A NEW GENUS FOR CORVINA GRUNNIENS SCHOMBURGK , 1843 ( PERCIFORMES , SCIAENIDAE ) FROM THE AMAZON AND ESSEQUIBO RIVER BASINS AND REDESCRIPTION OF PETILIPINNIS

Petilipinnis, new genus, is proposed for Corvina grunniens Schomburgk, from the Amazon and Essequibo river basins. It is distinguishable from other Sciaenidae genera with the exception of Menticirrhus, by the presence of only one spine in the anal fin. From Menticirrhus, Petilipinnis differs by the morphology of the swimbladder and the number of dorsal fin soft rays. Also a redescription of Petilipinnis grunniens is presented.


INTRODUCTION
The Sciaenidae (croakers, drums, pescadilhos, pescadas, and corvinas) is one of the largest perciform families in number of species and distribution, including approximately 78 genera and 287 species worldwide, in coastal and brackish waters of temperate and tropical areas of the world (Chao, 1986).The genera Aplodinotus Rafinesque, Pachypops Gill, Pachyurus Agassiz, Plagioscion Gill, and Boesemania Trewavas are restricted to freshwater.
The genus Corvina was proposed by Cuvier (1829) and the type species, C. nigra, was subsequently designed by Gill (1861).According to Trewavas (1966), Corvina is a pre-occupied name in Aves, and moreover is a synonym of Sciaena Linnaeus, 1758.Since Corvina is an invalid name in the zoological literature, Corvina grunniens Schomburgk cannot presently be placed in any valid genus.Notwithstanding this, the diagnosable external character (the presence of one anal spine) and the locality (Essequibo and Amazon River basins), among other characters pointed out in the discussion, show that this species cannot be assigned to any other genus in the Sciaenidae.
A new genus, Petilipinnis, is here proposed for Corvina grunniens with a redescription of Petilipinnis grunniens (Schomburgk).

METHODS
Counts and measurements were taken with calipers with 0,1 mm of resolution and follow Hubbs & Lagler (1958); lateral-line scales counts include the perforated scales to the hypural joint.Subunits of body are expressed as proportions of standard length (SL), except the anal fin spine length; the anal fin spine length and subunits of the cephalic region are expressed as proportions of head length (HD).One specimen was cleared and stained according the method proposed by Taylor & Van Dyke (1985), and compared with cleared and stained specimens of Pachypops and Pachyurus.The list of the material examined comprises the locality, including the country, state or department, river and complement, institutional number, number of specimens, in parentheses the standard length in mm, collection date and collector.
The examined specimens are deposited in the following institutions: AMNH, American Museum of Natural History; ANSP, Academy of Natural Sciences of Philadelphia; FMNH, Field Museum of Natural History; MZUSP, Museu de Zoologia da Universidade de São Paulo; and INPA, Instituto Nacional de Pesquisas da Amazônia.

DIAGNOSIS OF THE GENUS
Petilipinnis is distinguishable from all other Sciaenidae genera, with the exception of Menticirrhus, by having only one spine in the anal fin.From Menticirrhus it differs by having 29 to 32 soft dorsal-fin rays and a carrotshaped swimbladder with an anterior pair of short branched appendages (fig.3 a); in Menticirrhus the dorsal fin has 18 to 27 soft rays and the swimbladder, when present, is reduced to a simple vesicle.A single species is included in the genus: Petilipinnis grunniens (Schomburgk, 1843).
Etymology: From the Latin petilus (adjective), slender, and pinna (substantive), fin, in allusion to the slender morphology of the anal fin.Gender feminine.
Scales ctenoid, except on snout and preopercle, where they are cycloid.Lateral line curved until the anterior third of second dorsal fin, extending to the hind margin of the caudal fin.Small ctenoid scales on proximal 2/3 of dorsal fin, proximal 3/4 of pelvic fin, proximal 3/4 of caudal fin, and proximal 1/2 of pectoral and anal fins.
First dorsal fin with 10 spines, first very small; a notch present between first and second dorsal fins; second dorsal with an spine.Posterior tip of pectoralfin not reaching the posterior tip of pelvic-fin when depressed.Anal-fin spine slender, most commonly 3.3-3.6times in head length.Caudal fin rhombic in lateral view.
Swimbladder carrot-shaped with an anterior pair of short branched appendages, its posterior end slightly surpassing anus (fig.3 a).
Color in alcohol: Head and dorsal 3/4 of body light tan, with small brown irregular spots, concentrated anteriorly.Lower 1/4 of body silvery.First dorsal fin light tan with dark edges and second dorsal fin with numerous small brown spots forming irregular stripes.Pectoral, pelvic, anal, and caudal fins light yellow, slightly silvery.Juveniles are light tan with brown spots smaller than eye on body.
Distribution: Known from rivers of the Amazon River basin in Brazil, and Cuyuni and Essequibo River basins in Guyana (fig.4).

REMARKS
In the description of Corvina grunniens, a South American freshwater sciaenid from the Essequibo River, Guyana, Schomburgk (1843: 136) cited the presence of black dorsal and anal fins spots, elongate eye, terminal mouth, 32 dorsal-fin soft rays, 14 pectoral-fin rays, and anal-fin with one spine and 7 soft rays.Eigenmann (1912: 476-78), in his study of the freshwater fishes of Guyana, based on four specimens, also reported the presence of a single anal spine in a sciaenid species, identified by him as Pachypops grunniens.
The comparision of swimbladder and external morphology of Petilipinnis to the South American freshwater genera, Pachypops, Pachyurus, and Plagioscion, is presented in Table 2.
The single published phylogenetic hypothesis about sciaenid genera relationships was proposed by Sazaki (1989).In his work, Pachyurus and Pachypops, both included in the subfamily Pachyurinae, comprise a monophyletic clade supported by three synapomorphies: an anteriorly straight vomer, a pointed ventral tip of the nasal bone, and the dorsal surface of the third infraorbital lying almost parallel to the horizontal axis of eye.A fourth synapomorphy pointed out by Sazaki (1989: 40), a suspensorium strongly depressed, is here discarded as a character since the depression level is not comparable among other genera within the family.The sinapomorfies of the Pachyurinae shared by Petilipinnis reinforces its position in this subfamily.Petilipinnis also shares with Pachyurus the derived second pharyngobranchial plate enlarged, more than twice as large as that on the third epibranquial; in Pachypops the plate on the second pharyngobranchial is smaller than that on the third epibranquial (Sazaki, 1989: 56;Casatti, 2000: 29).Another derived Pachyurus character is a derived metapterygoid lateral margin overlapping the medial side of the lower arm of hyomandibular which, in contrast, is not found in Petilipinnis and Pachypops that present a straight metapterygoid lateral margin (Sazaki, 1989: 40;Casatti, 2000: 31)

Figure 4 .
Figure 4. Geographic distribution of Petilipinnis grunniens.Some symbols represent more than one examined lot.

Table 2 .
Comparision of external and swimbladder morphological characters the four among South American freshwater Sciaenidae genera.