Substratum choice for nesting in male Nile tilapia Oreochromis niloticus

Four substrata were offered to groups of adult Nile tilapia Oreochromis niloticus (one male and two females) simultaneously: pure sand, a mixture of sand and shells, stones and no substratum. The results showed that males chose to dig nests in a lighter and more homogeneous substratum.     

Clock-controlled endogenous melatonin rhythms in Nile tilapia (Oreochromis niloticus niloticus) and African catfish (Clarias gariepinus)

The purpose of this work was to investigate the circadian melatonin system in two tropical teleost species characterized by different behavioral habits, Nile tilapia (diurnal) and African catfish (nocturnal). To do so, fish were subjected to either a control photoperiod (12L:12D), continuous light (LL) or darkness (DD), or a 6L:6D photoperiod. Under 12L:12D, plasma melatonin levels were typically low during the photophase and high during the scotophase in both species. Interestingly, in both species, melatonin levels significantly decreased prior to the onset of light, which in catfish reached similar basal levels to those during the day, demonstrating that melatonin production can anticipate photic changes probably through circadian clocks. Further evidence for the existence of such pacemaker activity was obtained when fish were exposed to DD, as a strong circadian melatonin rhythm was maintained. Such an endogenous rhythm was sustained for at least 18 days in Nile tilapia. A similar rhythm was shown in catfish, although DD was only tested for four days. Under LL, the results confirmed the inhibitory effect of light on melatonin synthesis already reported in other species. Finally, when acclimatized to a short photo-cycle (6L:6D), no endogenous melatonin rhythm was observed in tilapia under DD, with melatonin levels remaining high. This could suggest that the circadian clocks cannot entrain to such a short photocycle. Additional research is clearly needed to further characterize the circadian axis in teleost species, identify and localize the circadian clocks, and better understand the environmental entrainment of fish physiology.     

Bone development in the jaw of Nile tilapia Oreochromis niloticus (Pisces: Cichlidae)

East African cichlids have evolved feeding apparatus morphologies adapted to their diverse feeding behaviors. The evolution of the oral jaw morphologies is accomplished by the diversity of bone formation during development. To further understand this evolutionary process, we examined the skeletal elements of the jaw and their temporal and sequential emergence, categorized by developmental stages, using the Nile tilapia Oreochromis niloticus as a model cichlid. We found that chondrogenesis started in Stage 17. The deposition of osteoid for the dermal bones commenced in Stage 18. The uptake of calcium dramatically shifted from the surface of larvae to the gills in Stage 20. The bone mineralization of the skeleton began in Stage 25. These data provide important information regarding the sequential events of craniofacial development in East African cichlids and lay the groundwork for studying the molecular mechanisms underlying adaptation of jaw structure to feeding behavior.     

Protein and energy needs for maintenance and growth of Nile tilapia (Oreochromis niloticus)

A trial was undertaken with juvenile Nile tilapia fed graded levels of dietary protein (0–35% DP) over 7 weeks (28°C). Measurements of nitrogen and energy utilization were made using data on ADC, comparative carcass analyses, nitrogen excretion and oxygen uptake. Data indicate that the daily protein intake for maximum N gain was c. 12 g/kg/d and the maintenance protein requirements were about of 2g/kg/d. The optimal DP/DE ratio was found to be 18mg/kJ. Differences were observed in the data on endogenous nitrogen and energy utilization depending on the dietary treatments as well as on the criteria used.     

