Plagioscion Gill

Genus Plagioscion Gill

Plagioscion Gill 1861: 82 (type species by subsequent designation by Jordan and Eigenmann 1889: 80: Sciaena squamosissima Heckel 1840).

Diplolepis Steindachner 1863: 2 (type species by original designation: Sciaena squamosissima Heckel 1840), name preoccupied in Hymenoptera (Neave 1939 cited in Chao 1978).

Diagnosis. A genus of freshwater Sciaenidae, distinguished from the other three South American sciaenid genera (Pachyurus, Pachypops, and Petilipinnis) by the following combination of characters: large scales in lateral line covered by smaller scales, conical teeth on premaxilla and dentary, elongate gill rakers on first gill arch, an oblique mouth in lateral view, a dark axillary spot present, and absence of mental barbels (vs. scales in lateral line covered by a single scale, conical teeth absent, short gill rakers on first gill arch, an inferior mouth in lateral view, absence of a dark axillary spot in Pachypops, Pachyurus, and Petilipinnis; and mental barbels present in Pachypops, Petilipinnis, and Pachyurus adspersus).

Comments. According to Sasaki (1989), monophyly of Plagioscion is supported by possession of an intercalar that projects ventrally and by three large scales on the lateral line that are covered by smaller scales. The author’s ongoing study on the phylogeny of South American freshwater sciaenids, reveals additional characters for Plagioscion: a parasphenoid with dorsal projections (except in P. magdalenae), the possession of fewer than five few ossified epipleural ribs, the presence of horned swimbladder appendages, and the presence of a dark axillary spot.

The species of Plagioscion are generally not easy to separate using only meristic characters or coloration, and instead one must rely on less subjective morphometric features. From nominal species previously assigned to Plagioscion, P. heterolepis (Bleeker) and P. microps Steindachner are currently associated with Johnius (Sasaki, 1992) and Nebris (pers. obs.), respectively. Based on meristic and morphometric features, five nominal species are herein considered valid: Plagioscion squamosissimus (Heckel), Plagioscion auratus (Castelnau), Plagioscion magdalenae (Steindachner), Plagioscion ternetzi Boulenger, and Plagioscion montei Soares and Casatti. Eight nominal species are considered to be junior synonyms: Johnius crouvina Castelnau (syn. of P. squamisissimus), Johnius amazonicus Castelnau (syn. of P. squamosissimus), Corvina monacantha Cope (syn. of P. auratus), Pseudosciaena surinamensis Bleeker (syn. of P. squamosissimus), Plagioscion auratus iquitensis Nakashima (syn. of P. squamosissimus), Plagioscion squamosissimus iquitensis Nakashima (syn. of P. squamosissimus), Plagioscion macdonaghi Daneri (syn. of P. ternetzi), and Plagioscion casattii Aguilera & Aguilera (syn. of P. squamosissimus). The status of Plagioscion pauciradiatus Steindachner and Sciaena rubella Schomburgk remains doubtful because the types were not located and the original descriptions are insufficient to allocate these species to any known species of Plagioscion (Casatti, 2003).

Key to the species of Plagioscion

1. Distance from anus to origin of anal fin 1.9-3.5 in HL (mean 2.7) (Fig. 1A).............. 2

- Distance from anus to origin of anal fin 3.6-5.6 in HL (mean 4.4 [10% of the individuals recognized as Plagioscion squamosissimus can show this value ranging from 3.1 to 3.5]) (Fig. 1B).......................................................................................................... 4

2. Pectoral fin short, when adpressed falling short of vertical through anus, its length 4.3-5.0 in SL; horizontal diameter of orbit 5.0-6.2 in HL; tip of longest pelvic soft ray reaching anus; body, including fins, generally dark-tan (rarely lighter)......................................................................... P. auratus (Orinoco, Essequibo, and Amazonas basins)

- Pectoral fin long, when adpressed reaching or surpassing vertical through anus, its length 2.7-4.2 in SL; horizontal diameter of orbit 3.1-5.2 in HL; tip of longest pelvic soft ray rarely reaching anus; head and dorsal portion of trunk tan or yellowish-tan, lighter and silvery ventrally........................................................................................ 3

3. Interorbital width 5.0-6.5 (mean 5.8) in HL; second spine of anal fin stout, its length 1.5-2.8 (mean 2.2) in HL; 10-13 scales (mode 12) above lateral line.................................................................................. P. magdalenae (Magdalena and Amazonas basins)

- Interorbital width 3.5-5.0 (mean 4.4) in HL; second spine of anal fin thin, its length 2.5-4.4 (mean 3.3) in HL; 8-10 scales (mode 10) above lateral line........................................................................................................................ P. montei (Amazonas basin)

4. At least 2/3 of lower part of soft dorsal-fin covered by scales (usually 15-20 longitudinal series); tip of pelvic fin reaching anus...................................................... P. ternetzi (Río Paraná basin below Itaipu reservoir, and Paraguay and Uruguay basins)

- No more than 1/2 of lower part of soft dorsal-fin covered by scales (usually 1-5 longitudinal series); tip of pelvic fin falling short of anus........................ P. squamosissimus (widespread in South America east of Andes)