Comparative Growth and Survival of Reciprocal Hybrids Between Clarias gariepinus and Clarias anguillaris

Reciprocal, interspecific hybrids of Clarias gariepinus and Clarias anguillaris were produced via hypophysation and dry stripping. Two-week-old fry (0.06 ± 0.02 g) of the parental Clarias species and their interspecific hybrids were stocked in duplicated 1 m³ hapas suspended in outdoor earthen ponds (16 m²) for a rearing period of 56 days. Growth and survival of the hybrids was intermediate to the faster growing and better surviving C. gariepinus and the more slowly growing and more vulnerable C. anguillaris parental lines. The weight gain attained by C. gariepinus, C. anguillaris, C. gariepinus (♂)?×?C. anguillaris (♀), and C. anguillaris (♂)?×?C. gariepinus (♀) were 3.18 g, 2.99 g, 3.16 g and 3.05 g, respectively. Among hybrids, the growth of C. gariepinus (♂)?×?C. gariepinus (♀) was higher than the reciprocal C. anguillaris (♂)?×?C. gariepinus (♀). Heterosis for growth was a negligible 0.65% and for survival was negative (?5%). C. gariepinus appears to be the best species for aquaculture in terms of growth rate and survival.     

Spawning performance and seed production from hybridization between Oreochromis spilurus and the GIFT strain of the Nile tilapia Oreochromis niloticus

This study compared the spawning performance of four crosses between the salt‐tolerant tilapia Oreochromis spilurus and the fast‐growing genetically improved farmed tilapia (GIFT) strain of the Nile tilapia Oreochromis niloticus: (1) O. spilurus♀×O. spilurus♂ (SS); (2) Nile ♀× Nile ♂ (NN); (3) Nile ♀×O. spilurus♂ (NS) and (4) O. spilurus♀× Nile ♂ (SN). In each cross, males and females having mean weights of 157 and 115 g, respectively were stocked in 0.43 m³ tanks at a male to female ratio of 1:3. Seeds (unhatched eggs, yolk‐sac fry and free swimming fry) were harvested biweekly for 127 days. Viable spawns resulted from all crosses. The NN cross produced the highest total seeds tank^?1 (2890), seeds (kg female)^?1 day^?1 (48.3), seeds m^?2 day^?1 (23.3) and seeds female^?1 day^?1 (7.9), followed by the SS and SN crosses. The NS cross had the lowest fecundity. Lower spawning parameters in the NS and SN crosses may be attributed to the lower social compatibility between the males and females compared with that of the pure parents. The survival and growth of the offspring from the four crosses will be compared in seawater (37–40 g L^?1) to assess whether hybrid vigor was produced.     

Preliminary observations on growth and survival of Oreochromis spilurus x GIFT Oreochromis niloticus F1 reciprocal hybrids in fresh and seawater

Tilapia has a significant potential for culture in saline environments. There is an increasing demand among tilapia producers to develop a tilapia hybrid that can survive well in marine water conditions. This study compared mean weight (MWT), daily growth rate (DGR), specific growth rate (SGR), survival, feed conversion ratio (FCR), condition factor (K), production rate (PR) and rate of skin lesions, and tail and fin rot as well as eye cataract in the Genetically Improved Farmed Tilapia strain (GIFT) of the Nile tilapia, Oreochromis niloticus (NN) and the salt‐tolerant Oreochromis spilurus (SS), and their F₁ reciprocal hybrids; O. niloticus ♀ x O. spilurus ♂ (NS) and O. spilurus ♀ x GIFT O. niloticus ♂ (SN) in freshwater (0 g L^?1) and seawater (40 g L^?1). Fish (3.5 g) were stocked at 150‐fish m^?3 and fed with sea bream pellets (47% protein) for 180 days. Results showed that in seawater, the SN hybrid had the highest values for MWT (165.9 g), DGR (0.9 g fish^?1 day^?1), SGR (2.14% day^?1), survival (96.3%), PR (23.9 kg m^?3) and best FCR (1.53) followed by the NS hybrid and the NN parent. Both hybrids had significantly lower rates of skin lesions and fin and tail rot than the NN genotype. In freshwater, the NN had the highest values for MWT (255.1 g), DGR (1.40 g fish^?1 day^?1), SGR (2.38% day^?1), K (2.13%) and PR (34.9 kg m^?3) followed by the NS and SN hybrids. High estimate of heterosis for MWT (41.3%), DGR (42.5%), SGR (10.7%) and survival (22.1%) was obtained in the hybrids reared in seawater, indicating that a hybrid vigor was produced and the fast growth trait from the GIFT parent was successfully combined with the salinity tolerance trait from the O. spilurus parent. The better growth performance and survival of the SN hybrid in seawater indicate that this hybrid is more suitable for culture in seawater than its reciprocal hybrid.     

