Chaetostoma anale

Ballen, Gustavo A., 2011, A New Species Of Chaetostoma Tschudi (Siluriformes: Loricariidae) From Colombia With A Definition Of The C. anale Species Group, Papéis Avulsos de Zoologia 51 (26), pp. 383-398 : 395-396

The Chaetostoma anale species group

Content: Three species. Chaetostoma anale (Fowler) , C. formosae Ballen , and C. jegui Rapp Py-Daniel.

The notable condition in the anal fin of mature males shared by C. anale , C. formosae and C. jegui is herein proposed as a synapomorphy for those species within Chaetostoma . All the remaining species from the genus have unmodified unbranched rays in the anal fin of mature males. Herein it is proposed that considering Chaetostoma (sensu Armbruster, 2004) as the ingroup and the remaining ancistrins as the outgroup; and the character as having two conditions, namely, second unbranched anal-fin ray unmodified and comparable in length to branched rays in mature males (Condition A; Loricariidae including most of Chaetostoma except C. anale , C. formosae and C. jegui , Fig. 7A, B ) and second unbranched anal-fin ray in mature males enlarged and bearing two posterior fleshy ridges (Condition B; C. anale , C. formosae and C. jegui , Fig. 7C, D ), then the derived character-state (B) in the transformation series A à B is evidence for a monophyletic group comprising those three species within Chaetostoma .

There is another particular structure present in those three species of Chaetostoma , namely, a parietosupraoccipital dermal keel ( Fig. 5A, B ). However this structure is found in other species of Chaetostoma (e.g., C. leucomelas , C. milesi , C. tachiraense and C. vagum ), and in some species of Cordylancistrus (G.A. Ballen, unpubl. data). Therefore both conditions are uninformative for the interrelationships among species of Chaetostoma given that both conditions (i.e., skin on parieto-supraoccipital unmodified in the form of a darkly pigmented dermal keel, Fig. 5C, D ; and skin on parieto-supraoccipital modified into a dermal keel) would be recovered as plesiomorphic given that both are present in both ingroup and outgroup. This would be enough to reject such a transformation series as a phylogenetic character pertinent as evidence for the systematics of the species within Chaetostoma . Nevertheless, this character has shown to be informative un- der a more inclusive question on interrelationships of fishes from the Chaetostoma group (Ballen and Vari, in prep.). In addition, Salcedo (2006) noted its presence in eleven species of the genus including C. anale and C. jegui , arguing that they conform a monophyletic group within Chaetostoma as evidenced by this synapomorphy and noting some ontogenetic variation also documented for C. formosae in the present paper. However, such a monophyletic assemblage does not stand as valid by the presence of the presumed derived state because some species outside Chaetostoma as currently understood do show this state (i.e., “ Dolichancistrus ” setosus and an undescribed species of Cordylancistrus from northern Colombia).

Recognition of subgeneric arrangements in the Hypostominae is rare. Armbruster (2004, 2008) Proposed substantial changes to the generic composition of the Hypostominae (including the Ancistrini ), and such modifications resulted in a striking and improved classification. The author provided diagnoses for most of his recognized genera, sometimes clumping together several monotypic or small genera traditionally believed to represent different units. One of the most interesting instances of such changes is the genus Panaque , now composed of three subgenera: Panaque , Panaqolus and Scobinancistrus ( Armbruster, 2004; Lujan et al., 2010). All three genera were considered different units under a non-phylogenetic framework (e.g., Isbrücker, 2001). However, Armbruster (2004) included Panaqolus and Scobinancistrus as subgenera of Panaque , making the genus diagnosable and allowing easy recognition of any species of Panaque as member of a particular subgenus, therefore allowing taxonomic arrangements that reflect evolutionary relationships and facilitate identification of species. This is one of the cases where formal subgeneric arrangements were made for hypostomine genera, but unfortunately it is not the rule in current taxonomy of this group of fishes. Although Hypostomus lacks an explicit and formal subgeneric arrangement, there are recognized species groups, as informal hierarchical categories. Although informal, such species groups facilitate identification. For instance, the recognition of the Hypostomus cochliodon species group ( Armbruster, 2003b) makes it easier to identify a species of Hypostomus by restricting the necessary comparisons among species of the genus.

Chaetostoma , in contrast to other large or moderate genera of the Hypostominae , lacks to date explicit subgeneric arrangements, and the present recognition of the Chaetostoma anale species group is the first attempt to organize the genus. Identification of Chaetostoma species is very difficult, and taxonomic decisions are often based on geographical distribution, a very questionable taxonomic strategy for groups with poorly-resolved taxonomies. It is anticipated that recognition of subgeneric units, either formal (i.e., subgenera) or informal (i.e., species groups) will improve our understanding of the taxonomy of Chaetostoma , one of the most speciose hypostomine genera. Despite its few included species, the recognition of the C. anale species group is a first step towards a more complete scheme of classification. Further work on some other subunits within Chaetostoma is in progress and hopefully will help to clarify the taxonomy of the genus.

Figures

FIGURE 5: Supraoccipital region in species of Chaetostoma in dorsal view, anterior portion of body upward; arrows indicate the parietosupraoccipital keel when present. A-B, species showing fleshy supraoccipital ridge; C-D, species with skin on supraoccipital region unmodified. A) Chaetostoma milesi, ICNMHN 16923, subadult male, 122.6 mm SL; B) Chaetostoma formosae sp. nov., ICNMHN 16364, paratype, adult female, 74.7 mm SL; C) Chaetostoma platyrhynchus, ICNMHN 5492, subadult male, 48.4 mm SL; D) Chaetostoma sovichthys, ICNMHN 16221, adult male, 96.4 mm SL. Scale bars equal 3 mm.

FIGURE 7: Sexual variation in the anal-fin length; ventral view, anterior to left. A-B represents the primitive condition (Condition A); C-D represents the derived condition (Condition B). A) Chaetostoma dorsale, ICNMHN 8011, adult male, 80.6 mm SL; B) Chaetostoma dorsale, ICNMHN 1183, adult female, 59.4 mm SL; C) Chaetostoma formosae sp. nov., holotype, ICNMHN 17114, adult male, 97.4 mm SL; D) Chaetostoma formosae sp. nov., paratype, ICNMHN 16364, adult female, 74.7 mm SL. Scale bars equal 10 mm.