Identification and partial characterisation of a chitinase from Nile tilapia, Oreochromis niloticus

Measurement of chitinase activity in extracts from stomach, intestine, and serum of Nile tilapia with the artificial substrates 4-methylumbelliferil beta-D-N,N'-diacetylchitobioside and 4-methylumbelliferil beta-D-N,N'N"-triacetylchitotrioside (4MU[GlcNAc](2,3)) showed that an endochitinase was involved in the liberation of the fluorophore 4-methylumbelliferone (MU). Enzymes were isolated from tilapia serum by a combination of gel filtration, ion exchange, and reverse-phase chromatography. The molecular mass of the enzyme was estimated to be 75 kDa by SDS-PAGE, suggesting that the enzyme occurs as a monomer. The partially purified enzyme showed maximal activity at pH 7.0 when assayed with 4MU[GlcNAc](2) and lost its activity below pH 5.0 and above pH 8.0. The optimal pH of the purified enzyme toward the substrate 4MU[GlcNAc](3) was pH 9.0 and activity was lost below pH 8.0 and above pH 9.0. Our study has revealed the presence of a chitinolytic enzyme in the gastrointestinal tract and serum that may play a role in digestion and/or defense.     

Comparisons of isotopic niche widths of some invasive and indigenous fauna in a South African river

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Molecular cloning and gene expression of Foxl2 in the Nile tilapia, Oreochromis niloticus

A Foxl2 cDNA was cloned from the Nile tilapia ovary by RT-PCR and subsequent RACE. Alignment of known Foxl2 sequences from vertebrates confirmed the conservation of the Foxl2 open reading frame and protein sequences, especially the forkhead domain and C-terminal region, while some homopolymeric runs of amino acids are found only in mammals but not in non-mammalian vertebrates. RT-PCR revealed that Foxl2 is expressed in the tilapia brain (B), pituitary (P), gill, and gonads (G), with the highest level of expression in the ovary, reflecting the involvement of Foxl2 in B-P-G axis. Northern blotting and in situ hybridization also revealed an evident sexual dimorphic expression pattern in the gonads. Foxl2 mRNA was mainly detected in the granulosa cells surrounding the oocytes. The ovarian expression of Foxl2 in tilapia begins early during the differentiation of the gonads and persists until adulthood, implying the involvement of Foxl2 in fish gonad differentiation and the maintenance of ovarian function.     

Taste preferences and feeding behaviour in the facultative herbivorous fish,Nile tilapia Oreochromis niloticus

Taste preferences in fishes are known mainly for carnivorous species, whereas herbivorous consumers were rarely used in such studies. The main goal of the present study was to evaluate the taste preferences in the herbivorous African cichlid fish, Nile tilapia Oreochromis niloticus. In laboratory settings, the palatability of widely used taste substances (four taste substances that are considered to be sweet, sour, bitter and salty for humans – sucrose, citric acid, calcium chloride and sodium chloride; 21 free L -amino acids; 12 sugars and artificial sweetener Na-saccharin; 0.1–0.0001 M) was evaluated. In each trial, a standard agar pellet flavoured with a substance was offered for fish individually. The consumption of pellet, the number of grasps and the retention time before the pellet was finally ingested or rejected were registered. Overall, 21 of 38 substances were palatable, whereas other substances did not shift consumption of pellets in relation to blank pellets. Pellets containing citric acid, L -cysteine, L -norvaline, L -isoleucine, L -valine, Na-saccharin and D -sorbitol were consumed in >85% of trials. Taste attractiveness of amino acids was highly species-specific and was not associated with the trophic category of the 19 species compared. Moreover, it did not correlate with dietary quantitative requirements of Nile tilapia (r_s = 0.27; P > 0.05). Palatability of sugars for O. niloticus and their sweetness for humans did not correlate as well (r_s = 0.21; P > 0.05); nonetheless, Na-saccharin has the most attractive taste for both O. niloticus and humans. The most palatable amino acids lost their effect if the concentration was lowered to 0.01 M for L -cysteine and 0.001 M for L -norvaline (lower than 242.3 μg and 23.4 μg per a pellet, respectively). Single pellet grasp was characteristic of O. niloticus feeding behaviour (>95% of trials), and this pattern may be related to the social lifestyle of this species. Fish spent 4–8 s on average for orosensory evaluation of pellet edibility. The retention time correlated with the palatability of substances and was significantly longer in trials that ended up with pellet swallowing. It is suggested that prolonged orosensory evaluation of food before swallowing provides a reliable and accurate sensory evaluation, which, in turn, can reduce the probability that inadequate food will be consumed.     