Studies on fish heterosis with DNA fingerprinting

One Tilapia hybridization, Oreochromis aureus, (Steindachner) ♂ × Oreochromis niloticus, (Linnaeus) ♀, and two common carp, Cyprinus carpio, (Linnaeus) ♂, hybridizations, Russian mirror carp ♂ × Xingguo red carp ♀, and German mirror carp ♂ × Xingguo red carp ♀ were used to study the feasibility of predicting heterosis using genetic distance from DNA fingerprinting. The results indicated that highly polymorphic DNA fingerprints could be obtained with human minisatellite 33.6 as a probe on the studied varieties. The within‐population similarity indices of O. aureus, O. niloticus, Russian mirror carp, German mirror carp and Xingguo red carp were 0.785, 0.577, 0.432, 0.348 and 0.339 respectively. The similarity indices between F1 and their parents of three hybridization combinations were not significantly different (P > 0.05). There were larger genetic distances between two hybridization combinations, O. aureus♂ × O. niloticus♀ and Russian mirror carp ♂ × Xingguo red carp ♀, which showed heterosis in production and were extensively used in Chinese aquaculture. The genetic distance between O. aureus♂ and O. niloticus♀ was 0.338, and that between Russian mirror carp and Xingguo red carp was 0.344. However, the genetic distance between German mirror carp and Xingguo red carp was 0.129; this corresponded with the fact that their F1 generation did not show heterosis in the Chinese fish hybridization experiment. The study suggested that genetic distance could be used to predict fish heterosis.     

DNA riboprinting analysis of Tilapia species and their hybrids using restriction fragment length polymorphisms of the small subunit ribosomal DNA

Morphometric, meristic and DNA riboprinting analyses of Tilapia species and their hybrids inhabiting the River Nile were examined. Morphometric data showed striking similarities and overlapping among Tilapia species, making it impossible to differentiate these species. Meristic characteristics revealed that Tilapia species could be identified into four major groups (Oreochromis niloticus, O. aureus, Sarotherodon galilaeus and Tilapia zillii). The lateral line scales differed significantly between the four Tilapia species, while the number of fin rays in the dorsal and anal fins differed significantly, differentiating three species (but not between O. niloticus and O. aureus). Restriction fragment length polymorphisms (RFLPs) of nuclear small sub‐unit ribosomal RNA (18S srRNA) gene were used to differentiate the species. Polymerase chain reaction‐restriction fragment length polymorphisms data provided a unique pattern for each species with a specific restriction enzyme. Two hybrids of Tilapia designated H₁ and H₂ were detected. The endonucleases SacII and ApaI differentiated H₁ and H₂. This research revealed a monophylogenetic relationship among all the studied Tilapia species.     

Comparison of the physiological energetics between Mytilus chilensis,Mytilus galloprovincialis and their hybrids,under laboratory conditions

Laboratory‐produced juvenile individuals of the species Mytilus chilensis, M. galloprovincialis and their hybrids were subjected to physiological measurements under an experimental diet of Isochrysis galbana (30 × 10⁶ cells L^?1), 13°C temperature and a salinity of 30 psu. Pure species individuals showed a higher clearance rate (CR). Mytilus chilensis had a CR of 1.13 L h^?1, while M. galloprovincialis registered only 0.78 L h^?1. Also, pure taxa registered higher values (above 70%) of absorption efficiency when compared with hybrid individuals. Ammonia excretion in M. chilensis and M. galloprovincialis was 1.5% and 0.4%, respectively, while hybrids registered significantly lower values. Under these experimental conditions, M. chilensis registered the highest scope for growth (P < 0.05), compared with M. galloprovincialis and their hybrids. However, the net growth efficiency index (K ₂) in hybridization type I (♀Mg × ♂Mc) was higher (P < 0.05) than other experimental groups. The invasive mytilid M. galloprovincialis showed values that are very similar to those obtained with the hybridization I group (♀Mg × ♂Mc). Finally, we discuss that water temperature is an important factor in the biogeographic separation of both species and the potential effects that the settlement of the invasive species may have for Chilean mussel production.     

