Regulation of splenic contraction persists as a vestigial trait in white-blooded Antarctic fishes

In fishes, the spleen can function as an important reservoir for red blood cells (RBCs), which, following splenic contraction, may be released into the circulation to increase haematocrit during energy-demanding activities. This trait is particularly pronounced in red-blooded Antarctic fishes in which the spleen can sequester a large proportion of RBCs during rest, thereby reducing blood viscosity, which may serve as an adaptation to life in cold environments. In one species, Pagothenia borchgrevinki, it has previously been shown that splenic contraction primarily depends on cholinergic stimulation. The aim of the present study was to investigate the regulation of splenic contraction in five other Antarctic fish species, three red-blooded notothenioids (Dissostichus mawsoni Norman, 1937, Gobionotothen gibberifrons Lönnberg, 1905, Notothenia coriiceps Richardson 1844) and two white-blooded “icefish” (Chaenocephalus aceratus Lönnberg, 1906 and Champsocephalus gunnari Lönnberg, 1905), which lack haemoglobin and RBCs, but nevertheless possess a large spleen. In all species, splenic strips constricted in response to both cholinergic (carbachol) and adrenergic (adrenaline) agonists. Surprisingly, in the two species of icefish, the spleen responded with similar sensitivity to red-blooded species, despite contraction being of little obvious benefit for releasing RBCs into the circulation. Although the icefish lineage lost functional haemoglobin before diversifying over the past 7.8–4.8 millions of years, they retain the capacity to contract the spleen, likely as a vestige inherited from their red-blooded ancestors.     

Interannual variation in condition of the mackerel icefish

Mackerel icefish Champsocephalus gunnari are widespread on the South Georgia (54° S, 36° W) shelf. Analysis of condition indicated a strong interannual variation. High condition indices, indicative of good feeding conditions, were present when krill were abundant in the region. Years when krill were scarce and condition index was consequently low, were consistent with years when indices from land-based krill predators also indicated that krill were scarce.     

Evolution after Whole-Genome Duplication: Teleost MicroRNAs

MicroRNAs (miRNAs) are important gene expression regulators implicated in many biological processes, but we lack a global understanding of how miRNA genes evolve and contribute to developmental canalization and phenotypic diversification. Whole-genome duplication events likely provide a substrate for species divergence and phenotypic change by increasing gene numbers and relaxing evolutionary pressures. To understand the consequences of genome duplication on miRNA evolution, we studied miRNA genes following the teleost genome duplication (TGD). Analysis of miRNA genes in four teleosts and in spotted gar, whose lineage diverged before the TGD, revealed that miRNA genes were retained in ohnologous pairs more frequently than protein-coding genes, and that gene losses occurred rapidly after the TGD. Genomic context influenced retention rates, with clustered miRNA genes retained more often than nonclustered miRNA genes and intergenic miRNA genes retained more frequently than intragenic miRNA genes, which often shared the evolutionary fate of their protein-coding host. Expression analyses revealed both conserved and divergent expression patterns across species in line with miRNA functions in phenotypic canalization and diversification, respectively. Finally, major strands of miRNA genes experienced stronger purifying selection, especially in their seeds and 3′-complementary regions, compared with minor strands, which nonetheless also displayed evolutionary features compatible with constrained function. This study provides the first genome-wide, multispecies analysis of the mechanisms influencing metazoan miRNA evolution after whole-genome duplication.     

Age and growth of ocellated icefish, Chionodraco rastrospinosus DeWitt and Hureau, 1979, from the South Shetland Islands

Age and growth of ocellated icefish, Chionodraco rastrospinosus, were investigated using counts of annual growth increments from sagittal otoliths. Samples were collected during research surveys by benthic trawl carried out around Elephant and South Shetland Islands in January–February 2002 and December 2006–January 2007. A total of 290 specimens were selected for the study, consisting of 120 females and 170 males. The age of fish was estimated by counting annuli on transverse sections obtained by grinding and polishing whole otoliths embedded in epoxy resin. The precision-of-age estimates within and between readers were tested applying both the average percent error (APE) and the coefficient of variation (CV). The estimated age-range was 1–12 for both sexes of C. rastrospinosus. Applying the von Bertalanffy growth function to the age–length data, a growth curve was obtained for each sex. The estimated values of VB growth parameters L _∞ and k were, respectively, 47.9 cm and 0.28 for females and 42.9 cm and 0.36 for males. Compared to other congeneric species, the growth performance of C. rastrospinosus was relatively high, being 2.82 and 2.81 in males and females, respectively. Age at sexual maturity was estimated to be about 4 years in both sexes. C. rastrospinosus captured in the studied area consisted mainly of adult specimens between 3 and 8 years, with few older fish.     

