The Three Rummynose Tetras are reclassified

Byron

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We had a thread a few weeks back in which we were discussing the differences in the three "rummynose" tetras. A new study published electronically on June 26, 2020 in the journal Neotropical Ichthyology proposes the taxonomic revision of the three "rummynose" tetra species Hemigrammus rhodostomus Ahl, 1924, Hemigrammus bleheri Gery & Mahnert, 1986 and Petitella georgiae Gery & Boutiere, 1964. The study is "Phylogenetic relationships of the neon tetras Paracheirodon spp. (Characiformes: Characidae: Stethaprioninae), including comments on Petitella georgiae and Hemigrammus bleheri," and the authors are Pedro Senna Bittencourt, Valéria Nogueira Machado, Bruce Gavin Marshall, Tomas Hrbek and Izeni Pires Farias. The paper can be referenced in abbreviated form as Bittencourt et al, 2020. I will link the paper at the end.

H. rhodostomus was the first rummynose tetra to be discovered and described. In 1924, a young German researcher, Gustav Ernst Ahl (who in later life was the Director of the Natural History Museum in Berlin), discovered the fish in the tributaries of the Rio Orinoco in Venezuela, and subsequently described the species. The species epithet is from the Greek and translates as "rose's mouth."

The second species discovered three or four decades later was Petitella georgiae in the Purus and Madeira rivers which are located in the southern Amazon basin. Following years of study, Jacques Gery and Henry Boutiere described the distinct species in 1964. The species epithet georgiae honours Gery's wife, Georgie. Gery & Boutiere erected the new genus Petitella, and up to the present this has been the only species assigned to the genus.

Gery, who was during his life (he died in June 2007) the unequalled authority on the characidae, enters the story again. In the 1960's the then-20 year old explorer Heiko Bleher discovered the third "rummynose" in the middle Rio Negro. Gery & Mahnert (1984) described the species as Hemigrammus bleheri, in honour of its discoverer. Heiko in his 60+ years as an explorer has discovered more than 6000 new species of fish across the globe--including a fourth "neon tetra" which is awaiting description.

The occurrence of three distinct species which nevertheless bear such remarkable similarity to one another has long intrigued ichthyologists. The external characteristics of these species are unique among all known characidae. "The genus Petitella is readily distinguished from all remaining characid genera by the possession of a distinctively bright red head, the presence of a black horizontal bar that extends from the end of the caudal peduncle to the middle rays of the caudal-fin, and the presence of an oblique black bar in each caudal-fin lobe, separated by white colored bands" (Bittencourt et al, 2020).

The prime focus of this study was to "test the Paracheirodon monophyly hypothesis using a molecular phylogenetic approach, through analysis of the mitochondrial genes cytochrome C oxidase subunit I (COI) and the 16S ribosomal RNA (16S rRNA)." The three species Paracheirodon innesi, P. axelrodi and P. simulans form a monophyletic clade, meaning that the three distinct species descended from a common ancestor. Other species were involved in the analysis, leading us to the three rummynose tetras. The paper's abstract is a good summary:

Neon tetras (Paracheirodon spp.) are three colorful characid species with a complicated taxonomic history, and relationships among the species are poorly known. Molecular data resolved the relationships among the three neon tetras, and strongly supported monophyly of the genus and its sister taxon relationship to Brittanichthys. Additionally, the sister-taxon relationship of the rummy-nose tetras Hemigrammus bleheri and Petitella georgiae was strongly supported by molecular and morphological data. Therefore, we propose to transfer the rummy-nose tetras H. bleheri and H. rhodostomus to the genus Petitella. Furthermore, Petitella georgiae is likely to be a species complex comprised of at least two species.​

Under the rules of the International Commission on Zoological Nomenclature (ICZN), publication of the paper revises the taxonomy. The genus Petitella Gery & Butiere, 1964 now holds three species; the type species Petitella georgiae Gery & Boutiere, 1964, and Petitella rhodostomus (Ahl, 1924) and Petitella bleheri (Gery & Mahnert, 1984). "Type species" is the species first described within the genus and all subsequent species in the genus will share the identical characteristics which cause them to be unique to this assemblage; note that this may include differing external markings which do not necessarily alter the distinct biological traits that the species in the genus must share. The describer's names when in brackets indicates that while they were the first to described the species as distinct, it has since been transferred to a different genus from the one in which it was initially described.


Edited July 7 to correct species epithet of P. rhodostomus.
 
