Introgressive hybridization of tilapias in Zimbabwe

Three species of tilapia native to Zimbabwe, Oreochromis mossambicus, O. mortimeri and O. macrochir are not naturally sympatric, but because of transplantation they have been brought into sympatry. Allozymes of fish examined from Lakes Kariba, Kyle and Chivero showed evidence of introgressive hybridization. The data showed non-random mating populations in Lakes Kariba and Kyle. All three reservoirs displayed individuals with intermediate principal component scores and hybrid index scores indicating hybridization.     

The consequences of habitat occupation and habitat overlap of the introduced tilapia Oreochromis mossambicus and indigenous fish species for fishery management in a Sri Lankan reservoir

Habitat occupation and habitat overlap of the introduced tilapia Oreochromis mossambicus and co-occurring indigenous fish species were studied in a shallow lowland reservoir in Sri Lanka. Results were used to discuss the possible existence of empty habitats in reservoirs and the position of the introduced O. mossambicus in the fish community. Predictions were made of the effects of a subsidiary small-meshed gillnet fishery for indigenous minor cyprinids on the yield of tilapia. The zooplanktivorous halfbeak Hyporamphus gaimardi, an indigenous invader from brackish water estuaries and lagoons, has successfully colonized the pelagic habitat in this reservoir. The habitat of the introduced O. mossambicus overlapped significantly with those of the indigenous minor cyprinids. Only the smallest size classes of O. mossambicus (below 45 mm) arc spatially segregated from these cyprinids. Spatial distribution patterns are influenced by the rainy seasons which trigger an inshore movement of O. mossambicus and several other species around November and May. Barbus spp, (Cyprinidae) need riverine habitats for spawning, but for all other species the available habitats within the reservoir are suitable to complete their full life cycle. Exploitation of Barbus spp. with gillnets with a mesh size of 30-mm stretched mesh is predicted to have considerable detrimental effects on the existing fishery for O. mossambicus. However, a gillnet fishery with 15-mm stretched mesh to exploit Amblypharyngodon melettinus, Rasbora daniconius and H. gaimardi in the open water zone is expected to be successful without harming the existing fishery for O, mossambicus or the populations of Barbus spp.     

Otolith microstructure pattern in Oreochromis niloticus (L.)

The microstructure of otoliths from young and old Oreochromis niloticus (L.) were studied. Otoliths were prepared histologically except for those from newly hatched fish. Hatching results in the formation of a check in the otoliths, which appeared 1 day later. Other checks are rare in juvenile otoliths but common in adult otoliths. Faint and non-daily increments were observed within the hatching check. After hatching, increments were deposited daily. Sub-daily increments were faint and narrow, they were present in the area along the dorso-ventral axis of the otolith but did not continue into the lateral region. Discontinuous zones in the medial area appeared different from those in the lateral area. New growth centres were not only found in the juvenile fish otoliths, but also in adult fish otoliths.     

Effects of the organophosphate insecticide,monocrotophos, on acetylcholinesterase activity in the nile tilapia fish (Oreochromis niloticus) brain

The neurotoxic effects of monocrotophos on the brain of the nile tilapia fish (Oreochromis niloticus) were examined, using a static bioassay under laboratory conditions. By probit analysis the 96 h LC₅₀ value of monocrotophos was 4.9 mg/l. After 96 h exposure to acute levels of monocrotophos, the brain acetylcholinesterase (AChE) activity decreased progressively as the concentration of monocrotophos increased. In addition, four weeks following transfer to toxicant-free water after exposure to 1 mg monocrotophos, nile tilapia fish brain regained 95% of control AChE activity. The results indicate that inhibition of AChE activity in fish exposed to monocrotophos may serve as an indicator of hazard due to application of this chemical in the natural environment.Special issue dedicated to Dr. Robert Balazs.     

微囊藻毒素对尼罗罗非鱼原代肝细胞致毒机理的探讨

采用离体细胞培养诱导方法,研究微囊藻毒素-LR(microcystin-LR,MC-LR)对尼罗罗非鱼(Oreochromis niloticus)原代肝细胞的毒性效应.尼罗罗非鱼原代肝细胞经10、50、150、500 μg/L MC-LR体外诱导24h后,单细胞微量凝胶电泳(SCGE)检测显示,与对照组相比处理组出现明显的彗星拖尾现象,说明MC-LR可引起尼罗罗非鱼肝细胞DNA的损伤,并随着剂量的增加,DNA的损伤程度增大.PI/Annexin V双染色流式细胞仪(FCM)检测表明MC-LR能明显引起肝细胞凋亡,与SCGE结果一致,且DNA损伤程度越大,细胞早期凋亡率越高,呈现明显的时间、剂量依赖性.本研究为进一步从分子、细胞水平阐明MC-LR的毒性以及致毒机理提供重要的理论依据.     