The obvious heterosis and genetic characters of intergeneric cross and backcross juveniles between blunt snout bream (Megalobrama amblycephala) and topmouth culter (Culter alburnus)

In this study, we used artificial insemination to generate hybrid groups of fish [MC‐F₁(MA♀×CA♂) and MC‐F₂(MC‐F₁♀×♂)] by intergeneric crosses of Megalobrama amblycephala (MA) and Culter alburnus (CA); sequential backcrosses [CAM‐B₁ (CA♀×MC‐F₁♂) and MCC‐B₁ (MC‐F₁♀×CA♂)] were also performed. All these hybrids showed high rates of fertilization, hatching and survival (p > 0.05). For genetic traits, compared with those of the M. amblycephala and C. alburnus parental lines (Table 1), the fertilization rate, hatching rate and 7‐day survival rate of MC‐F1(MA♀×CA♂), MC‐F2(MC‐F1♀×♂), CAM‐B1 (CA♀×MC‐F1♂) and MCC‐B1 (MC‐F1♀×CA♂) by artificial insemination exhibited similar high rates (p > 0.05). The morphology of the four hybrids MC‐F₁/F_2, CAM‐B₁ and MCC‐B₁ were intermediate between those of their parents. Compared with their parents of MA and CA, weight gain rate (WG), specific growth rate (SGR) and protein efficiency ratio (PER) of hybrids MC‐F₁/F₂, CAM‐B₁ and MCC‐B₁ were significantly (p < 0.05) increased and feed conversion ratio (FCR) was significantly (p < 0.05) decreased after 3 months feeding. Moreover, protein content of muscle for MC‐F₁/F₂, CAM‐B₁ and MCC‐B₁ was significantly (p < 0.05) higher and carbohydrate content of muscle was significantly (p < 0.05) lower than their parents. The females and males of the four hybrids had normal gonadal development. In this study, we successfully generated intergeneric and backcross hybridization lines with fertile potential among fish of the Cultrinae subfamily and these hybrids had obvious heterosis in terms of growth performance, feed utilization and muscle quality.     

Rapid invasion of a subtropical lake fishery in central Mozambique by Nile tilapia,Oreochromis niloticus (Pisces: Cichlidae)

• 1. Lake Chicamba, Mozambique (19°08′S; 33°08′E) is a large (116 km²) impoundment in the headwaters of the Buzi River system, which was invaded by Oreochromis niloticus in 1996 from a small (<0.3 km²) upstream reservoir.
• 2. Experimental and artisanal catch data showed no O. niloticus until January 1996; after this O. niloticus was recorded in up to 83% of experimental seine net catches, 33% of experimental gill net catches, 43% of boat angling and 23% of shore angling catches, and in 48% of artisanal gill net catches.
• 3. During the period January to March 1997, O. niloticus mean (upper, lower 95% confidence interval) yields in the artisanal fishery were 5.2 (3.6, 7.0) t month^?1.
• 4. The rapid invasion of this lake illustrates the significant invasion threat that small point‐sources of this species pose to southern African freshwater systems.
• 5. The study recommends: (1) that this species should not be used for aquaculture or fisheries enhancement in catchments that have not been invaded, and (2) that the eradication of potential point sources of O. niloticus in non‐invaded catchment systems should be considered.

Copyright © 2007 John Wiley & Sons, Ltd.     

Effect of stocking density on the growth performance and yield of Nile tilapia [Oreochromis niloticus (L., 1758)] in a cage culture system in Lake Kuriftu,Ethiopia