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Barry Tetra

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Thanks for interesting info as always Byron :)
 
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Byron

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The question has been asked in the past and undoubtedly will again be asked, how to tell the three rummy's apart, so here are the characteristics used by ichthyologists, cited from the paper.

Comparative remarks. Petitella georgiae is distinguished from its congeners by a long and wide maxillary (vs. very short and round in P. bleheri, and short in P. rhodostoma); single row of premaxillary teeth (vs. two in P. bleheri and P. rhodostoma); dentary with 9–11 teeth with 5 cuspids (vs. 6, with 6 or 7 cuspids, followed by 1 or 2 tricuspidate ones in P. bleheri, and 5–6 with 5 cuspids, usually followed by 4 conical teeth in P. rhodostoma); absence of black spot on lower posterior border of caudal peduncle (vs. present in P. bleheri and P. rhodostoma).​
Petitella bleheri is distinguished from its congeners by the much more intense and widespread red color of the head, extending up to the humeral region (vs. limited red coloration and not extending to humeral region in P. georgiae and P. rhodostoma);horizontal black bar on the end of the caudal peduncle is never prolonged forward (vs. prolonged up to the anal-fin in P. georgiae and P. rhodostoma); anal-fin hyaline (vs. a black bar on the base of the anterior part of the anal-fin, continuing obliquely on the branched rays in P. georgiae and P. rhodostoma).​
Petitella rhodostoma is distinguished from its congeners by the red head color not extending to the humeral region and the presence of a black spot on the lower posterior border of the caudal peduncle (vs. head color not extending to humeral region with only one black spot on caudal peduncle in P. georgiae, and head color extending to humeral with two black spots on caudal peduncle in P. bleheri); dentary with 5–6 teeth, with 5 cuspids, usually followed by 4 conical ones (vs. 9–11 teeth, with 5 cuspids in P. georgiae, and 6 teeth, with 6 or 7 cuspids, followed by 1 or 2 tricuspidate ones in P. bleheri).​

You will note there are external differences and internal differences. I also missed the gender change in the species epithet of P. rhodostoma so this has been corrected in the original post. Under the rules of the ICZN when a species changes genus, the epithet may sometimes be altered to agree in gender with the genus name, and that is why rhodostomus becomes rhodostoma here. I won't go into the complications of this, why it doesn't always. ;) But for those interested, the entire Code (18 chapters) can be accessed free of charge on the ICZN site:
 
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PheonixKingZ

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Very interesting.

It is also interesting how so many fish keepers out there are confused about how many types of Rummy nose tetras there are.
 

essjay

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I knew that the fish most often found in shops was not the true rummy nose tetra, and recently found out that there is also a third species that goes by the same common name in shops. It is often hard to convince people that they don't have the true rummy nose tetra, although in practice it doesn't really matter which they have. Now we need to say that the name of two has changed, though it'll take a long time to appear in the literature.
 
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Byron

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The California Academy of Sciences maintains Eschmeyer's Catalog of Fishes [link below]. This project was initiated by Dr. William Eschmeyer in 1980, and as of yesterday's monthly update it contains the scientific names of 11,116 fish genera and 63,610 fish species with the help of 31,363 references. Leading to a total of 60,639 available species names for currently 35,567 valid recent fish species [about 17.940 species from freshwater and 17.627 from marine environments]. Eschmeyer's Catalog of Fishes is currently edited by Ronald Fricke (on staff of the Department of Zoology, State Museum of Natural History Stuttgart, Germany), William N. Eschmeyer (Curator Emeritus, California Academy of Sciences, San Francisco, California; Research Associate, Florida Museum of Natural History, Gainesville, Florida), and Richard van der Laan (on the faculty of Hogeschool Utrecht, Netherlands).

The "rummynose" species revisions were officially incorporated with yesterday's update to the Catalog. I noticed that the species epithet of Petitella rhodostomus had not been changed to P. rhodostoma as proposed in the paper, so I contacted the technical officer to confirm; I will be updating the Seriously Fish entries and these things must be accurate. This morning I received a response from Richard van der Laan that rhodostomus is correct under the Code of the ICZN because it is a noun and as such cannot be declined (as can adjectives) so it does not change according to the gender of the genus.

The ICZN Code is very complex, and I have often consulted the CAS for clarification. A great resource in ichthyology, and extremely helpful professionals.

