Ilyodon furcidens

Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Ilyodon furcidens
Original Description: 

  JORDAN, D. S. & C. H. GILBERT (1883): Catalogue of fishes collected by Mr. John Xantus at Cape San Lucas, which are now in the U.S. National Museum, with descriptions of eight new species. Proceedings of the United States National Museum 5 (1883): pp 353 - 371


  This species name is derived from the Latin and means "with forked teeth". This species has got bifid teeth.


  Collection-number of the types: United States National Museum, Cat. No. USNM-9571 and USNM-30971.

English Name: 
Goldbreast Splitfin
Mexican Name: 
Mexclapique del Armería

Characodon furcidens   Jordan & Gilbert, 1883

Ilyodon paraguayense   Eigenmann, 1907

Balsadichthys xantusi   Hubbs, 1926 (nomen nudum)

Balsadichthys xantusi   Hubbs & Turner, 1939

Ilyodon xantusi   Miller & Fitzsimons, 1971

Ilyodon furcidens amecae   Kingston, 1979 (nomen nudum)

Ilyodon amecae   Radda, 1989 (nomen nudum)


  The Karyotype describes the number and appearance of chromosomes during the phase of condensation, classified by the position of the centromere (Levan et al., 1964).

The following abbreviations are employed:


M = large metacentric chromsome (a result of Robertsonian fusion)

m = small metacentric chromsome (centromere at medium position)

sm = submetacentric chromsome (centromere at submedian position)

smst = submetacentric-subtelocentric chromosome (continous series)

st = subtelocentric chromosome (centromere at subterminal region)

stt = subtelocentric-acrocentric chromosome (continous series)

t = acrocentric chromosome (centromere at terminal region)


The Karyotype of Ilyodon furcidens, following Uyeno, Miller & Fitzsimons, 1983:

2n = 48    8st/ 40t

The maximum known SL is 103mm (Miller et al, 2005).
Terra typica: 

Following the original description, the type locality should be the Cape San Lucas in Lower California (Baja California), which is definitely incorrect. Most probably there had been done mistakes when the types had been recorded: The item under "Locality " is blank for the cotype series (No. 30971) reported as from San Lucas, and someone assumed without warrant that this and nearby blanks signified dittos from an entry above of Cape San Lucas. Two series in the National Museum, all collected by Xantus, are cotype lots (No. 5093 and 35338), entered as from Colima, Mexico, so this seems be the right terra typica for the holotypes, too.  

Status after IUCN: 

not mentioned

Status following other sources: 

Low risk? Comment: Declining in Ayuquila River

Distribution and ESU's: 

This species comes from the Pacific Slope, inhabiting the basins of the ríos Ameca, Armería, Cihuatlán (=Marabasco) and Coahuayana in the states Colima, Michoacán and Jalisco.


  The habitats are pools and riffles of streams and rivers over substrates of sand, gilt, gravel, rocks, bedrocks and boulders in depths to 1.5m. The water is usually clear to murky with no vegetation or with green algae, Potamogeton and Nasturtium. The currents are not homogeneous, going from none to slow to fairly swift.


GWG survey at the Arroyo Dávalos

Arroyo Dávalos


Río de las Bolas

rocky section of the Río de las Bolas


Río Potrero Grande I

Rio Potrero Grande II


Hubbs & Turner pointed 1939 at the much variation in the colouration of this species, with age, sex and individuals. Typical for young fish is a lateral row of irregular blackish spots and bars crossing an inconspicuous axial streak as well as an irregular dorsolateral row of smaller spots and traces of other dark markings. These bars are occasionally lacking in larger young, rarely fused into a lateral band, faint to intense, small and roundish to high and narrow, often restricted to the anterior, posterior or median section.  As these several variations are independent, many types of pattern are developed. In the males the bars fade out more or less completely at a length of about 35mm and are then replaced by rather dense speckling, which is strongest forward. In the females, the bars show the same variations but can be seen faintly in even the largest specimens. The dorsolateral spots usually become obsolete in the smaller females, but occasionally persist rather strongly to a length of at least 56mm. The speckling is is usually little developed in smaller females, but gradually increases in intensity. Nevertheless, females are more faintly and in a finer pattern speckled than males. The ground colouration of both sexes is mainly greenish or grayish-brown on the back, becoming darker with age, and yellow to orange on the belly. This can be overwhelmed with grayish-green or brown in old individuals. The fins are normally coloured yellow to orange, the males show a dark submarginal band in the dorsal and a (most times broad) submarginal or marginal band in the caudal fin with 1 to 3 additional vertical rows of spots. Females show a narrow to wide submarginal band on the anal fin as well as in the caudal fin, where is becomes disrupted with age and fades in older females. Pelvic and Pectoral fins are usually yellow. All in all, both sexes display a very striking colouration. 


Following Miller, young are born from February to July (possibly through autumn) as indicated by captures. The adult fish prefer swimming in a strong current, the young fish can be found in pools near the shore.