This research was conducted to investigate the effect of stocking density on the growth performance and yield of Oreochromis niloticus in cage culture in Lake Kuriftu. The treatments had stocking densities of 50 (50F), 100 (100F), 150 (150F), and 200 (200F) fish per m^?3. All treatments were in duplicate. Juveniles with an average weight of 45. 76±0.25 g were stocked in the treatments. The fish were fed a composite mixture of mill sweeping, cotton seed, and Bora food complex at 2% of their body weight twice per day using feeding trays for 150 days in powdered form. The growth performance of O. niloticus was density dependent. The final mean weight of O. niloticus ranged 147.76±0.28–219.71±1.42 g and the mean daily weight gain was 0.69±0.01–1.15±0.02 g day^?1. Fish held in cages with lower density were heavier than the ones held at higher densities, and showed higher weight gain and daily weight gain. The most effective stocking density, in terms of growth parameters, was 50 fish m^?3. The gross yield (4.5–20.55 kg cage^?1) showed a significant difference with increasing stocking density (P<0.05). Moreover, the apparent food conversion ratio (2.48–7.22) was significantly affected by stocking density (P<0.05). However, survival rate was not affected by stocking density (P>0.05). It can be concluded that the most effective stocking densities were at 50 fish m^?3 cage for larger size fish demand in a short period and 200 fish m^?3 for higher gross production with supplementary feed.     

Intergeneric Hybridization between Kutum, Rutilus frisii kutum, and Bream, Abramis brama orientalis, of the Caspian Sea

Artificial hybridization was performed between Rutilus frisii kutum and Abramis brama orientalis of the Caspian Sea. Synchronization of spawning of female broodstock of both species was induced by injection of carp pituitary extract. Reciprocal crossings between R. frisii kutum ♀×A. brama orientalis ♂ (RA) and A. brama orientalis ♀×R. frisii kutum ♂ (AR) produced viable hybrid larvae without any clear particular pre‐ or postzygotic isolation phenomena. RA and AR hybrid larvae were reared to fingerling stage with survival rates of 22.5 and 28% and average weight of 6.8 ± 0.17 g and 9.0 ± 0.79 g, respectively. A heterosis of 45% was calculated for weight at fingerling stage. RA and AR hybrid fingerlings were cultured in polyculture along with Chinese carps for 6–7 mo and reached an average weight of 190–195 g and 235–255 g, respectively. Karyotyping of these hybrids revealed a modal diploid number of 2n = 50 for both groups, which is similar to those of the parental species. Discriminant function analysis on 28 morphometric and meristic characteristics of two parental species as well as their hybrids could separate these groups at highly significant level (P < 0.001). These results indicated an overall intermediate inheritance of the studied characters.     

Euclinostomum heterostomum and E. ardeolae in tilapia species of Aswan Governorate,Egypt: morphological,molecular, and histopathological characterization

Euclinostomum heterostomum and Euclinostomum ardeolae, both encysted metacercariae (EMC), were found to infect farmed Oreochromis niloticus in the Sahary fish hatchery and wild O. niloticus, Sarotherodon galilaeus, and Tilapia zillii in Lake Nasser, Aswan Governorate, Egypt, at a prevalence of 25.25% and an infection intensity of 1–14 EMC/fish. Macroscopic and microscopic examinations were used to identify them morphologically. PCR amplification, sequencing of the 28S large ribosomal RNA region, and phylogenetic analysis were used for molecular characterization. E. heterostomum and E. ardeolae were isolated from farmed O. niloticus kidneys, peritoneum, abdominal cavity, and under the gills attached to the head bone at prevalence of 13.5% and 6%, respectively, and from wild O. niloticus, S. galilaeus, and T. zillii at a prevalence of 15% and 4.5%, 18% and 10%, and 22.5% and 11.5% respectively. The molecular analyses of rRNA-28S marker revealed that the two Euclinostomum species examined are related yet distinct. This is the first molecular and phylogenetic evidence linking E. ardeolae to Euclinostomum. The present study added two new sequences to the GenBank databases for the genus Eulinostomum from Egypt, with accession numbers MW604803 and MW604806. Euclinostomum spp. have been shown to have detrimental and destructive effects on the kidneys of infected fish; these effects may eventually result in the parasites’ cysts replacing the kidney’s tissues. The kidneys of tilapia spp. infected with Euclinostomum spp. displayed degenerative alterations, with dilated renal tubules associated with migratory tunnels composed primarily of leucocytic infiltrations. Euclinostomum spp. exhibited clear hemophagic characteristics.

