Plagioscion ternetzi Boulenger 1895

Lilian Casatti, 2005, Revision of the South American freshwater genus Plagioscion (Teleostei, Perciformes, Sciaenidae)., Zootaxa 1080, pp. 39-64 : 55-57

Plagioscion ternetzi Boulenger 1895

(Fig. 8)

Plagioscion ternetzi Boulenger 1895 : 523, fig. 3 (type-locality: Río Grande, Paraguay).

Plagioscion macdonaghi Daneri 1954 : 179, fig. 1 (type-locality: Río de La Plata, Argentina).

Material examined. Type specimens: BMNH 1895.5.17.1, lectotype of Plagioscion ternetzi herein designated (390.0 mm SL), Remanso, Río Grande, Paraguay ; BMNH 1895.5.17.2, paralectotype of Plagioscion ternetzi (210.6 mm SL), same locality as lectotype ; MACN 4197, holotype of Plagioscion macdonaghi (113 mm SL), Río de La Plata, Buenos Aires Prov., Argentina .

Non-types (41 specimens). Brazil: Río Paraná basin: MZUSP 51112 (1) , MZUSP 50759 (1) , MZUSP 51111 (3) , MZUSP 51110 (8) , MZUSP 38196 (3) , MZUSP 40094 (1) , MZUSP 21145 (4) , MHNG 2120.95 (1) , CAS 78519 [10094] (1) , MHNG 2206.18 (1) , USNM 181602 (1) , USNM 181731 (2) , USNM 181695 (2) , MHNG 2157.20 (1) , MHNG 2414.32-33 (2) ; Uruguay: Río Uruguay basin: NMW 85547 (1) , NMW 85541 (2) , MZUSP 45841 (1) , MHNG 2414.26 (1) , MHNG 2414.26 (2) , MHNG 2414.27 (2) .

Diagnosis. A species of Plagioscion defined by the following combination of characters: anus close to anal-fin origin (anus to anal-fin length 3.6-5.3 in HL); horizontal diameter of orbit 3.9-5.4 in HL; interorbital broad (width 3.8-4.2 in HL); pectoral fin short, when depressed tip does not reach vertical through anus; second anal-fin spine relatively short (2.8-3.8 in HL); 12-15 series of scales above lateral line and 11-14 series of scales below lateral line; more than 2/3 of lower part of soft dorsal-fin covered with scales (usually 15-20 longitudinal series).

Description. Morphometric and meristic data are presented in Table 3. Body elongated; maximum body depth at origin of dorsal fin. Dorsal profile of body convex. Ventral profile flattened from prepelvic region to anal-fin origin. Snout slightly rounded in lateral view. Length of snout greater than horizontal diameter of orbit. Mouth terminal, oblique in lateral view. Teeth conical, visible externally; premaxilla with outer row of larger teeth and several inner rows of smaller teeth; dentary with 2 or 3 outer rows of smaller teeth and 1 inner row of larger teeth. Posteriormost tip of the premaxillary bone reaching vertical through posterior margin of orbit. Orbit lateral; eye round. Interorbital septum dorsally developed. Nostrils situated dorsolaterally; anterior circular, posterior crescent-shaped, and close to anterior margin of orbit. Laterosensory canal segments on head externally visible on lacrymal, suborbital, and preopercle. Preopercle slightly serrated on right corner. Tip of opercle located posterior to vertical through pectoral-fin base. Posterior margin of postemporal bone covered with small ctenoid scales, appearing as bony flap above dorsal limit of gill slit. Gill rakers developed. Scales ctenoid, except for cycloid scales on snout, lacrymal, second to fourth infraorbitals, and preopercle. Lateral line extending to posterior margin of caudal fin, with anterior third concave. Lateral line complex, formed of single larger basal scales covered by 4 or 5 smaller scales. Anal, pectoral, and pelvic fins with 1 or 2 rows of small ctenoid scales along their bases, and with few scales on basal half of membranes. Second dorsal and caudal fins densely scaled. Spinous dorsal-fin low, with longest spine falling short of soft dorsal fin origin when spinous dorsal fin is depressed. First dorsal-fin spine very small. Notch present between spinous and soft dorsal fins. Origin of soft dorsal-fin located along vertical through pectoral-fin tip. Anal fin truncate, the first spine reduced and the second spine strong, longer than one-half of longest soft ray. Caudal fin rhomboidal, with median rays longer in juveniles. Pectoral-fin falcate, and falling short of vertical through tip of pelvic fin. Pelvic-fin origin on vertical through pectoral-fin origin. First soft pelvic-fin ray longer, reaching anus. Gas bladder fusiform, with anterior pair of horn-shaped appendages.

Color in alcohol. Head and dorsal portion of trunk tan, lighter ventrally. Fins yellowish-tan, lighter and silvery ventrally. Axillary dark blotch present.

Distribution. Plagioscion ternetzi is known from the Paraná-Paraguay-Uruguay River system, in Brazil, Argentina, Paraguay and Uruguay, below the former Sete Quedas Falls now drowned by the Itaipu Reservoir (Fig. 3).

Remarks. Daneri (1954) described P. macdonaghi from the Río de La Plata, based on a juvenile specimen (113.0 mm). The diagnostic characters emphasized by Daneri (depth of body, interorbital width, horizontal orbit diameter, and number of gill rakers) all fall within the range of those values for P. ternetzi (Table 3). This justifies the recognition of P. macdonaghi as a junior synonym of the latter species.

The morphometric and meristic similarities between P. ternetzi and P. squamosissimus make it difficult to discriminate some individuals of the two species, and for this reason a lectotype and paralectotype are herein designated. The larger of the two syntypes (BMNH 1895.5.17.1) was chosen as lectotype because it probably was the one used by Boulenger (1895) in writing the original description, in which he indicated “total length 450 millim” (Boulenger, 1895: 523).

The densely scaled base of the dorsal fin (with 15-20 longitudinal series of scales) in both juveniles and adults of P. ternetzi serves to separate this species from P. squamosissimus , which lacks extensive series of scales on the dorsal-fin base. In addition, although ranges in length of the second anal-fin spine overlap (2.8-3.8 in HL in P. ternetzi ; 2.6-6.0 in HL in P. squamosissimus ) the spines of P. ternetzi average significantly longer than those of P. squamosissimus (t-Test, p ≤ 0.0008).