022) [14] , who observed reduced reproductive performance of Brachionus rotundiformis at higher salinity levels. Therefore, maintaining optimum salinity is important for successful mass culture of B. plicatilis in aquaculture systems.

Effect of Salinity on Population Growth Rate of

Brachionus plicatilis

The present study showed that salinity significantly affected the specific population growth rate of Brachionus plicatilis. The maximum growth rate was observed at 15 ppt salinity, followed by 20 ppt, indicating that moderate salinity conditions were favourable for rotifer reproduction and population increase. Similar results were reported by Snell (1986) [19] and Miracle and Serra (1989) [17] , who observed that salinity influences reproductive activity and growth rate of Brachionus species. The higher growth rate at moderate salinity may be due to lower energy requirements for osmoregulation, allowing more energy to be utilized for feeding, reproduction and population development. At higher salinities (25-30 ppt), the decrease in growth rate may be related to osmotic stress, which affects physiological activities and reduces reproductive efficiency. Lubzens et al. (2001) [15] also reported that optimum salinity conditions are essential for maintaining high rotifer productivity in aquaculture systems.

The findings suggest that 15-20 ppt salinity is suitable for improving population growth and mass culture of B. plicatilis as a live feed organism for aquaculture.

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