     

Cold tolerance of tilapia species and hybrids

Several experiments were conducted,involving Oreochromis mossambicus, O. aureusand their F₁ and F₂ hybrids to studythe genetic basis of cold tolerance in tilapiinefishes. Groups of fish, of similar age and grown underidentical conditions, were housed in replicated netenclosures in a controlled cooling water system.Survival time through a regime of controlledtemperature reduction was the observed parameter forcold tolerance. Intra-population variation anddifferences among species, hybrids and repeated spawnswithin a species were examined. There was nocorrelation between cold tolerance and fish size(within the range of 23–105mm standard length), andthe distribution for the trait was not normal. O. mossambicuswas the most cold-sensitive group,followed by the F₂, and the F₁ which wassimilar to O. aureus. Genetic variation in coldtolerance seems to have a large dominance component,based on the similarity of the F₁ hybrid to the O. aureus parent.     

Effects of azithromycin on tilapia (Oreochromis niloticus): health status evaluation using biochemical,physiological and morphological biomarkers

Bacterial diseases cause tilapia's high‐mortality outbreak. This study investigated the toxicity of azithromycin (AZT), a macrolide antibiotic that has been considered a possible therapeutic drug for tilapia aquacultural use. The 48‐h acute toxicity (50% lethal concentration, LC50; 48 h) of AZT was determined for Oreochromis niloticus. Thereafter, fish were exposed to 0, 1, 50 and 100 mg L^?1 AZT during 14 days (chronic exposure) and measured the haematological variables, the activity of superoxide dismutase, catalase, glutathione peroxidase, glutathione‐S‐transferase (GST) and the concentration of glutathione (GSH), protein carbonyl and lipid peroxidation in the liver; histopathology was analysed the liver, gills and kidneys. The LC50; 48 h was >100 mg L^?1. No fish died during chronic exposure. Haematocrit and haemoglobin concentration increased in fish exposed to 50 and 100 mg L^?1, and the total number of leucocyte and thrombocyte increased after exposure to 100 mg L^?1 AZT, suggesting a stimulation of defence cell production. In the liver, the antioxidant enzyme activities did not change, but GST activity and the GSH level increased in fish exposed to 100 mg L^?1 AZT. Oxidative stress did not occur. Histopathological index (HI_L) indicates moderate liver damage; minor histological changes in the gill and no change in the kidneys. AZT was considered non‐toxic for O. niloticus after acute exposure and, although it causes moderated histopathology in the liver after chronic exposure, this antibiotic may be an alternative against bacterial infections, depending on its efficacy to control bacterial disease in fish.     

Ultraviolet-induced androgenesis in Nile tilapia, Oreochromis niloticus (L.), and hybrid Nile × blue tilapia, O. aureus (Steindachner)

Fertilization of ultraviolet (UV) irradiated oocytes of Nile tilapia, Oreochromis niloticus (L.), with sperm from O, niloticus or blue tilapia, O, aureus (Steindachner), and subsequent suppression of the first cleavage of fertilized eggs successfully induced androgenesis in Nile and blue tilapia. The optimal doses of UV irradiation to denucleate a female genome of Nile tilapia prior to androgenesis ranged from 5940 to 6930 erg mm^?2 for 54-63 s at a fixed intensity of 110 erg mm^?2 s^?1. Putative androgenetic fish were created from eggs which were irradiated at various times and several durations of heat-shock. Eggs which were treated for 5 min at 41.6 ^oC at 2 7.5 min after fertilization were the most successful at suppressing the first cleavage and producing viable androgenetic diploids in Nile or hybrid Nile X blue tilapia. The maximal survivals of putative androgenetic diploids in relation to the control were 1.60% and 0.90% in Nile and hybrid Nile X blue tilapia, respectively. The androgenetic offspring established exhibited active feeding and normal growth.     

Growth performance and feed utilization of Nile tilapia Oreochromis niloticus (Linnaeus, 1758) and tilapia galilae Sarotherodon galilaeus (Linnaeus, 1758) fingerlings fed plant protein-based diets