有明银鱼耳石显微结构和微化学研究

1998年从鸭绿江捕取有明银鱼成鱼 ,生物学测量后剖出耳石。每尾鱼 1对矢耳石中的 1个用于制片 ,显微镜下观察其显微结构 ,另 1个制片镀碳膜后 ,用扫描电子微探针测定Sr和Ca的浓度比率。Sr和Ca分析的标准样用SrSO4 和CaSO4 。 31尾鱼 (体长 116～ 15 7mm)耳石长半径 (Y)与鱼体长 (X)成线性关系 ,其方程式为Y =2 72 16X +9 2 2 92 ,r =0 9178,P <0 0 1。耳石中心和耳石原基的平均直径分别为 2 2 5 5± 2 88μm和6 95± 1 30 μm。 13尾鱼耳石上的日轮数为 30 5～ 35 4,大约在 45～ 70日轮处出现过渡轮。日轮间距 0 94～ 1 14μm ,最大间距在 190～ 2 2 0日轮处 ,最小间距出现在 2 80日轮之后。耳石中Sr、Ca浓度比例在第 5至第 7测点出现第 1个高峰 ,这与稚鱼为了越冬洄游到海中时间相吻合。第 2个高峰出现在 2 8～ 32测点 ,这与鱼产卵上溯洄游时间相符。个体发育过程中生理和生态条件的变化 ,可引起耳石中Sr、Ca比率的改变 ,但栖息水域盐度的突然改变是导致Sr、Ca比率显著增高的主导因素。     

Shedding new light on the life cycle of mackerel icefish in the Southern Ocean

Mackerel icefish have a widespread distribution in the Atlantic and Indian Ocean sectors of the low-Antarctic region. Biological characteristics differ considerably between populations in the southern Scotia Arc and those living further to the north. Fish living in the north mature 1 year earlier than in the south. They have a much shorter life span and die after they have spawned two to three times. The number of eggs produced per gram of body mass is higher in the north. Stocks have declined in most parts of the distributional range due to the impact of fishing and due to natural causes. Increases in populations of Antarctic fur seals at South Georgia and parts of the Indian Ocean appear to have led to increased predation on stocks of icefish. Shifts in hydrological regimes in the northern part of the distributional range have either started to lead or will lead to deteriorating living conditions for mackerel icefish in the near future. Fish stock assessment needs to take these constraints into consideration when providing advice on total allowable catches for fisheries management.     

江湖阻隔对短吻间银鱼空间发生格局的影响

基于2001–2003年长江流域86个水体短吻间银鱼资源的连续调查,本文报道了短吻间银鱼(Hemisalanxbrachyrostralis)在流域尺度上的分布格局和年度动态,分析了其时空分布格局与湖泊面积和隔离因子间的关系,探讨了流域江-湖水网系统破碎化对短吻间银鱼种群结构和动态的影响。结果显示短吻间银鱼仅分布于葛洲坝以下的长江中下游,发生率低(年均21.3%),空间发生格局年度变动频率高,种群稳定性低。调查湖泊内目标种发生率、种群稳定性均与湖泊面积呈正相关关系,而与湖泊隔离系数呈负相关关系;三年的发生格局始终极显著地正相关于湖泊的面积因子,同时负相关于湖泊的隔离因子。表明江湖阻隔所引发的湖泊水面缩减和湖间隔离度增大对短吻间银鱼资源产生了深刻的影响,对其空间格局的形成贡献显著。     

A vast icefish breeding colony discovered in the Antarctic

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Distribution and ecology of <Emphasis Type="Italic">Chaenocephalus aceratus</Emphasis> (Channichthyidae) around South Georgia and Shag Rocks (Southern Ocean)

Chaenocephalus aceratus (Family Channicthyidae) is one of the dominant species of demersal fish living on the South Georgia shelf where it is caught in low numbers as by-catch in the mackerel icefish and Antarctic krill commercial fisheries. Data collected during 14 demersal fish surveys, from 1986 to 2006, are analysed to investigate biomass, distribution, growth and diet. Biomass estimates from a swept area method ranged from 4,462 to 28,740 tonnes on the South Georgia and Shag Rock shelves although few fish were caught at Shag Rocks. Analysis of length frequency data indicated that growth was fast in the first five years with males and females attaining lengths at first spawning of 440 mm TL and 520 mm TL. The diet was comprised of fish and crustaceans, with an ontogenetic shift in diet from Euphausia superba and mysids to benthic fish and decapods observed to begin at 250 mm TL. In larger fish (>500 mm TL) the diet was dominated by fish. C. aceratus diet is sufficiently different from the other species of channichthyids around South Georgia to suggest that these species have undergone resource partitioning.