摘要
研究者自2002年10月至2005年7月間，在33個淡水水域採集到Simocephalus（低額溞屬）枝角類（Cladocera: Daphniidae）：Simocephalus vetulus（老年低額溞）、Simocephalus vetuloides（擬老年低額溞）、Simocephalus mixtus（混型低額溞）、Simocephalus serrulatus（鋸頂低額溞）、Simocephalus heilongjiangensis（黑龍江低額溞），其中S. mixtus及S. heilongjiangensis是台灣新記錄種。
S. vetulus、S. vetuloides、S. mixtus在傳統分類學上有許多爭議，本研究以形質分析、分子分析進行探討。
形質分析樣本來自45隻S. vetulus、72隻S. vetuloides、70隻S. mixtus，總計有187個成熟帶有夏卵的雌性個體。以地標點法取得8個特徵點，轉換成24段特徵值進行主成份分析，結果顯示S. vetuloides與S. vetulus、S. vetuloides與S. mixtus形態重疊；階層群集分析結果大致上可分為S. vetulus、S. vetuloides、S. mixtus三個群，但有混入其他類型的個體。S. vetulus平均體長最小、平均卵數最少、平均背角最小，S. vetuloides平均體長、平均卵數、平均背角居中，S. mixtus平均體長最大、平均卵數最多、平均背角最大，由SPSS計算得到體長與卵數、背角與卵數之皮爾森相關係數為0.700、0.725，兩值皆達顯著相關，且為正曲線相關。
分子分析共有98個低額溞屬樣本，以mtDNA之COI核酸序列當作分子遺傳標記，獲得S. vetulus、S. vetuloides、S. mixtus、S. serrulatus、S. heilongjiangensis 641 bp的序列長度，各類型基因型多樣性（Hd）：0.374 ~ 0.802，核苷酸多樣性（π）：0.00140 ~ 0.00806，遺傳距離（Dxy）：0.00671 ~ 0.16046，其中S. vetulus、S. vetuloides、S. mixtus的基因型重疊，三者間遺傳距離僅0.00671 ~ 0.00785，為種內差異；由親緣關係樹形圖得知S. heilongjiangensis是較原始種，繼而有S. serrulatus，最後是S. vetulus、S. vetuloides、S. mixtus群，而S. heilongjiangensis與S. serrulatus則有較近的親緣關係。
研究者透過形質分析、分子分析，皆未能支持S. vetulus、S. vetuloides、S. mixtus三者為獨立種，推論S. vetulus、S. vetuloides、S. mixtus應屬同一種，建議以優先法則命名為Simocephalus vetulus（O. F. Műller, 1776），所以，台灣產低額溞屬有S. vetulus、S. serrulatus、S. heilongjiangensis三種。

Abstract
From October 2002 to July 2005, samples of Simocephalus species（Cladocera: Daphniidae）were collected from 33 sites in various freshwater bodies. These samples contained Simocephalus vetulus, Simocephalus vetuloides, Simocephalus serrulatus, Simocephalus mixtus, and Simocephalus heilongjiangensis. The last two species are new records for Taiwan.
There are some controversial issues in the traditional taxonomy concerning Simocephalus vetulus, S. vetuloides and S. mixtus. I condoucted this study to differentiate these species by using molecular and morphological analyses. By the landmark method, eight characters were selected and then transferred into 24 characteristric values for the Principal Components Analysis. The PCA result indicated overlapping of morphometry among these species. The result of hierarchical cluster showed that there were three groups each composing of one of these three species, but some individuals of one species were mixted with the group of other species. Dorsal angle, brood size, and body length were smallest in S. vetulus, largest in S. mixtus, and medium in S. vetuloides. Pearson’s correlation coefficient was 0.725 between dorsal angle and brood size and 0.700 between body length and brood size. Both coefficients were correlatively significant in quadratic curve.
In the molecular analysis, 98 specimens of Simocephalus were used. The present study applied the mtDNACOI sequence 641 bp as genetic maker to analysis of the population genetic structure of S. vetulus, S. vetuloides, S. mixtus, S. serrulatu, and S. heilongjiangensis. The haplotype diversity（Hd）ranged from 0.374 to 0.802; nucleotide diversity（π） from 0.00140 to 0.00806, and genetic distance（Dxy） from 0.00671 to 0.16046. The haplotypes of S. vetulus, S. vetuloides, and S. mixtus overlapped. The genetic distance between each other was merely 0.00671-0.00785, pertaining to intraspecific difference. Based on phylogenetic analysis, S. heilongjiansis was the most primitive among the five species. S. serrulatus was most closely related to S. heilongjiansis, having the shortest genetic distance at 0.14391.
From the mophormetric and molecular comparisons, it is concluded that Simocephalus vetulus, S. vetuloides and S. mixtus are the same species. S. vetuloides and S. mixtus are considered to be synonymous with S. vetulus. The number of species of Simocephalus in Taiwan is thus reduced to three, namely, S. vetulus, S. serrulatus and S. heilongjiangensis.

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