Two new species of Knodus (Characidae: Stevardiinae) from the upper rio Tocantins basin, with evidence of ontogenetic meristic changes

ABSTRACT Two new species from the upper rio Tocantins basin are described in Knodus based on the traditional definition of the genus. The new species are distinguished from other congeners by meristic and morphometric characters, such as the number of cusps in the premaxillary and dentary teeth, the number of scale series between dorsal-fin origin and lateral line, the orbital diameter and the body depth. With the two new species, the number of endemic species in the upper rio Tocantins basin upstream of the mouth of the rio Paranã, rises to 53 (89 to the confluence with rio Araguaia). The existence of a meristic character that changes through ontogeny (allomery), viz. the number of scale series between dorsal-fin origin and lateral line, was detected in some species of Knodus through a regression analysis. Additionally, this paper describes an unambiguous, more informative and precise new method for counting vertebrae, which will enhance the efficacy of this trait in species comparisons.

摘要
本研究依据Knodus属的传统分类定义，对托坎廷斯河（rio Tocantins）上游流域的两个新种开展了分类描述。该两个新种可通过多项计数分类性状与可量分类性状与其他同属物种相区分，包括前颌骨与齿骨齿的齿尖数量、背鳍起点至侧线间的鳞列数、眼径以及体高。
纳入这两个新种后，帕拉南河（rio Paranã）河口以上的托坎廷斯河上游流域的特有物种数量增至53种；若研究范围延伸至与阿拉瓜亚河（rio Araguaia）的汇流处，则该流域的特有物种总数可达89种。
本研究通过回归分析，在部分Knodus属物种中检测到一种随个体发育发生改变的计数分类性状，即背鳍起点至侧线间的鳞列数，该现象属于异速生长变异（allomery）。
此外，本研究还提出了一种清晰直观、信息更为丰富且精度更高的椎骨计数新方法，该方法将提升该性状在物种比较研究中的应用效能。