Differentiation of steroid-producing cells and folliculogenesis in the developing ovary of the Nile tilapia Oreochromis niloticus

Differentiation and development of steroid-producing cells (SPCs) and folliculogenesis during ovarian differentiation in the Nile tilapia Oreochromis niloticus were immunohistochemically and ultrastructurally examined. Clusters of immunopositive cells (IPCs) against antibodies (ABs) of cholesterol side-chain cleavage cytochrome P450 (P450scc), 3β-hydroxysteroid dehydrogenase (3βHSD), and cytochrome P450aromatase (P450arom) only appeared in the area near blood vessels in the fish ovaries at 50-60 days after hatching (dah). Ultrastructural results showed that differentiation and development of SPCs from undifferentiated to maturation occurred in the area near blood vessels, indicating that it would be the original site of SPCs. At 70-80 dah, IPC clusters invaded the interstices among oocytes at the perinucleolar stage from the area near the blood vessels. IPCs increased in number in the interstices among the previtellogenic oocytes, and some clusters began to enclose the outer thecal layer of the previtellogenic oocytes at 90 dah. The process of folliculogenesis was ultrastructurally observed. SPCs enclosed by fibroblastic cells invaded the interstitial areas among oocytes and some reached the surfaces of oocytes. The upper portions of these elongations opened and began to enclose the outer surfaces of developed oocytes to become thecal layer. Later, newly migrated SPCs reach the thecal layer to become thecal cells. These results indicate that steroid-producing thecal cells originate from the SPCs in the area near blood vessels. After thecal layer formation, an immunopositive reaction against P450arom AB, but not against P450scc or 3β-HSD ABs, appeared first in the granulosa cells enclosing the vitellogenic oocytes at 100 dah. At this time, estrogen production in serum levels rapidly increased. Thus, folliculogenesis could be essential for active production of estrogen in the ovary.     

Behavioural and physiological effects of methadone in the perioperative period on the Nile tilapia Oreochromis niloticus

Through the analysis of behavioural changes, this study demonstrates that methadone has behavioural, but not analgesic, effects on Oreochromis niloticus. It provides information that suggests the drug has sedative abilities, as the recovery time was shorter in the fish receiving methadone. Future research, with different doses and stimuli, is required to provide more information about analgesia.     

Effects of exogenous melatonin on the reproductive activities of Nile tilapia,Oreochromis niloticus

Gonadal development and sexual maturation of fish are often related to photic conditions. We attempted to utilise the direct application of melatonin to elicit similar effects in the absence of photoperiod manipulation. We found no significant differences in somatic growth between melatonin-treated groups and controls, indicating that melatonin administration did not negatively affect the growth of the experimental fish. Treatment with low-dose melatonin (0.3 mg/kg BW) resulted in a decrease in spawning frequency, number of spawned eggs and gonadosomatic index in female tilapia. In male tilapia, low-dose melatonin was associated with a decrease in sperm count, spermatocrit and spermatozoa activity index, implying that exogenous melatonin might mimic the effects of shortened light photoperiod, which suppresses reproductive activity in this species. These results suggest the possibility that an optimised dose of melatonin treatment could be used to control the spawning behaviour of this species.     

Muscle growth in Nile tilapia (Oreochromis niloticus): histochemical,ultrastructural and morphometric study

Muscle growth in Nile tilapia (Oreochromis niloticus) was studied focusing on histochemical, ultrastructural, and morphometric characteristics of muscle fibers. Based on body length (cm), we studied four groups: G1=1.36+/-0.09, G2=3.38+/-0.44, G3=8.90+/-1.47, and G4=28.30+/-3.29 (mean+/-S.D.). All groups showed intense reaction to NADH-TR in subdermal fibers and weak or no reaction in deep layer fibers. In G3 and G4, an intermediate layer was also observed with fibers presenting weak reaction; in G4, groups of fibers with intense reaction were observed in the subdermal region. The myosin ATPase (m-ATPase) activities were acid-stable and alkali-labile in subdermal fibers; most deep layer fibers were alkali-stable and acid-labile. Intermediate fibers were acid-labile and alkali-stable. Two fiber populations were observed near deep muscle layer: one large presenting weak acid- and alkali-stable and the other small alkali-stable.During growth, muscle fiber hypertrophy was more evident in intermediate and white fibers for G3 and G4. However, in these groups, the presence of fiber diameters < or =21 microm suggested that there is still substantial fiber recruitment, confirmed by ultrastructural study, but hypertrophy is the main mechanism contributing to increase in muscular mass.     