Here is the link to the catalogue. Each species entry provides the initial name of the species with the name of the assignor and the date, and a reference to the publication containing the description of the species. The species are listed alphabetically by species epithet since this cannot change once assigned. All subsequent uses of the name in the scientific literature, including any changes, with the author and date and a reference to the paper, follow. Each entry ends with the valid name now accepted, with the Family, Sub-family if relevant, distribution and habitat. Here are the entries for the rummynose species as an example.

bleheri, Hemigrammus Géry [J.] & Mahnert [V.] 1986:41, Fig. [Tropical Fish Hobbyist v. 34 (no. 11); ref. 8088] Middle Rio Negro basin, Brazil (probably near Rio Jufaris). Holotype: MZUSP 37369. Paratypes: MHNG 2179.87 (1), 2179.91 (1), 2179.90 and 2239.51 (5), 2239.52 (1). Type catalog: Oyakawa 1996:467 [ref. 23140]. •Valid as Hemigrammus bleheri Géry & Mahnert 1986 -- (Taphorn 1992:224 [ref. 23654], Lima & Oyakawa in Reis et al. 2003:131 [ref. 27061], Carvalho et al. 2010:253 [ref. 30862], Mirande 2010:508 [ref. 31006], DoNascimiento et al. 2017:48 [ref. 35633]). •Valid as Petitella bleheri (Géry & Mahnert 1986) -- (Bittencourt et al. 2020:7 [ref. 37634]). Current status: Valid as Petitella bleheri (Géry & Mahnert 1986). Characidae: Stethaprioninae. Distribution: Negro and Meta River basins: Brazil and Colombia. Habitat: freshwater.​
georgiae, Petitella Géry [J.] & Boutière [H.] 1964:474, Fig. 1 [Vie et Milieu Suppl. No. 17; ref. 1599] Lagunas village, lower Río Huallago, Loreto District, Peru. Holotype: MHNG 2150.28 [ex Géry coll. M.314,1]. Paratypes: ANSP 139710 (6); MHNG 2150.28 (1), 2150.29-64 (37), 2150.6588 (31); MNHN 1980-1447 (2); ZFMK 1419 (1); ZSM 26332 (3). Type catalog: Busse 1984:218 [ref. 28342], Böhlke 1984:47 [ref. 13621], Weber 1998:8 [ref. 23532], Herder et al. 2010:118 [ref. 31087], Neumann 2011:247 [ref. 31652]. •Valid as Petitella georgiae Géry & Boutière 1964 -- (Ortega & Vari 1986:9 [ref. 9708], Lima in Reis et al. 2003:153 [ref. 27061], DoNascimiento et al. 2017:52 [ref. 35633], Bittencourt et al. 2020:7 [ref. 37634]). Current status: Valid as Petitella georgiae Géry & Boutière 1964. Characidae: Stethaprioninae. Distribution: Amazon River basin: Brazil, Colombia and Peru. Habitat: freshwater.​
rhodostomus, Hemigrammus Ahl [E.] 1924:405, fig. [Wochenschrift für Aquarien- und Terrarienkunde v. 21 (no. 18); ref. 15433] Rio Tapajoz, near Santarem, Pará, Brazil. Lectotype: ZMB 22626 [apparently not 22616]. Paralectotypes: USNM 273367 [ex ZMB 22626] (2); ?ZMB 20804 (1), 32434 (3). Types mentioned: Paepke 1995:92 [ref. 21847], Zarske & Géry 1995:111, Fig. 7 [ref. 22674] but as 22616. Lectotype selected by Zarske & Géry 1995:112 [ref. 22674]. Type locality described in more detail by Ramsperger 1924:810 [ref. 35915]. Specific name is an undeclinable noun. •Valid as Hemigrammus rhodostomus Ahl 1924 -- (Taphorn 1992:224 [ref. 23654], Zarske & Géry 1995:118 [ref. 22674], Lima & Oyakawa in Reis et al. 2003:133 [ref. 27061], Bertaco & Carvalho 2005:147 [ref. 28675], Marinho et al. 2008:59 [ref. 29547], Carvalho et al. 2010:253 [ref. 30862], DoNascimiento et al. 2017:49 [ref. 35633]). •Valid as Petitella rhodostomus (Ahl 1924) -- (Bittencourt et al. 2020:7 [ref. 37634] as rhodostoma). Current status: Valid as Petitella rhodostomus (Ahl 1924). Characidae: Stethaprioninae. Distribution: Amazon and Orinoco river basins: Brazil, Colombia and Venezuela. Habitat: freshwater.​

 
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