日本一个研究小组在新一期英国《生物学通讯》杂志网络版上发表文章说，他们通过基因分析发现，鳗鱼的祖先很可能是深海鱼。
鳗鱼生活在河川、湖泊等淡水中，但成年鳗鱼会不远千里到深海产卵。鳗鱼这种独特的洄游行为一直未得到很好的解释。
东京大学海洋研究所等机构的研究人员以包括鳗鱼科在内、亲缘关系较近的19科56种鱼类为对象进行了研究。通过比较它们的全基因序列，研究人员绘制出表示它们进化轨迹的系统树。结果显示，鳗鱼科鱼类与生活在热带海洋深处的线鳗科和锯犁鳗科鱼类的亲缘关系非常近。属于线鳗科和锯犁鳗科的一些鱼类拥有巨大的嘴或者像喙一样的颚，之前人们认为它们与鳗鱼的亲缘关系较远。
据此，研究人员推断鳗鱼科和线鳗科等科鱼类拥有共同的深海鱼祖先。研究人员说，在鳗鱼科鱼类祖先所栖息的热带和亚热带，淡水区域比海洋拥有更丰富的食物和营养。为了保证获得充足食物和营养，鳗鱼科鱼类的祖先选择在淡水中生活，但产卵场所依然是深海，这一习惯一直延续到今天。（生物谷Bioon.com）
生物谷推荐原始出处：
Biology Letters January 6, 2010, doi: 10.1098/rsbl.2009.0989
Deep-ocean origin of the freshwater eels
Jun G. Inoue1,*, Masaki Miya2,*, Michael J. Miller1, Tetsuya Sado2, Reinhold Hanel3, Kiyotaka Hatooka4, Jun Aoyama1, Yuki Minegishi1, Mutsumi Nishida1 and Katsumi Tsukamoto1
1Ocean Research Institute, The University of Tokyo, Tokyo 164-8639, Japan
2Natural History Museum and Institute, Chiba, Chiba 266-8682, Japan
3Johann Heinrich von Thünen-Institut 22767, Hamburg, Germany
4Osaka Museum of Natural History, Osaka 546-0034, Japan
Of more than 800 species of eels of the order Anguilliformes, only freshwater eels (genus Anguilla with 16 species plus three subspecies) spend most of their lives in freshwater during their catadromous life cycle. Nevertheless, because their spawning areas are located offshore in the open ocean, they migrate back to their specific breeding places in the ocean, often located thousands of kilometres away. The evolutionary origin of such enigmatic behaviour, however, remains elusive because of the uncertain phylogenetic position of freshwater eels within the principally marine anguilliforms. Here, we show strong evidence for a deep oceanic origin of the freshwater eels, based on the phylogenetic analysis of whole mitochondrial genome sequences from 56 species representing all of the 19 anguilliform families. The freshwater eels occupy an apical position within the anguilliforms, forming a highly supported monophyletic group with various oceanic midwater eel species. Moreover, reconstruction of the growth habitats on the resulting tree unequivocally indicates an origination of the freshwater eels from the midwater of the deep ocean. This shows significant concordance with the recent collection of mature adults of the Japanese eel in the upper midwater of the Pacific, suggesting that they have retained their evolutionary origin as a behavioural trait in their spawning areas.
