全文获取类型
收费全文 | 5452篇 |
免费 | 481篇 |
国内免费 | 205篇 |
专业分类
6138篇 |
出版年
2024年 | 23篇 |
2023年 | 58篇 |
2022年 | 56篇 |
2021年 | 181篇 |
2020年 | 226篇 |
2019年 | 182篇 |
2018年 | 159篇 |
2017年 | 178篇 |
2016年 | 206篇 |
2015年 | 180篇 |
2014年 | 203篇 |
2013年 | 286篇 |
2012年 | 176篇 |
2011年 | 175篇 |
2010年 | 185篇 |
2009年 | 247篇 |
2008年 | 259篇 |
2007年 | 248篇 |
2006年 | 209篇 |
2005年 | 203篇 |
2004年 | 197篇 |
2003年 | 187篇 |
2002年 | 195篇 |
2001年 | 185篇 |
2000年 | 171篇 |
1999年 | 171篇 |
1998年 | 182篇 |
1997年 | 151篇 |
1996年 | 114篇 |
1995年 | 128篇 |
1994年 | 107篇 |
1993年 | 80篇 |
1992年 | 82篇 |
1991年 | 64篇 |
1990年 | 100篇 |
1989年 | 58篇 |
1988年 | 25篇 |
1987年 | 33篇 |
1986年 | 19篇 |
1985年 | 41篇 |
1984年 | 40篇 |
1983年 | 19篇 |
1982年 | 32篇 |
1981年 | 21篇 |
1980年 | 26篇 |
1979年 | 17篇 |
1978年 | 18篇 |
1977年 | 14篇 |
1976年 | 10篇 |
1973年 | 3篇 |
排序方式: 共有6138条查询结果,搜索用时 0 毫秒
31.
32.
Comparison of the results of ichthyoplankton surveys conducted at 97 stations in the eastern part of the Peter the Great Bay of the Sea of Japan, in June–July 2007 with the similar research data of the 1950s shows that at present, as was the case 50 years ago, flatfish eggs belonging mainly to the yellowfin sole Limanda aspera and brown sole Pleuronectes herzensteini prevail in the local ichthyoplankton (up to 86%). The highest concentrations of these species’ eggs were recorded in the Vostok Bay and Strelok Bay. The spawning activity of flatfish in 2007 is found to be lower than in the mid 1900s, but the significance of the eastern part of the Peter the Great Bay for flatfish reproduction remains large. The importance of long-term monitoring in this area, which is being subjected to steadily growing anthropogenic impacts, is also proven. 相似文献
33.
Linking molecular evolution to biological function is a long‐standing challenge in evolutionary biology. Some of the best examples of this involve opsins, the genes that encode the molecular basis of light reception. In this issue of Molecular Ecology, three studies examine opsin gene sequence, expression and repertoire to determine how natural selection has shaped the visual system. First, Escobar‐Camacho et al. ( 2017 ) use opsin repertoire and expression in three Amazonian cichlid species to show that a shift in sensitivity towards longer wavelengths is coincident with the long‐wavelength‐dominated Amazon basin. Second, Stieb et al. ( 2017 ) explore opsin sequence and expression in reef‐dwelling damselfish and find that UV‐ and long‐wavelength vision are both important, but likely for different ecological functions. Lastly, Suvorov et al. ( 2017 ) study an expansive opsin repertoire in the insect order Odonata and find evidence that copy number expansion is consistent with the permanent heterozygote model of gene duplication. Together these studies emphasize the utility of opsin genes for studying both the local adaptation of sensory systems and, more generally, gene family evolution. 相似文献
34.
James H. Barrett 《Journal of fish biology》2019,94(6):1033-1044
This paper explores the past and potential contribution of archaeology to marine historical ecology. The primary focus is European fishing of marine and diadromous taxa, with global comparisons highlighting the wider applicability of archaeological approaches. The review illustrates how study of excavated fish bones, otoliths and shells can inform our understanding of: (a) changes in biogeography, including the previous distribution of lost species; (b) long-term fluctuations in the aquatic environment, including climate change; (c) the intensity of exploitation and other anthropogenic effects; (d) trade, commodification and globalisation. These issues are also relevant to inform fisheries conservation and management targets. Equally important, the long (pre)history of European fishing raises awareness of our ecological heritage debt, owed for centuries of wealth, sustenance and well-being, and for which we share collective responsibility. This debt represents both a loss and a reason for optimism, insofar as it is a reservoir of potential to be filled by careful stewardship of our rivers, lakes, seas and oceans. 相似文献
35.
B. Austin 《FEMS microbiology letters》1987,43(3):295-300
A technique was developed, which permitted the rapid determination of antibiotic resistance of bacterial pathogens in diseased material. The method involved use of an antibody-based antigen capturing system, exposure to antibiotic solutions, and thence the determination of viability by reduction of thiazolyl blue. 相似文献
36.
37.