This study was designed to determine the effect of complete substitution of fish meal (FM) by three plant protein sources including extruded soybean meal (SBM), extruded full‐fat soybean (FFSB) and corn gluten meal (CGM) on growth and feed utilization of Nile tilapia Oreochromis niloticus and tilapia galilae Sarothrodon galilaeus. Four isonitrogenous of crude protein (ca. 28.0%) and isocaloric (ca. 19 MJ kg⁻¹) experimental diets were formulated. The control diet (diet 1) was prepared with FM as the main protein sources. Diets 2–4, each FM control diet, were completely substituted with SBM (diet 2), FFSB (diet 3) and CGM (diet 4). l ‐lysine and dl ‐methionine were added to plant protein diets to cover the nutritional requirements of tilapia. Each treatment was allocated to three net pens and fed for 17 weeks. Nile tilapia fed the control diet showed significantly higher (P≤0.05) values for final body weight (FBW), feed intake (FI), weight gain (WG) and specific growth rate (SGR), whereas fish fed the diet with CGM achieved the lowest values. Tilapia galilae fed SBM diet recorded the highest (P≤0.05) values for growth performance. Better feed conversion ratio (FCR) for both Oreochromis niloticus and Sarothrodon galilaeus was observed when fish were fed SBM diet, whereas the worse FCR was recorded for FFSB diet. Feed utilization parameters including protein productive value (PPV), fat retention (FR) and energy retention (ER) showed significant differences (P≤0.05) for both the species fed different dietary protein sources. The present results suggest that, for Nile tilapia, both SBM and FFSB supplemented with dl ‐methionine and l ‐lysine can completely replace dietary FM. Meanwhile, S. galilaeus fed SBM diet exhibited comparable growth and feed utilization with those fish fed a fish‐meal‐based diet.     

Melanosis Inhibition and SO2 Residual Levels in Farmed Tiger Shrimp (Penaeus monodon) Following Different Sulfite-Based Treatments

Inhibition of melanosis was studied in farmed tiger shrimp (Penaeus monodon) treated with different concentrations of sulfite-based formulations. Treatments, applied by dipping for 0.5 h, were carried out immediately following the harvest. Melanosis, sensory quality, and the level of SO₂ residues in the muscle were determined in a selection of the most effective treatments. The minimum dose to avoid melanosis and keep quality for at least 10 days was found to be 30 g kg^?1. With all the treatments, the statutory limit of 0.3 g kg^?1 SO₂ in the edible part was not exceeded by any of the samples analyzed.     

Fluoride retention after dietary fluoride exposure in Siberian sturgeon Acipenser baerii

This study evaluated the uptake of fluoride ions (F^?) in tissues of juvenile Siberian sturgeon (Acipenser baerii). Fish were fed diets containing 75.2 (Control), 162.6, 360.8, 710.2, 1478.3 mg F^? kg^?1 in triplicate for 12 weeks with F^? added as NaF. Growth was inhibited in fish fed diets with 710.2 or 1478.3 mg F^? kg^?1, but was not inhibited in fish fed diets with 162.6 or 360.8 mg F^? kg^?1. The F^? concentration in scute, cartilage, skin and gill increased with time and dietary F^? concentration, and with the order of concentration at the end of the experiment, highest to lowest levels: scute (7721.3 mgF^? kg^?1), cartilage (2324.2), gill (721.4) and skin (262.5). Muscle, liver, gut and pylorus did not accumulate F^? with increasing F^? level in the diet. Thus, F^? up to 360.8 mg F^? kg^?1 can be included in the Siberian sturgeon diet without an adverse effect on growth or survival. The F^? as low as 162.6 mg F^? kg^?1 in diet increased F^? concentrations in scute and cartilage, and the high concentration of F^? in cartilage is possibly a danger to the consumption of cartilage in sturgeon.     

Heterosis analysis at early generations for complete diallel crosses in three different geographical culture populations of Sinonovacula constricta (Lamarck 1818) in Zhejiang,China

The paper reports a study focused on breeding new rapidly growing and heat‐tolerant strains in a commercially important Asian bivalve, Sinonovacula constricta. To breed a new strain of S. constricta with fast growth and high temperature tolerance, complete diallel crosses were carried out using three different geographical culture populations from Zhejiang (Z), Fujian (F) and Guangdong (G) provinces, China. Larval and spat growth and the survival and heat tolerance of spats were compared in detail. The experimental serial growth traits extracted from larval and spat measurements (shell length and shell height) indicated that there was no difference between six hybrid crosses and parental crosses at the larval stage; while at the spat stage, some differences were observed. The hybrid crosses ZF (Z♀ × F♂), ZG (Z♀ × G♂) and FG (F♀ × G♂) outperformed the parental groups in shell length at the end of intermediate culture, with lengths of 21.77 ± 0.28 mm, 21.55 ± 0.29 mm and 21.44 ± 0.20 mm respectively. For spat survival rate, the hybrid cross ZF (Z♀ × F♂) had a significantly larger value (88.89 ± 4.16%) than all the other groups, and the hybrid cross ZG (Z♀ × G♂) had a significantly larger rate than the cross GZ (G♀ × Z♂) and parental crosses ZZ (Z♀ × Z♂) and GG (G♀ × G♂). The hybrid crosses ZF (Z♀ × F♂), ZG (Z♀ × G♂) and FG (F♀ × G♂) exceeded the parental groups in heat tolerance, but there was no significant difference. The growth, survival and heat tolerance results showed that the crosses ZF, ZG and FG in this study were the best hybridization combinations for genetic improvement, which could have significant implications for the development of razor clam aquaculture. This work provides a theoretical basis for breeding new varieties (strains) that grow rapidly and are heat tolerant.     

