Ilyodon whitei (incl. cortesae, lennoni and parts of xantusi)

   The Balsas Splitfin was originally placed in the genus Goodea by Meek because of a similar dentition, but Hubbs recognized sufficient differences in the structure of ovaries and trophotaenia and erected therefore the genus Balsadichthys in 1926. The genus Balsadichthiys is one of only five that have been erected exclusively for Goodeid fish and lateron became synonyms. It was seen as genus similar to Ilyodon, but with loose teeth instead of firmly attached, that's why Hubbs and Turner described in 1939 out of a sampling from Ilyodon furcidens from the Colima region the species Balsadichthys xantusi for fish from the ríos Armería and Coahuayana with loose teeth. Lateron the species included additionally fish from the Río Coahuayana basin. After understanding the existance of different trophic forms of one species in even one river, the genus was withdrawn and the species Balsadichthys xantusi seen as a synonyme of Ilyodon furcidens. The etymology of the generic name goes partly back to the ancient Greek. The Río Balsas gave it the name, and combinded with ἰχθύς (íchthús, íchthýs), the Greek word for fish, the generic name can be translated with "Fish from the Balsas (river)". 

  In her doctor thesis in 1979, Dolores Kingston split the species Ilyodon furcidens and xantusi in several subspecies, based on morphological differences. Following her, the nominotypical subspecies Ilyodon furcidens furcidens became restricted to the ríos Armería, Salado and El Tule drainages, Ilyodon furcidens amecae to the Río Ameca system, Ilyodon furcidens tuxpan to the upper Río Coahuayana, the Río Tuxpan, and Ilyodon furcidens variabilis to the Río Jiquilpan (Río La Labora in her thesis). Ilyodon xantusi again, she split into Ilyodon xantusi xantusi from the ríos Chalaca, Armería, Salado, El Tule and Coelcomán drainages, while Ilyodon xantusi latos became a distinct form from the Río El Tule drainage, living sympatric with the other subspecies. However, the results have never been published officially. The reason therefore is the US-American jurisdiction, that a thesis doesn't count as an official publication. Nevertheless, Alfred Radda took over one of these name and gave it as Ilyodon amecae species rank in 1989. Today, all of these species and subspecies descriptions count es Nomina nuda (names without value), and some of them would be either fully or partially regarded as members of Ilyodon whitei. Concerning the taxonomic status of Ilyodon whitei, Kingston wrote the following sentences in her thesis: "The lack of material from the Balsas system makes comparisons of populations of Ilyodon whitei difficult, as presently there is access only to two areas at opposite ends of the drainage. Until new material becomes available from throughout the drainage, Ilyodon whitei is best considered as one species including all the Ilyodon in the Balsas drainage."

  The upper row shows pictures of the Kingstons types of Ilyodon furcidens furcidens, furcidens amecae and furcidens tuxpan, from left to right, the lower row those of Ilyodon furcidens variabilis, xantusi xantusi and xantusi latos, also from left to right. Ilyodon furcidens tuxpan and xantusi latos would belong completely to Ilyodon whitei following the recent classification, Ilyodon furcidens furcidens and xantusi xantusi only with populations from the Rio Coahuayana except for the Salado river. Ilyodon furcidens amecae and furcidens variabilis would belong completely to Ilyodon furcidens. However, all these subspecies are not validly described:

    In 1980, Turner & Grosse hypothesized that Ilyodon furcidens and xantusi are conspecific, representing different trophic forms as they sampled a dichotomous population in the Río Armería and one in the Río Coahuayana and discovered little or no allozyme differentiation between the presumptive species in either location. They suggested intraspecific polymorphism. In 1984, Grudzien et al. collected Ilyodon at four locations along the Río El Terrero and one of its tributaries, the Río El Tule, and examined 36 gene loci. They detected 15 loci being polymorphic, but the detected variation has not been connected with one of the both trophic forms. In contrary: they varied independently from both forms in the same value, so there could not been detected any genetic difference between furcidens and xantusi. A similar result gave the examination of the kind of chromosomes in Ilyodon through Turner et al. in 1985: surprisingly they found a different number of metacentric chromosomes in fish of the Río Coahuayana basin, so-called cytotypes, starting with 0-2 up to 16! Such a variation is very rare among teleosteen fish at all. However, genic and chromosomal divergence are clearly uncoupled in Ilyodon and a certain number of metacentric chromosomes is not typical for one of the both trophic forms. So finally, it is neither genetically nor by the number of metacentric chromosomes possible to distinguish furcidens from xantusi. Considering that in many habitats both trophic forms can be found (even in the type material of furcidens), the final consequence would have to be to see both forms as members of one species, what Turner et al. did, but only for the Río Coahuayana populations. Ilyodon xantusi stayed valid for the Río Armería population until Lyons & Navarro-Perez (1990) synonymized both species completely. However, we point on the fact, that the taxon xantusi was in use for fish that belong nowadays to Ilyodon furcidens and whitei.