RAINE KORTET ANSSI VAINIKKA MARKUS J. RANTALA JOUNI TASKINEN 《Biological journal of the Linnean Society. Linnean Society of London》2004,81(1):111-117
According to sperm competition models, a male spawning in a disfavoured role should have spermatozoa with higher velocity but shorter longevity compared with a male spawning in a favoured role. Moreover, immunosuppressive androgens are needed to produce both secondary sexual characters and sperm cells. The 'sperm protection' hypothesis suggests that the immunosuppressive action of androgens has evolved to protect haploid spermatozoa, which are antigenic, from autoimmune attacks. Therefore, a male with high sexual ornamentation may be more susceptible to diseases but may possess better quality ejaculate than his less ornamented rival. We studied sexual ornamentation (breeding tubercles), ejaculation quality (sperm concentration, longevity and spermatozoal velocity) and intensity of parasitism in the cyprinid, Rutilus rutilus . Sperm longevity and spermatozoal velocity were positively correlated. Males having elaborated sexual ornamentation had longer-lived sperm and more Myxobolus mülleri parasites in the liver compared with males with low ornamentation. However, no difference was found between males with different degrees of ornamentation with respect to sperm concentration, spermatozoal velocity or other parasites. Since the highly ornamented males had higher sperm longevity, the present results are partly consistent with the predictions of the sperm competition models and the 'sperm protection' hypothesis. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 81 , 111–117. 相似文献
38.
M. Kawasaki J. Prather Y. -X. Guo 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1996,178(4):453-462
The sensory cues for a less known form of frequency shifting behavior, gradual frequency falls, of electric organ discharges (EODs) in a pulse-type gymnotiform electric fish, Rhamphichthys rostratus, were identified. We found that the gradual frequency fall occurs independently of more commonly observed momentary phase shifting behavior, and is due to perturbation of sensory feedback of the fish's own EODs by EODs of neighboring fish. The following components were identified as essential features in the signal mixture of the fish's own and the neighbor's EOD pulses: (1) the neighbor's pulses must be placed within a few millisecond of the fish's own pulses, (2) the neighbor's pulses, presented singly at low frequencies (0.2–4 Hz), were sufficient, (3) the frequency of individual pulse presentation must be below 4 Hz, (4) amplitude modulation of the sensory feedback of the fish's own pulses induced by such insertions of the neighbor's pulses must contain a high frequency component: sinusoidal amplitude modulation of the fish's own EOD feedback at these low frequencies does not induce gradual frequency falls. Differential stimulation across body surfaces, which is required for the jamming avoidance response (JAR) of wave-type gymnotiform electric fish, was not necessary for this behavior. We propose a cascade of high-pass and low-pass frequency filters within the amplitude processing pathway in the central nervous system as the mechanism of the gradual frequency fall response.Abbreviations
EOD
electric organ discharge
-
f
frequency of EOD or pacemaker command signal
-
JAR
jamming avoidance response
-
S
1
stimulus mimicking fish's own EOD
-
f
1
frequency of S1
-
S
2
stimulus mimicking neighbor's EOD
-
f
2
frequency of S2 相似文献
39.
DAVID D. HUFF LOREN M. MILLER CHRISTOPHER J. CHIZINSKI BRUCE VONDRACEK 《Molecular ecology》2011,20(20):4246-4258
Reintroductions are commonly employed to preserve intraspecific biodiversity in fragmented landscapes. However, reintroduced populations are frequently smaller and more geographically isolated than native populations. Mixing genetically, divergent sources are often proposed to attenuate potentially low genetic diversity in reintroduced populations that may result from small effective population sizes. However, a possible negative tradeoff for mixing sources is outbreeding depression in hybrid offspring. We examined the consequences of mixed‐source reintroductions on several fitness surrogates at nine slimy sculpin (Cottus cognatus) reintroduction sites in south‐east Minnesota. We inferred the relative fitness of each crosstype in the reintroduced populations by comparing their growth rate, length, weight, body condition and persistence in reintroduced populations. Pure strain descendents from a single source population persisted in a greater proportion than expected in the reintroduced populations, whereas all other crosstypes occurred in a lesser proportion. Length, weight and growth rate were lower for second‐generation intra‐population hybrid descendents than for pure strain and first‐generation hybrids. In the predominant pure strain, young‐of the‐year size was significantly greater than any other crosstype. Our results suggested that differences in fitness surrogates among crosstypes were consistent with disrupted co‐adapted gene complexes associated with beneficial adaptations in these reintroduced populations. Future reintroductions may be improved by evaluating the potential for local adaptation in source populations or by avoiding the use of mixed sources by default when information on local adaptations or other genetic characteristics is lacking. 相似文献
40.
In this paper, the first documentation of egg-guarding behaviour in an artedidraconid species, Pogonophryne scotti, through in situ photographic imagery obtained during video transects is provided. The male specimen was observed closely guarding a well-defined multi-layered egg mass deposited on the sea floor at 240 m on the southern South Orkney Islands shelf in the northern Weddell Sea. Egg-guarding parental care is present in species that are distributed among all of the major lineages of Antarctic notothenioids; however, lack of information on egg-deposition behaviours in Bovichtidae and Pseudaphritis prevents assessment of whether parental care originated prior to the origin of the Antarctic notothenioid radiation. 相似文献