Effect of dietary phosphorus sources and varying levels of supplemental phosphorus on survival,growth and body composition of postlarval shrimp (Litopenaeus vannamei)

Two experiments were conducted for 30 days each to investigate the effective phosphorus source and supplemental phosphorus levels for postlarval Litopenaeus vannamei. The first experiment was performed in postlarval shrimp (mean initial wet weight 2 mg) fed four isoenergic and isonitrogenous diets containing three supplemented inorganic phosphorus sources [D1: no supplemental phosphorus, D2: NaH₂PO₄·2H₂O, D3: KH₂PO₄·2H₂O, D4: Ca(H₂PO₄)₂·2H₂O]. The quantities of the three supplemental NaH₂PO₄·2H₂O, KH₂PO₄·2H₂O and Ca(H₂PO₄)₂·2H₂O were 11.6, 12.8 and 10 g kg^?1 of the diet, respectively in order to make the three diets have the same total phosphorus. Growth performance (final mean body weight, FBW; weight gain, WG; specific growth ratio, SGR) of shrimp in D3 treatment was the highest and had significant difference with the D1 treatment. The survival of shrimp in D3 treatment was the highest and had significant difference with the other treatments. The mineral concentration and body composition of shrimp were not significantly different among treatments. We could conclude that KH₂PO₄·2H₂O was the optimal phosphorus source for postlarval L. vannamei from the growth performance and survival. The second experiment was performed in postlarval shrimp (mean initial wet weight 0.88 mg) fed four isoenergic and isonitrogenous diets containing four supplemental KH₂PO₄·2H₂O levels (d1, d2, d3 and d4 with 0, 5, 10 and 20 g kg^?1, respectively). Shrimp in d2 treatment showed the highest growth performance and survival and also showed significant difference with other diet treatments. The whole body content of zinc (Zn) increased with the increase of dietary KH₂PO₄·2H₂O and significant differences were observed when dietary KH₂PO₄·2H₂O reached 5 g kg^?1, excess KH₂PO₄·2H₂O supplementation (10 and 20 g kg^?1) had a negative effect on Zn content, the Zn content significantly decreased when KH₂PO₄·2H₂O was 20 g kg^?1. We can conclude that the amount of total phosphorus in the diet should be maintained between 20.9 and 22.0 g kg^?1, the amount of supplemental KH₂PO₄·2H₂O in the diet is less than 10 g kg^?1.     

Mass production of hybrid magur and its culture potential in Bangladesh

Fertility, survival rate and hatching rate of hybrids of local magur Clarias batrachus and African catfish C. gariepinus were compared with the purebred species. Hybridization was conducted by hormone injection of purebreds and F₁ hybrids (F₁‐A), and artificial fertilization. Pure parental crosses as well as possible hybrid combinations were obtained. The fertility of F₁ hybrids (F₁‐A), purebred C. batrachus and C. gariepinus was 71.0%, 46.5% and 84.0% respectively. The fertility of an F₂ hybrid was 32.0%. The timing of embryonic developmental stages of F₁ hybrids (F₁‐A) was similar to that of purebred species. The hatching rate and survival rate of F₁ hybrids (F₁‐A) up to first feeding stage were 33.0% and 48.3%, respectively, which were significantly higher (P > 0.05) than the purebred C. batrachus. Only three embryos of an F₂ hybrid hatched successfully, and only one of these survived. The embryonic development of F₁ hybrids (F₁‐A) and their hatchability and survivability ensured the viable capacity of their progeny for mass culture. Hybrid morphology, both external and internal, was intermediate to that of the parents.