Two trophic types can be distinguished (by their mouth) in this species: One with a narrow mouth (being suggested to be insectivore or planktivore), and one with a broad mouth (being suggested to feed from aufwuchs and algae). Both types have been seen feeding in the habitat from aufwuchs, the narrow-mouthed form additionally from the surface (Kingston, 1979). The gut of both types is long and suggests herbivorous feeding habits.


The confusion about the origin of this species is not alone shown in the original description and the wrongly assigned type locality (see above in Terra typica), but also in the description of Ilyodon paraguayense by Eigenmann in 1907: The two (crushed) types of Ilyodon paraguayense were assigned to Paraguay because they were mixed in with some Characins, collected by E. Palmer in this country. Hubbs & Turner examined in 1939 these types , and stated, that they "agree very closely not only in characters but also in superficial appearance with the types of Characodon furcidens, and it is probable that they were part of the same material, collected by Xantus in Colima." 


In 1939, Hubbs & Turner described Balsadichthys xantusi in honour of John Xantus, who collected the first specimens. Even in the original description - though they pointed at the narrow relationship with Balsadichthys whitei - they discussed hybrids between Ilyodon furcidens and the new described species. They mentioned an intimate relationship of the both genera, "as indicated by their common ovarian and trophotaenial characters and evidenced by the discovery that these genera commonly hybridize in nature." They wrote furthermore: "This circumstance might even be considered a reason for synonymizing Balsadichthys and Ilyodon, but to do so would violet the consistent judgement of authors that the species with weak teeth movably set in loosely conjoined jaws of a wide, transverse mouth should be separated generically from those with stronger teeth tightly set in the firmly joined jaws of narrower mouths with better-developed lateral gape." and "It is not all improbable that it will be found expedient to synonymize Balsadichthys and Ilyodon...".

So Hubbs & Turner recognized instinctively the narrow relationship of all these forms, they decided to let the consistent system of judging the fish after their teeth-structure persist, maybe because they saw no other, or better way to judge the fish. However, they saw this judgement not very pleasable.

Nevertheless, it took another 30 years till this synonymizing had been done finally (Miller & Fitzsimons, 1971), mainly because both trophic forms can be found at the same locations, but not always. Sometimes there is only the furcidens-type represented, sometimes the xantusi-type, and sometimes both with many intermediates, so hybridisation seemed to be very likely.


In 1979, Kingston added another subspecies to Ilyodon furcidens, calling it Ilyodon furcidens amecae. Though this subspecies hadn't been described officially, Radda took over this name and gave it species rank in 1989. Domínguez (pers. comm. 2010) values the population from the Río Ameca - though Omar Domínguez had found some differences (2004) and had placed this form in the vicinity of Ilyodon whitei, but relativated these results later - as belonging to Ilyodon furcidens without subspecies status. Due to the difficult situation in Ilyodon (Kingston..."a taxonomic nightmare"...), we treat this form as Ilyodon furcidens from the Río Ameca till we get other information.


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 Terrero and one of its tributaries, the Río del 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 had to be to see both forms as members of one species.


Last phylogenetical results by Domínguez et al revealed that the fish from the Río Coahayana basin, traditionally furcidens, belong to whitei as well as some populations from the Río Armería, destroying more or less all accepted kinds of classification until now, restricting the "real" furcidens to the ríos Ameca, Marabasco and most parts of the Armería. 

Well, comparing at least the population from the Río Tamazula with "typical" furcidens or whitei, it resembles nevertheless in colour, shape, size and behaviour rather furcidens than whitei, so also in this case, the last word isn't spoken.

An interesting excursus to the Poeciliid genus Xiphophorus, where the species clemenciae is definitely a Platy, according to results in comparing the Cytochrome b gene, but testing some nuclear genes, it belongs to the Southern Swordfish around hellerii, a species, it resembles obviously. Following the scientists, this species might be the product of hybridisation between hellerii and a Platy species, most probably maculatus. Maybe a similar process had happened in the genus Ilyodon, so we might have populations of interspecific hybrids in the transition zone (southern Río Armería and Río Coahuayana), and contemplating the very close relationship with little genetic distances of all Ilyodon-populations, the situation might create confusing results.

But like a professor of mine used to say: The situation may be completely different...


Hieronimus (1989) sampled Ilyodon furcidens in a small creek above San Antonio, finding differences mouth forms in the colour of their caudal fin (orange without markings = furcidens; yellow with black makings = xantusi). keeping the fish in tanks, the differences disappeared and the fish looked similar to each other. 


Comparing all Ilyodon-forms (including whitei and cortesae), there is not much difference (even phylogenetically) between them all. The genetic distances (intraspecific pairwise uncorrected "p" distances) comparing the Cytochrom b gene between Ilyodon furcidens, the former xantusi and whitei is low, only p=0.3 to 1.7% (Domínguez et al, 2004). This value is even lower than between some populations of Ilyodon whitei (1.5 to 1.7%, which is still very low)!

This genus is definitely in a specification process, which makes it very difficult for scientists to classify the different forms.