尼罗罗非鱼Orexin前体基因的克隆、组织分布及其在摄食调控中的表达

该文采用RT-PCR和cDNA末端快速扩增技术(rapid-amplification of cDNA ends,RACE)的方法,从尼罗罗非鱼(Oreochromis niloticus)下丘脑总RNA中获得了尼罗罗非鱼Orexin前体基因的cDNA全长序列。该cDNA全长648bp,其中开放阅读框的长423bp,编码Orexin前体蛋白为140个氨基酸,包括37个氨基酸的信号肽、43个氨基酸的Orexin-A、28个氨基酸的Orexin-B和末尾32个氨基酸组成的功能不详的多肽。采用Real-time PCR技术对尼罗罗非鱼Orexin前体基因的组织表达模式以及在摄食前后、饥饿和再投喂状态下的表达量变化进行了研究。结果显示,在脑部和外周等18个组织中都检测到了Orexin前体基因的表达,其中在下丘脑中表达量最高;在摄食前后,尼罗罗非鱼Orexin前体基因的表达量显著低于在摄食状态中;饥饿2、4、6和8d后,Orexin前体基因在下丘脑中的表达量与正常投喂组相比均显著升高,饥饿4d再投喂后,表达量又恢复至正常水平。这些结果表明,Orexin在尼罗罗非鱼摄食中可能有着重要的调节作用。     

Fine‐tuning for the tropics: application of eDNA technology for invasive fish detection in tropical freshwater ecosystems

Invasive species pose a major threat to aquatic ecosystems. Their impact can be particularly severe in tropical regions, like those in northern Australia, where >20 invasive fish species are recorded. In temperate regions, environmental DNA (eDNA) technology is gaining momentum as a tool to detect aquatic pests, but the technology's effectiveness has not been fully explored in tropical systems with their unique climatic challenges (i.e. high turbidity, temperatures and ultraviolet light). In this study, we modified conventional eDNA protocols for use in tropical environments using the invasive fish, Mozambique tilapia (Oreochromis mossambicus) as a detection model. We evaluated the effects of high water temperatures and fish density on the detection of tilapia eDNA, using filters with larger pores to facilitate filtration. Large‐pore filters (20 μm) were effective in filtering turbid waters and retaining sufficient eDNA, whilst achieving filtration times of 2–3 min per 2‐L sample. High water temperatures, often experienced in the tropics (23, 29, 35 °C), did not affect eDNA degradation rates, although high temperatures (35 °C) did significantly increase fish eDNA shedding rates. We established a minimum detection limit for tilapia (1 fish/0.4 megalitres/after 4 days) and found that low water flow (3.17 L/s) into ponds with high fish density (>16 fish/0.4 megalitres) did not affect eDNA detection. These results demonstrate that eDNA technology can be effectively used in tropical ecosystems to detect invasive fish species.     

The nutrition of stomachless phytoplanktivorous fish in comparison with Tilapia

The contents of alimentary tracts of stomachless phytoplanktivorous fish, including silvercarp (Hypophthalmichthys molitrix), Esomus danrica thermoicos, and Amblypharyngodon meletinus were investigated microscopically and compared with those of Tilapia (Oreochromis mossambicus, a fish with stomach).The predominance of detritus in the food and the low efficiency with which phytoplankton is utilized prove, that stomachless fish are not primarily herbivorous. The possibility of omnivorous feeding as an alternative is discussed. These results may contribute to our understanding of the role of filterfeeders in aquaculture and ecosystems.     

酸碱胁迫对不同体质量尼罗罗非鱼幼鱼能量代谢的影响

利用6×3析因试验设计,设置6组pH(5.0、6.0、7.0、8.0、9.0和10.0)和3组体质量(1.02、5.13和10.31 9),在试验室条件下研究了酸碱胁迫对不同体质量尼罗罗非鱼幼鱼的呼吸、排泄、乳酸脱氢酶(LDH)和Na+-K+ATPase活力的影响.结果表明:尼罗罗非鱼幼鱼的耗氧率和排氨率随pH值的升高先上升后下降,在pH7.0～8.0时达到最高值.耗氧量与排氨量随体质量的增加而增加并呈幂函数相关.体质量的一次和pH的一次、二次效应对尼罗罗非鱼幼鱼的耗氧率和排氨率有极显著的影响(P＜0.0l);体质量与pH之间无互作效应(P＞0.05);建立了pH与体质量对耗氧率、排氨率之间的回归方程,决定系数分别达到0.942和0.936;体质量对氧氮比(O:N)值有显著影响(P＜0.01),酸碱胁迫使尼罗罗非鱼幼鱼的能源物质利用模式发生改变.高pH条件不利于LDH活力的增加,但有利于Na+-K+ ATPase活力的上升,pH的一次和二次效应对酶活有极显著影响(P＜0.01),体质量对Na+-K+ ATPase活力没有显著影响,且pH与体质量对酶活无互作效应(P＞0.05).     

In vitro stimulation and inhibition of steroid hormone release from postovulatory follicles of the tilapia,Oreochromis mossambicus

Summary Postovulatory follicles of the tilapia, Oreochromis mossambicus, were incubated with graded doses of salmon gonadotropin to identify the steroid hormones released by this tissue. In addition, the effects of either cytochalasin B or colchicine on steroid hormone release were studied. After the incubation, the tissue was examined by electron microscopy. Postovulatory follicles released testosterone and estradiol-17B in a dose-dependent manner with gonadotropin. There was no detectable release of progesterone or 17a-OH-progesterone. When stimulated with high doses of gonadotropin, the steroidogenic cells showed an increase in smooth endoplasmic reticulum, Golgi complexes, and lipid droplets. Also, microfilaments became arranged in orderly bundles and were found close to the numerous secretory vesicles and lipid droplets. Upon incubation with gonadotropin and either colchicine or cytochalasin B, the cells still appeared steroidogenic, but the filaments were not organized nor associated with vesicles or lipid droplets. Release of steroid hormone decreased significantly. Also in these tissues, vesicles were no longer numerous in the apical region of the granulosa cells, but were located primarily near smooth endoplasmic reticulum and Golgi complexes. This suggests that disruption of the cytoskeleton results in reduced steroid hormone synthesis or release.