  In 1983, Meyer and Förster described Ilyodon lennoni in honour of the famous British musician and former member of the "Beatles", John Lennon. Following the describers, this species should be forming a complex with Ilydodon xantusi in contrast to a complex formed by furcidens and whitei. However, the noticed differences in dentition and trophotaenial structure are not big enough to separate lennoni from whitei. Furthermore, a described gender related pair of chromosomes seemed to rather reflect differences that can be found routinely in condensation and fixation of chromosomes than a real chromosomal heteromorphism (Turner, 1985). In 2000, Ilyodon cortesae was described by Paulo-Maya & Trujillo-Jiménez, differing from other populations of Ilyodon whitei by its strongly convex cephalic profile, its small size, the thinner caudal peduncle, more scales in the longitudinal series, form and size of the neural prezygapophyses of the caudal vertebrae, the strongly bifid teeth and the disposition of the pores of the lateral line of the preorbital system. However, last phylogenetical results (Beltrán-Lopez et al., 2017) are clear, that both populations are just populations of Ilyodon whitei.

    The last phylogenetic studies on Ilyodon were done by Beltrán-Lopez et al. in 2017. While the authors found evidence of accepting just one clade in Ilyodon furcidens, they erected several ones for Ilyodon whitei with the populations from the Río Coahuayana placed closer to Ilyodon whitei in this study and therefore treated as this species. However, the fact remains, that we find different trophic forms exclusively in the ríos Armería and Coahuayana basins which makes it an interspecific phenomenon. The Río Coahuayana populations (except for the Río Salado population) remain however difficult in denotation. Anyway, the Balsas Splitfin is widely distributed. Its eastern habitats in the Río Nexapa drainage are about 70km SSE of Mexico City, while its western habitats in the Río Coahuayana basin are more than 500km beeline from the Río Nexapa habitats. Looking at its distribution along the Río Balsas basin, it is conspicuous, that it inhabits mostly the right tributaries of this river, leading to separation of populations along the river. This might be caused partly by a higher temperature in the Río Balsas lowlands and western affluents, that makes other species of fish adopted in a better way to these habitats than Ilyodon whitei. A complexe biogeographical history, including different events of dispersion and vicariance and this huge distribution range lead to different clusters. Roughly spoken, there seems to be a separation in a western group of clades encompassing the Río Coahuayana and western respectively central Río Balsas populations, and an eastern Clade B formed by populations from the Río Atoyac, Grande de Amacuzac and the Río Cutzamala or several eastern affluents of it. This eastern group (Clade B) can be split in a Subclade B1, these are fish from the Río Atoyac basin, meaning the eastermost populations of this species. Subclade B2 again shows a separation in populations from the Río Grande de Amacuzac and affluents (?) of the Río Cutzamala (only fish from the Río Zitácuaro were included in the studies). The western group of clades is composed by the two Río Coahuyana clades, namely Clade D for fish from the upper river drainage (ríos Tamazula and Tuxpan) and Clade C for fish from the lower river drainage (only fish from the Río El Tule were included in the studies). This separation is one, we may also find between populations of Allodontichthys tamazulae (see the chapter Remark there). The last Clade E encompasses fish from the Río Tepalcatepec in the west to the Río Cutzamala (o western affluents of it) in the east (only fish from the Río Chinapa were included in the studies). The Río Cutzamala situation is actually not to explain as only two populations were included and both fell into different clades. Further results including more populations from the whole Balsas river drainage may bring answers, eventually river piracy events caused by the high tectonic activity in this area played an important role, but again, for conclusions of that kind, we have far too little data.