Despite its designation as type species this is among the most frequently-misidentified members of the genus.
This is in part because its natural distribution was earlier considered even more extensive than the still-substantial range recognised today, with populations from the Río Orinoco, Guianas and rio Tocantins now recognised as species in their own right.
Further confusion has been caused by undescribed Geophagus species from various parts of the Amazon basin being traded as geographic forms of G. altifrons or G. sp. aff. altifrons despite possessing prominent midlateral markings or vertical body stripes, whereas in G. altifrons sensu stricto these features are greatly reduced and absent, respectively.
It's a member of the putative G. surinamensis 'group' of closely-related species within the genus and can be identified by a combination of characters including: no dark preopercular marking; no vertical bars on the flanks even when stressed or preserved; dark midlateral marking reduced to 1-2 scales width or even absent in adults.
The latter is important because it's unique to the species and can be used to distinguish it from all other congeners as things stand.
Juveniles are very difficult to identify though as the size of the midlateral markings is comparable to that seen in other species at the same age.
Representatives of the G. surinamensis assemblage are diagnosed by their relatively deep head and body shape, possession of a variably-sized dark spot on each flank and presence or absence of a small, dark preopercular marking.
There are currently thirteen described members but diversity is predicted to eventually prove much greater with a number of undescribed forms known some of which, such as G. sp. 'orange head' and G. sp. 'Pindare', are regularly available in the aquarium trade.
The described members of the group are G. abalios, G. altifrons, G. brachybranchus, G. brokopondo, G. camopiensis, G. dicrozoster, G. megasema, G. neambi, G. parnaibae, G. proximus, G. surinamensis, G. sveni and G. winemilleri.
Only five Geophagus species aren't included, namely G. argyrostictus, G. gottwaldi, G. grammepareius, G. harreri and G. taeniopareius.
These are most easily separated on the basis that they all possess a complete infraorbital stripe but oddly have not had their own group name assigned, usually being referred to simply as 'non-G. surinamensis group' species.
Accurate identification of G. surinamensis group members has been problematic for a number of reasons, not least that prior to Kullander (1986) G. surinamensis itself was thought to range throughout the Orinoco, Amazon and Guianas river systems but is now considered endemic to the Rio Suriname and Rio Maroni/Marowijne watersheds in eastern Suriname.
Several former 'populations' have thus been described as species in their own right since the turn of the century, meaning older literature can be somewhat unreliable.
Further, the majority of Geophagus species in the hobby are sold as G. surinamensis regardless of origin - a situation which continues to cause confusion and exacerbated by the fact that juveniles of most are virtually identical.
G. surinamensis is in fact almost never traded and very few privately-owned specimens exist.
The number of species awaiting description is unconfirmed but it seems feasible that certain rivers within the Río Orinoco, rio Amazonas, rio Tocantins and rio Parnaíba basinsm, plus coastal drainages of the Guianas, contain one or two endemic forms.
Of those known in the aquarium hobby some are traded as G. sp. aff. altifrons which can be misleading as they don't always resemble that species.
More preferable is to label with locality data when available.
The genus Geophagus was rediagnosed by Kullander (1986) who restricted it to include only those species with paired caudal extensions to the swimbladder lined by 6-12 epihemal 'ribs' plus a greater number of caudal than abdominal vertebrae.
Some former species were moved into the resurrected genus Satanoperca while others, such as the 'Geophagus' brasiliensis and 'G.' steindachneri groups represent distinct groupings still in need of definitive classification.
Geophagus and a number of related genera are often included in the putative subfamily Geophaginae.
Kullander (1998) conducted a morphology-based phylogenetic study in which the neotropical Cichlidae was divided into six subfamilies of which the Geophaginae contained 16 genera divided among three 'tribes':
Acarichthyini - Acarichthys and Guianacara.
Crenicaratini - Biotoecus, Crenicara, Dicrossus and Mazarunia.
Geophagini - Geophagus, Mikrogeophagus, 'Geophagus' brasiliensis group, 'Geophagus' steindachneri group, Gymnogeophagus, Satanoperca, Biotodoma, Apistogramma, Apistogrammoides and Taeniacara.
Later molecular studies by Farias et al. (1999, 2000, 2001) resulted in the additions of Crenicichla and Teleocichla to the Geophaginae, a result supported by López-Fernández et al. (2005) who conducted the most detailed molecular analysis of the grouping to date including 16 of the 18 genera and 30 species.
However their conclusions regarding interrelationships between genera did vary somewhat from previous hypotheses and can be summarised by the following loosely-defined groups:
- a weakly-supported sister group relationship between Acarichthys and Guianacara.
- a well-supported 'Satanoperca clade' comprising Satanoperca, Apistogramma, Apistogrammoides and Taeniacara.
- a 'big clade' with Geophagus, Mikrogeophagus, 'Geophagus' brasiliensis group, 'Geophagus' steindachneri group, Gymnogeophagus, Biotodoma, Crenicara and Dicrossus.
- a 'crenicarine clade' with Biotoecus and Crenicichla.
No representatives of Teleocichla or Mazarunia were included in the study but the former is well-established as sister to Crenicichla while the latter has grouped closely with Dicrossus and Crenicara in earlier works.
The other main conclusions of the paper are confirmation that Geophaginae is a monophyletic group exhibiting strong signs of having undergone rapid adaptive radiation.
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