Retroculus acherontos

Retroculus: from the Latin retro, meaning 'back, behind', and oculus, meaning 'eye', in reference to the position of the eye in the posterior portion of the head.

acherontos: meaning 'of Acheron' from the Ancient Greek Ἀχέρων (Akhérōn), in reference to the 'stream of woes' which flows to Hades, land of the souls, in Greek mythology. In reference to the type locality, Rio das Almas, which means 'river of the souls' in Portuguese.

Known only from affluents within the upper rio Tocantins basin draining into the enormous Serra da Mesa hydroelectric reservoir in Goiás state, Brazil. It has been collected from the rios Maranhão, das Almas, Traíras, and Palmerinha/Palmeira(?).

Type locality is 'Brazil, Goiás, Rio Tocantins basin, Rio das Almas, tributary of left margin of Rio Maranhão, 14˚37′51.2″S, 49˚1′56.6″W'.

This species has been collected in small- to medium-sized tributaries containing either clear, slightly humic water or sediment-filled 'whitewater', where it is sympatric with R. xinguensis. Based on its morphology (see 'Notes'), it is hypothesised to be less rheophilic than congeners although images of one habitat depict a fast-flowing river with many boulders of varying sizes, and still pools with substrates of sand or gravel in marginal zones.

The natural diet of the congener R. xinguensis is dominated by benthic aquatic invertebrates, particularly chironomid (midge), trichoptera (caddisfly), and ephemeroptera (Mayfly) larvae.

Retroculus spp. are primarily benthivorous, but also feed readily on items suspended in the water flow, small fishes, etc., in addition to sifting mouthfuls of substrate through the gills to extract edible items.

This species is unknown in the ornamental trade.

It can be distinguished from congeners by the following combination of unique characters: presence of a nuchal hump in specimens of both sexes larger than 121.4 mm5 in SL (vs. no such hump); a black blotch on the median portion of soft dorsal-fin rays 1-7 (vs. blotch basal); 33–37 scales in the midlateral series (vs. 38 or more midlateral scales); 15–16 pectoral-fin rays (vs. 17–18); ventral scales not smaller than flank scales and with posterior margin free from skin (vs. scales much smaller and embedded); fourth vertebra hypapophysis elongate (vs. not elongate); first epibranchial deeper (vs. shallower).

It is further distinguished by: absence of gill rakers on the fifth ceratobranchial (vs. presence in R. lapidifer and R. septentrionalis); presence of faint, slender, dark bars on the caudal-fin (vs. thicker, more conspicuous bars in R. septentrionalis and R. xinguensis, no bars in R. lapidifer); anterior series of papillae on the vomero-palatine apparatus regular (vs. irregular and occasionally forming a second series in R. xinguensis, absent in R. septentrionalis); spiny process on the first hypobranchial weakly pronounced(vs. strongly pronounced in R. septentrionalis); vertebral count 31–32 (vs. 33–34 in R. lapidifer and R. xinguensis).

Retroculus was originally diagnosed by Eigenmann and Bray (1894)[1], and this was expanded by Landim et al. (2015) due to their new species R. acherontos displaying less-specialised morphological features than congeners, specifically a less depressed body, less acute snout, more anterior eye, rounded ventral body shape, and ventral scales which are not distinctively smaller than the flank scales.

Following the rediagnosis, members of this genus are distinguished from other cichlid taxa by possession of two unique characters: epibranchial lobe supported by a thickening of the first epibranchial (vs. epibranchial lobe supported by the second epibranchial or a laminar expansion of the first epibranchial); a vomero-palatine apparatus on the roof of the mouth (vs. roof of mouth not distinctly specialised).

The genus is further diagnosed by a combination of the following characters: two lachrymal plates plus five tubular infraorbitals; first tubular infraorbital only associated with the orbit posteriorly; no overlap between lip folds at the corner of the mouth (African lips sensu Kullander 1986, type-I lips sensu Stiassny 1987); two supraneurals; seven lateralis canal foramina in the preopercle; five lateralis canal foramina on the dentary; a black blotch on the anterior soft dorsal-fin rays; orobranchial mucosa covered by papillae laterally; presence of skin flaps and absence of microbranchiospines along the external margin of the first ceratobranchial; attachment of the ligament hypohyal-hypobranchial 1 along the antero-medial spiny process of the first hypobranchial; supracleithrum laterosensory canal angled posteroventrally; posterior portion of the dentigerous arm of the dentary slender; rakers at base of epibranchial lobe.

Retroculus is regarded as the earliest diverging lineage of Neotropical cichlids and sister taxon to the genus Cichla, with this clade considered basal to the remaining neotropical cichlids. Despite the apparent resemblance, it is not a member of the putative subfamily Geophaginae.