Metzia formosae and Metzia lineata

Two very distinct congeners from Hainan Island

There is deep taxonomical confusion around the genera Metzia Jordan & Thompson 1914 and Rasborinus Oshima 1920. Correct identification of specimens is difficult. The fact, even the quite helpful and reliable fish species database Fishbase at www.fishbase.com shows a picture of Metzia formosae (herein called under its junior synonym Rasborinus formosae) in their species fact sheet of Metzia lineata. This is an example of the comprehensive misunderstanding of the relationships within this species’ group.

The story: Jordan and Thompson created the genus Metzia in 1914 using Acheilognathus mesembrinum Jordan & Evermann 1902 as the type species. Thus the new name for Acheilognathus mesembrinum is Metzia mesembrinum (Jordan & Evermann 1902). Mesembrinum derived from Greek and is a noun in apposition and therefore do not need to follow the feminine gender of the genus Metzia. Apart from Acheilognathus mesembrinum Jordan and Thompson did not enclose any other species in Metzia, so that the generic status was monotypic at this time.

Six years later, in 1920, the Japanese ichthyologist Oshima created the genus Rasborinus defining R. takakii as the type species.

Following KOTTELAT (2001b), who compared the original descriptions and figures, Metzia mesembrinum and Rasborinus takakii are conspecific, means R. takakii is a junior synonym of M. mesembrinum. They are one and the same species. Since a type species defines the characteristics for the entire genus, means for every species belonging to this genus, all the species of former Rasborinus should be placed into Metzia. The genus Rasborinus should be a junior synonym of Metzia (see also Berg, 1932; Myers, 1934).

This is the widely unknown Metzia lineata from West Hainan nearby Dongfang (Code: EZ-CN 13-06)

Nowadays the genus Metzia incorporates the following recent species: 

Metzia alba (Nguyen, 1991)

Metzia formosae (Oshima, 1920)

Metzia hautus (Nguyen, 1991)

Metzia lineata (Pellegrin, 1907)

Metzia mesembrinum (Jordan & Evermann, 1902)

The status of the species Metzia alba and M. hautus is uncertain, since Nguyen’s species descriptions are issued in Vietnamese language and are not accompanied by an illustration (KOTTELAT, 2001b). Future taxonomical surveys will unveil that the very distinct species Metzia formosae does not belong into Metzia, since the species’ habitus is simply too different. Thus KOTTELAT uses the taxon only as ‘Metzia’ formosae indicating the comparatively distinct relationship.

Metzia species are widely unknown to aquarists. On my collection trip to Hainan Island I could catch Metzia formosae and M. lineata. Both species were living sympatrically in the biotope EZ-CN 13-06 located in the western part of the Island near Dongfang (东方市). If you are interested in biotope details please check in the biotope side the Hainan 2006 report.

The dominating species was Puntius semifasciolatus, the second most frequent was Metzia lineata. Metzia formosae was quite rare. Other species caught in this biotope were Macropodus opercularis, Gambusia affinis and a yet undetermined sea needle (order Syngnathiformes). In the river bank there were freshwater decapods of at least three different species in abundance. Each net draw at the bank conveyed dozens of these shrimps. Some of those might be escaped from the aquaculture ponds located some kilometers down the river. The aquaculture farmers usually use brackish water for raising the shrimps, but since these species are euryhaline they adapt easily the hard, alcaline freshwater.

Metzia formosae from EZ-CN 13-06. 

Back home I introduced Metzia formosae and M. lineata into my 280 x 55 x 40 cm living room tank. This tank did not have any water plants, but plenty of pebbles and bigger round rocks. The bottom was covered by a thin layer of sand. Due to the lack of direct lighting the entire setup was very shady. The center of the tank was very dark and contained a couple of drift wood and roots in order to provide hiding places and refugia. I used common hard and alcaline tap water without any special treatment. Since I did not used any heating equipment the resulting temperatures were ranging from 20ºC to 28ºC in very hot summer months. Other tank mates were wild caught Rhinogobius guirinus, Yaoshanicus normalis, Macropodus opercularis, Gambusia affinis and Rasbora steineri from Hainan Island. In addition I used the big tank as a grow up setup for juvenile Pseudomugil furcatus.

Metzia formosae and M. lineata behaved very shy during the entire time. When somebody approached the aquarium they were immediately retracting into the hiding places at the tank’s center. They congregated in this area forming a dense school. Both species mixed when congregating. If the fish keeper remains still they were hesitatingly leaving the shelters. Metzia lineata were always the first followed by M. formosae. One by one were leaving the darker areas to enter brighter’s in order to search for some food on the bottom. They mainly feed from the bottom than from the free water. In terms of food they did not have high demands; blood worms, frozen or living, living Tubifex and Daphnia, frozen Artemia and beef heard and also flake food were taken.

Both species are fishes of the lower water layers swimming some centimeters above the bottom. They like to school up in groups of 4 to 6 fishes and never show any intra- or interspecific aggression. Unfortunately their coloration is relatively pale, thus they probably never make an aquaristic career.

Marco Endruweit, 2007-10-20