Effect of melatonin on GnIH precursor gene expression in Nile tilapia,Oreochromis niloticus

Sexual maturation and gonadal development of fish is greatly influenced by photic information, an external environmental factor, and melatonin mediates this information to regulate gonadotropin (GTH) secretion and gonadal activation. The relationship between gonadotropin inhibiting hormone (GnIH) and melatonin in fish, however, has not been studied to date. Here, the GnIH expression pattern and daily change of melatonin levels were compared to each other in mature tilapia (body length 16.1 ± 0.2 cm, body weight 77.7 ± 3.43 g), and the effect of melatonin injection on GnIH gene expression was investigated. GnIH gene expression increased at night when the secretion of melatonin increased, whereas gene expression decreased during the day when melatonin secretion decreased. Injecting tilapia intraperitoneally with melatonin increased GnIH gene expression and decreased the expression of gonadotropin releasing hormone (GnRH) and GTH. Furthermore, the injection decreased the 11-KT concentration in male tilapia. These results indicate that melatonin is likely to suppress the hypothalamus-pituitary-gonad (HPG) axis via the action of GnIH in this species.     

Induction of diploid androgenetic and mitotic gynogenetic Nile tilapia (Oreochromis niloticus L.)

Androgenesis is a potentially valuable technique for recovering fish from gene banks composed of cryopreserved sperm, developing inbred lines, and analyzing patterns of inheritance. The procedure for producing diploid organisms whose nuclear DNA is wholly of paternal origin is dependent on: (1) the denucleation of host eggs, and (2) the inhibition of the first mitotic division in order to double the haploid sperm chromosome complement following fertilization of host eggs. Denucleation of tilapia (Oreochromis niloticus L.) eggs was carried out using UV irradiation. Treatment durations of 5–8 min (total dose of 450–720 J/m²) produced acceptable yields of viable denucleated eggs [22.9±1.6% (±SE) of controls] as estimated by the survival of haploid androgenetic tilapia to 48 h post-fertilization. Successful mitotic inhibition was accomplished using a heat-shock of 42.5 °C for 3–4 min, applied at 2.5-min intervals from 22.5 to 30 min post-fertilization (mpf). The mean survival of androgenetic diploid fish to yolk-sac absorption for treatment groups varied from 0.4% to 5.3%, relative to the controls. Differences in the suceptibility of eggs from different females to UV irradiation were a significant factor in the overall yield of androgenetic diploids. Paternal effects did not significantly influence the androgenetic yield, suggesting that individual males would not be selected against. For comparative purposes mitotic gynogenetic mitogyne diploids were produced from UV-irradiated sperm. Mean survival to yolk-sac absorption varied from 0.5% to 10.64%, relative to controls. Similar optima for androgenetic and gynogenetic induction were found in the period 25–27.5 mpf (minutes post-fertilization). Induction treatments would appear to be operating on the same developmental events in both these techniques, and the results suggest that the UV irradiations used do relatively little damage to the eggs beyond nuclear inactivation. The results indicate that the production of androgenetic O. niloticus is possible on a consistent basis and that the application of this technique may be useful in quantitative and conservation genetics.     

