Metzia formosae

Metzia: named for Charles William Metz, Stanford University, who helped authors identify fishes collected in Japan.

formosae: 'of Formosa [Taiwan]', in reference to the type locality.

Known from northern Taiwan, Hainan Island, Hong Kong, the Pearl River (Zhu Jiang) basin in Guangxi and Guangdong provinces of southern China, and the Red River watershed in Yunnan province, China, and northern Vietnam. It may also occur in Laos.

Type locality is 'small pond near Manka, Taihoku, Taiwan'.

Inhabits slow-moving to still habitats, typically containing clear to turbid water and abundant aquatic or riparian vegetation, including streams and other minor tributaries, rice paddies, marginal ditches, and canals.

Typical sympatric species include the congener Metzia lineata, Barbodes semifasciolatus, Rasbora steineri, and Macropodus opercularis.

Sexually mature females tend to be deeper-bodied than males.

This species can be distinguished from congeners by the following combination of characters: mouth superior; 43-44 lateral line scales; colour pattern comprising a longitudinal black stripe on the flank, extending from the posterior extremity of the gill opening to the caudal-fin base.

Although it is included in the genus Metzia this grouping remains somewhat unresolved in terms of taxonomy and it may contain more than a single genetic lineage. For example, members share a number of characters with the genera Ischikauia Jordan and Snyder 1900, Hemiculterella Warpachowski 1887, and several species of Anabarilius Cockerell 1923, including a bipartite gas bladder, a non-spinous dorsal-fin ray, and a sharp ventral keel between the pelvic-fin insertion and anus.

Metzia was also recovered together with the genus Hemigrammocypris in a phylogenetic study published in 2010, thus rendering it paraphyletic. In addition, M. formosae and M. longinasus differ from most other members of the genus in mouth position (upturned vs. terminal in putative congeners) and number of lateral line scales (43–48 vs. 35-39), and M. bounthobi differs from all congeners in number of branched anal-fin rays (18-20 vs. 10-18) and lateral line scales (49-55 vs. 35-48).

The current concept of Metzia can be diagnosed as follows: body compressed; abdomen with a sharp ventral keel between pelvic-fin insertion and anus; no barbels; no symphysal projection on the lower jaw fitting into a depression on upper jaw; lateral line complete, curved ventrally anterior to pelvic-fin insertion, running parallel with ventral profile and extending through centre of caudal peduncle above anal-fin base; gas bladder bipartite, posterior chamber with a rounded distal extremity; last simple dorsal-fin ray not spinous, origin closer to caudal-fin base than tip of snout or posterior to a vertical through pelvic-fin insertion; 10-20 branched anal-fin rays; 35-55 lateral-line scales.