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：本研究基于克诺得斯脂鲤属（Knodus）的传统分类定义，对托坎廷斯河（rio Tocantins）上游流域的2个新物种展开分类描述。该新种可凭借多项计数性状与形态测量性状，与同属其余物种相区分，具体包括前颌骨与齿骨的齿尖数量、背鳍起点至侧线间的鳞列数、眼径以及体高。加上这两个新种后，位于帕拉尼亚河（rio Paranã）河口上游的托坎廷斯河上游流域的特有物种数量增至53种（至与阿拉瓜亚河（rio Araguaia）汇流处的特有物种总数为89种）。本研究通过回归分析，在部分克诺得斯脂鲤属（Knodus）物种中发现了一种随个体发育发生变化的计数性状——异数发育（allomery），具体表现为背鳍起点至侧线间的鳞列数随个体发育改变。此外，本文还提出了一种清晰明确、信息量更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中的应用效能。