Metabolic and molecular responses in Nile tilapia,Oreochromis niloticus during short and prolonged hypoxia

The strictly aquatic breathing Nile tilapia, Oreochromis niloticus is an extremely hypoxia-tolerant fish. To augment our understanding of the effects of hypoxia on anaerobic glycolysis in the Nile tilapia, we studied the effect of short-term for 1 day (trial 1) and long-term for 30 days (trial 2) hypoxia on a selected glycolytic enzymes activity and mRNA expression in liver and white muscle. The hypoxic oxygen concentrations used in the two trials were 2, 1, and 0.5 mg O₂ L^?1 for comparison with a control normoxic group 8 mg O₂ L^?1. The activity of phosphofructokinase (PFK), pyruvate kinase (PK), and lactate dehydrogenase (LDH) in liver and white muscle except liver LDH decreased in trial 1 and increased in trial 2. Assessments of mRNA levels in trial 1 revealed that PFK was downregulated and LDH was upregulated in liver and white muscle, while PK fluctuated between upregulation in liver and downregulation in white muscle. Meanwhile, PK and LDH were upregulated while PFK was similar to control values in both tissues in trial 2. Comet assay results demonstrated an increase in DNA damage that was directly proportional to increasing hypoxic concentrations. This damage was more pronounced in trial 1. This suggests that the Nile tilapia cope better with long-term hypoxic conditions, possibly as an adaptive response.     

The impacts of invasive Nile tilapia (Oreochromis niloticus) on the fisheries in the main rivers of Guangdong Province,China

Nile tilapia (Oreochromis niloticus) is one of the most widespread invasive fish species, and this species has successfully established populations in the major rivers of Guangdong Province, China. Field surveys and manipulative experiments were conducted to assess the impacts of Nile tilapia on fisheries. We determined that the increase of Nile tilapia in these rivers not only affects the CPUE (catch-per-unit-per-effort) of the fish community and native fish species but also reduces the income of fishermen. In the manipulative experiments, we observed that the growth of native mud carp decreased in the presence of Nile tilapia. Our results suggest that the invasion of Nile tilapia negatively affected the fishery economy and native fish species, and suitable control measurements should be taken.     

Immunotoxicity and hepatic function evaluation in Nile tilapia (Oreochromis niloticus) exposed to diazinon

The LC(50) of the organophosphorus pesticides (OPs) diazinon to Nile tilapia (Oreochromis niloticus) was determined, thereafter, hepatic activity, phagocytic index, percentages of active cells, relative spleen weight, total IgM concentration and lymphoproliferation rates were compared between diazinon exposed groups (LC(50) and (1/2)LC(50)) and non-exposed control group. Experimental data show that diazinon is highly toxic for juvenile Nile tilapia (LC(50)=7.830 ppm) and presents immunotoxic properties which affect both the innate and cellular adaptive immune responses of this fish, as revealed by the fact that splenocyte proliferation and phagocytic indices were significantly decreased after acute exposure to the pesticide. However, the hepatic biochemical parameters and the total circulating IgM concentrations were not affected in this experimental model.     

尼罗罗非鱼早期发育形态及其在珠江水系的空间分布

【背景】罗非鱼作为联合国粮农组织（FAO）向全世界推广养殖的优良品种之一,有多个品系,其养殖范围已遍布85个国家和地区。1956年我国从越南引进莫桑比克罗非鱼,经过养殖及推广,2006年我国罗非鱼产量达到100万t。但是,该外来物种在给我国带来良好经济价值的同时,对土著种类及水环境造成了极大的影响。【方法】对近几年珠江水系渔获物的调查数据进行整理,并观察尼罗罗非鱼早期发育形态,统计尼罗罗非鱼苗对其他鱼苗的最大捕食量及捕食规格,以分析罗非鱼生物学特性及其在珠江水系的入侵现状。【结果】尼罗罗非鱼早期发育快,卵黄营养非常丰富,比珠江水系土著种类更有竞争性;罗非鱼苗呈现很强的攻击性与捕食性;尼罗罗非鱼已经扩散到珠江水系各主要河流,并在部分江段形成优势种群。【结论与意义】尼罗罗非鱼种群快速扩张,对土著种已构成严重威胁,有必要将其列为珠江水系高危入侵种。