Effects of dietary protein and energy levels on growth and lipid composition of juvenile snail

Journal of Shellfisheries Research, Jan, 2005 by Sang-Min Lee, Tae-Jun Lim

ABSTRACT A feeding trial of 5 dietary protein levels (12%, 22%, 32%, 42%, and 52%) and 2 dietary energy levels (3.3 and 3.9 kcal/g diet) factorial design with three replicates was conducted to investigate the proper dietary protein and energy levels for the growth of the snail (Semisulcospira gottschei). Snails, initial averaging 37 mg, were fed the experimental diets for 12 wk. Survival of each group was all above 80% and no significant difference among treatments. Mean weight gain of the snails was improved with increasing dietary protein level up to 22% and 32% at 3.3 and 3.9 kcal/g diets (P < 0.05), respectively, and reached a plateau above these levels (P > 0.05). Mean weight gain of snails fed the 22% protein diets with 3.3 kcal/g diet was not significantly (P > 0.05) different from that of snails fed the 32% to 52% protein diets with both energy levels. Lipid content of snails fed the 3.9 kcal/g diets showed higher values than that of the 3.3 kcal/g diet at the same protein level. Snails fed the 3.9 kcal/g diets showed a tendency toward to higher in 18:1 n-9, 18:2n-6, 18:3n-3, and 22:6n-3 and lower in 20:4n-6 and 22:1n-9 than those of snails fed the 3.3 kcal/g diets energy diets. The results of this study indicate that a diet containing 22% protein and 3.3 kcal/g diet with P/E ratio of 69 mg protein/kcal was recommended for snail growth.

KEY WORDS: dietary protein and energy, snail, Semisulcospira gottschei

INTRODUCTION

Protein is one of the most important nutrient affecting animal growth and the feed cost. Protein requirements have been studied in aquaculture species with the aim of determining the minimum amount required to produce maximum growth and not be used for energy. The protein requirement of fish varies with fish species, fish size, dietary protein quality, and environmental conditions (NRC 1993). The non protein energy levels may also influence the dietary protein requirement of fish. When insufficient non-protein energy is available in feeds, dietary protein is deaminated in the body to supply energy for metabolism rather than being used for tissue growth, and excreted ammonia can reduce water quality (Phillips 1972, Shyong et al. 1998). Because fish consume food to satisfy their energy requirement, excess dietary energy may limit intake of essential nutrients like protein and amino acids. Thus, excesses of energy can lead to growth reduction and increase fat deposition in fish (Daniels & Robinson 1986).

The snail (Semisulcospira gottschei) is becoming a candidate shellfish for aquaculture, because this species has high consumer's demand as the health food in Korea. However, limited study has been performed on nutritional requirements of this species except for essential fatty acids requirement and carbohydrate use (Lee et al. 2002, Lim et al. 2003). Therefore, this study investigates the effects of dietary protein and energy levels on growth and lipid composition of snails.

MATERIALS AND METHODS

Experimental Diets

A 5 x 2 factorial design using three replications was used. Ten experimental diets were formulated to contain 5 protein levels (12%, 22%, 32%, 42%, and 52%) and 2 energy levels (3.3 and 3.9 kcal/g diet). Gross energy levels of diets were calculated based on 4 kcal/g protein, 9 kcal/g lipid and 4 kcal/g N-free extract (Garling & Wilson 1976). Ingredients and nutrients contents of the experimental diets, and fatty acid compositions of dietary lipid sources are presented in Table 1 and Table 2, respectively. Casein increased mainly at the expense dextrin to increase the protein level in diets. The mixture of squid liver oil, linseed oil, and corn oil was added 4.5% and 12% for low and high-energy diets, respectively, in each protein level. Procedures for feed preparation were adapted from the method of Mat et al. (1995a) who studied for abalone feed. Experimental diets were dried at room temperature and stored at -25[degrees]C until used. Crude lipid contents in some experimental diets were somewhat reduced during the process of rolling and dipping into the solution of Ca[Cl.sub.2] to make a flake type of feed.

Experimental Conditions

Juvenile snails (Semisulcospira gottschei) were obtained from a private hatchery (Pyungchang, Korea). They were acclimated to a recirculating system in our laboratory for 1 wk by feeding a commercial abalone diet containing 30% protein and 5% lipid. They were then randomly redistributed into 25-L tanks (20 L of water each) at a density of 100 snails (37 mg/snail) per tank. Three replicate groups of snails were fed once in 2 days for 12 wk. Before feeding, uneaten diets in each tank were cleaned by siphoning and 20% of water volume in system was replaced with fresh water every 2 days. Fresh water was supplied at a flow rate of approximately 0.3 L/min in the recirculating system. Photoperiod was left at the natural condition, and water temperature ranged from 15[degrees]C to 22[degrees]C during the feeding trial. Snails in each tank were collectively weighed on the day of initiation and on the day of termination of the feeding trial, after being fasted for 24 h.

Sample Collection and Chemical Analysis

Three hundred snail samples at the beginning and all snails at the end of the feeding trial were sacrificed and stored at -70[degrees]C for chemical analysis. Proximate composition of experimental diets and snails were analyzed as follows. Crude protein was determined by Kjeldahl method using Auto Kjeldahl System (Buchi B-324/ 435/412, Switzerland). Crude lipid was determined with ether extraction in a soxhlet extractor, moisture was determined by dry oven (105[degrees]C for 12 h), and ash was determined by a muffle furnace (550[degrees]C for 4 h). Lipid for fatty acid analyses was extracted by the method of Folch et al. (1957), and fatty acid methyl esters were prepared by transesterification with 14% B[F.sub.3]-MeOH (Sigma, Chemical Co., USA). Fatty acid methyl esters were analyzed by using a gas chromatography (HP 5890, Hewlett-Packard, USA) with flame ionization detector, equipped with HP-INNOWax capillary column (30 m x 0.32 mm i.d., film thickness 0.5 [micro]m, Hewlett-Packard, USA). Injector and detector temperatures were 250[degrees]C and 270[degrees]C, respectively. The column temperature was programmed from 170[degrees]C to 225[degrees]C at a rate of 1[degrees]C/min. Helium was used as the carrier gas. Fatty acids were identified by comparison with retention times of the standard fatty acid methyl esters (Sigma, USA).