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1.
Recirculating aquaculture systems (RAS) can be installed indoors, allowing year-round production of tropical animals in nearly any climate. A nursery phase is commonly used in Litopenaeus vannamei production since it allows for enhanced biosecurity and better quantification of animals while reducing space requirements. However, it is unclear whether animal density and inclusion of artificial substrate may improve shrimp performance during the nursery phase. In this experiment, we compared shrimp production parameters in two stocking densities with or without the use of an artificial substrate by creating four treatments: low-density LD; 1500 PL/m−3, low-density with substrate LDS, high-density HD; 3000 PL/m−3), and high-density with substrate (HDS). The LDS and HDS treatments included 0.46-m2 of high-density polyethylene 2.5-cm mesh as a substrate, which increased the tank surface area by 21 %. Each treatment was randomly assigned to four 160-l culture tanks, each with a biofilter. The shrimp had an initial weight of 4 mg and were grown for 50 days. The low-density treatments had significantly higher dissolved oxygen (DO) and pH than the high-density treatments (P ≤ 0.001). Specifically, LDS had the highest DO and pH followed by the LD, HD, and HDS treatments, respectively. High-density treatments had significantly higher NO2-N levels than low-density treatments during week 2 of the experiment when an unusually high concentration of nitrite was observed. FCR was significantly lower in both low-density treatments than in high-density treatments. At harvest, the total biomass (kg m−3) was significantly higher in high-density treatments than in low-density treatments (P ≤ 0.001), and the HDS treatment had a significantly greater biomass output than HD. Producers should consider artificial substrate and higher densities during nursery production to maximize shrimp production; however, the effects on water quality should also be taken into account. 相似文献
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金定鸭的外型分化,受自然选择和性选择的双重影响。在生长发育过程中,依羽域在适应水域环境的重要性,雌雄金定鸭的正羽都按共同的羽域顺序陆续更换生长;与此同时,一日龄外型相同的异性雏鸭,逐渐表现出性双态,包括喙色、羽色、声音、性羽、体重和行为的表型差异,以满足繁殖竞争的需要。金定鸭性行为的出现以其性双态分化为基础,而性行为的分化又是其生理成熟的前提条件。对雄鸭的观察分析结果表明,性双态分化和性行为发育速度的快慢与其生理成熟的早迟是一致的。黑色尾羽的雄鸭,体重较小,其发育速度和性成熟均快于体重较大的白喙或端白尾羽者,因此,建议将尾羽颜色、性双态分化和性行为发育速度作为雄鸭的选育指标。 相似文献
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为改善中锰白口铸铁的韧性以扩大其应用范围,本文探讨了C, Si, Mn等合金元素、变质处理和热处理工艺对中Mn白口铸铁组织和性能的影响. 相似文献
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向日葵[IIelianlhus anuus (L.)]是我国的重要经济作物。其花粉通常呈深浅程度不同的黄颜色。据我们掌握和查新资料,目前世界上尚未见有关白花粉方面的报道。1989年7月下旬至8月上旬,我们在吉林农业大学向日葵育种试验田的恢复系C8711群体中,先后发现4株白花粉突变体。暂分别定名为C8711—1、C8711—2、C8711—3和C8711—4。在进行人工套袋之前,此4株突变体已经在一定程度上发生了开放授粉,其中C8711—4开放授粉程度较轻。成熟时单独收获这4株突变体的种子,以进行有关遗传学研究,本文拟对白花粉突变的遗传学机理进行初步的探讨。 相似文献
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本文利用透射电镜研究了寄生于家鸭小肠上皮细胞的菲莱氏温扬球虫的受精和卵囊壁的形成过程。作者在感染后96 h 的样品中发现一例大、小配子已受精;小配子已进入大配子中,大配子胞质中有一根纵切的鞭毛,小配子的核呈流体状,位于大配子核附近。卵囊壁是由成囊颗粒内含物进入合子表面的膜之间沉积成层而形成。成囊颗粒Ⅰ形成外层卵囊壁,成囊颗粒Ⅱ形成内层卵囊壁。沉积于膜之间的成囊颗粒Ⅰ内含物逐渐浓缩,堆积成颗粒状,使卵囊壁外层外表面形成颗粒状突起。成熟的卵囊壁由三层组成:即薄的外周膜、外层和内层。其内的合子原生质还被两层单位膜包裹。 相似文献
8.
Sugarcane yields have been severely reduced by white leaf and grassy shoot phytoplasma diseases in many parts of Asia. Australian sugarcane crops are not known to be affected by these diseases, but plant pathogenic phytoplasmas found in other introduced and native grasses in northern Australia could pose a serious threat to the Australian sugarcane industry. To further evaluate this threat, leaves from plants of 20 grass species, with and without symptoms, were collected during field surveys in northern Australia and tested to determine whether phytoplasmas were present and whether symptoms were reliable indicators of phytoplasma presence. Molecular tools were used to detect and characterize phytoplasmas. Four different phytoplasmas were found in seven grass species known to grow near healthy sugarcane crops. All the phytoplasmas were closely related to sugarcane white leaf phytoplasma (SCWL), one of the phytoplasmas that causes disease in sugarcane in Asia. Four of the host plant species and two of the phytoplasmas were new records. The relationship between symptoms and phytoplasma presence was poor. Because some plants with symptoms tested negative for phytoplasmas, a series of surveys was carried out in which flowers, leaves, roots and stems of two known host plant species, Whiteochloa cymbiformis and Sorghum stipoideum, were tested separately on nine occasions during two wet seasons. This was done to investigate the distribution of phytoplasmas within plants over time. Results showed that spatial and temporal variation of phytoplasmas occurred in these two host plant species. Hence, evaluation of disease distribution within a region requires repeated testing of all plant parts from plants without symptoms, as well as those with symptoms. To date, there is no report of a vector capable of transmitting to Australian sugarcane the phytoplasmas found in grasses in this study. If one is present, or occurs in the future, then native and introduced grasses could constitute a large reservoir of phytoplasma for vectors to draw on. This work provides an early warning for the sugarcane industry that the potential for infection exists. 相似文献
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