Plesiolebiasini Costa

Wilson J. E. M. Costa, 2007, Taxonomy of the plesiolebiasine killifish genera Pituna, Plesiolebias and Maratecoara (Teleostei: Cyprinodontiformes: Rivulidae), with descriptions of nine new species., Zootaxa 1410, pp. 1-41 : 38-40

[[ Tribe Plesiolebiasini Costa ]]

Discussion

The present study indicates that the genera Pituna , Plesiolebias and Maratecoara are more diverse and widespread in central and northeastern Brazil than previously recorded (e. g., Costa, 1998a, 1998b, 1998c). In addition, species in the three genera have congruent distributions, suggesting the existence of biogeographic patterns constituting areas of endemism determined by historical factors. However, phylogenetic hypotheses among species of these genera, which would be useful to erect hypotheses of biogeography, are not included here because the intra-generic plesiolebiasine relationships are still controversial. Pituna is hypothesized to be sister to the other plesiolebiasines, based on morphology (Costa, 1998a), but according to molecular data, Plesiolebias would be the sister group to a clade comprising at least Pituna plus Maratecoara (Murphy et al., 1999) or containing Pituna plus Maratecoara plus Papiliolebias (Hrbek & Larson, 1999). Furthermore, both morphological (Costa, 1998a) and molecular (Hrbek & Larson, 1999) studies have resulted in ambiguous hypotheses concerning the phylogenetic position of plesiolebiasines among rivulids. Each of these available plesiolebiasine tree topologies could drastically affect polarization of character states within genera, making possible opposite interpretations on character evolution. It would therefore be necessary to re-evaluate phylogenetic relationships among all plesiolebiasines, which is in progress but is not the aim of the present study. The discussion below focuses only on relevant aspects of the taxonomy.

Monophyly of Pituna has been defined by the following synapomorphic features: 1) fourth pectoral radial expanded ventrally; 2) anal fin elongate and pointed in males; 3) flank and unpaired fins darkly pigmented in females; 4) bright red pigmentation on the distal portion of dorsal fin in males; 5) a green and red stripe on the distal margin of the anal fin in males; and, 6) black spots on the pectoral fin in males (Costa, 1998a). However, none of these conditions are uniquely diagnostic of Pituna , since character states 1, 2, 3, and 4 occur in other plesiolebiasines, and character states 5 and 6 do not occur in all species of Pituna . However, the color pattern on the flanks in males of all species of Pituna (i. e., dark brown with light blue to golden small spots), is unique among plesiolebiasines, thus providing a uniquely diagnostic feature.

Pituna poranga was formerly placed in the synonymy of P. compacta by Costa (1998b), in part because P. poranga was known only from females and juvenile specimens. Examination of larger collections of both nomina has shown that P. poranga is a valid species, and differs from P. compacta in having a less robust and more slender body (for values see the key above). In addition, these species differ in details of color pattern: there are four or five bars or transverse rows of spots on the pectoral fin in males of P. poranga , instead of the seven or eight rows found in P. compacta ; and the dark marks on the flanks in females are usually coalesced in P. poranga to form a color pattern distinct from those of all other congeners (Fig. 4).

Pituna obliquoseriata was first identified as P. compacta by Costa (1998b). However, P. obliquoseriata is distinguished from both P. compacta and P. poranga , the two other congeners inhabiting the same composite biogeographic area (i. e., the das Mortes -Araguaia -Tocantins river basin), by the former having more caudalfin rays (28-31 vs. 25-28), a distinct flank color pattern in females (dark brown dots arranged in oblique rows, vs. small dark brown spots mainly arranged in longitudinal rows).

Costa (1998a) divided Plesiolebias into two groups (i. e., clades A and B). The first one (clade A), comprising P. glaucopterus (from the rio Paraguay basin) and P. aruana (from the rio Araguaia basin), was diagnosed by two derived conditions: a reduced number of pectoral fins (usually 11-12) and a black zone on the anterior portion of the anal fin in males. However, neither character seems to be useful for diagnosing either clade, since some species of clade B may have twelve pectoral-fin rays and the black zone is not always found among specimens of clade A. On the other hand, species of clade A also may be diagnosed by their possession of bright green eyes in males, a derived condition also occurring in Maratecoara , and a red stripe on the anterior part of the flanks that extends to the orbit in males, a condition not found elsewhere among rivulids. Plesiolebias canabravensis should be considered a member of this clade, since it shares these diagnostic features.

The second group of Plesiolebias (clade B), according to Costa (1998c), includes P. xavantei from the rio Tocantins basin and P. lacerdai from the rio das Mortes basin, and is diagnosed by eight synapomorphies: supraoccipital long, contacting first neural spine; second proximal radial of dorsal fin between neural spines of vertebrae 9-13; filamentous ray on tip of pelvic fins in males; dark dots coalesced to form oblique stripes on flank in females; black oblique bar on preopercle in males; basal half of dorsal fin intensely pigmented with red in males; oblique black bars on anterior portion of flank in males; and melanophores concentrated on posterior portion of anal fin in males. Plesiolebias fragilis , P. altamira and P. filamentosus possess the synapomorphies of clade B, except that the black bar on the preopercle is absent in P. fragilis .

Three of the four new species of Plesiolebias herein described were formerly misidentified (Costa, 1998c). Plesiolebias fragilis was first identified as P. lacerdai , which is also a miniature species (Costa, 1998c) occurring in the same hydrographic basin (Fig. 30). However, they may be easily distinguished by several body and head measurements: body depth (23.3-25.4 % SL in males and 22.2-25.7 % SL in females of P. fragilis , vs. 28.6-31.4 % SL in males and 28.0-30.0 % SL in females of P. lacerdai ); head depth (64.9-74.0 % of head length in males and 67.6-77.1 % SL in females of P. fragilis , vs. 75.6-85.8 % of head length in males and 73.9-83.7 % of head length in females of P. lacerdai ). They may also be distinguished by male color patterns(compare Figs. 14 and 29), including the presence of three white bars on the basal portion of the anal fin in male P. fragilis (vs. four bars in P. lacerdai ). The anteriormost anal-fin bar of P. fragilis seems to correspond to a fusion of the two anteriormost bars of P. lacerdai . In addition, in P. fragilis there are few bright greenish dots scattered on flank in males, in contrast to numerous dots arranged in oblique rows in P. lacerdai , and diffuse bars on the caudal peduncle side in males, instead of well-defined bars as in P. lacerdai .

Plesiolebias filamentosus was first identified as P. xavantei (Costa, 1998a), which is endemic to the same river basin (i. e., rio Tocantins), although separated by about 600 km (Fig. 13). Both species share a similar flank color pattern and the presence of a long black filament on the tip of each pelvic fin in males, but P. filamentosus may be distinguished from P. xavantei by the posterior position of the dorsal-fin origin (dorsal finorigin at vertical between base of 5th and 6th anal-fin rays, vs. between base of 7th and 8th anal-fin rays in P. xavantei ), fewer anal-fin rays (15-16 vs. 17-18), and a distinct color pattern of the male dorsal fin (red pigmentation restricted to basal third of fin and dorsal-fin base, with small white spots, vs. red pigmentation extending to basal two thirds of fin and dorsal-fin base with white bars). In addition, P. filamentosus seems to be a smaller species, reaching about 20 mm SL, even in individuals kept in aquaria by G. C. Brasil (pers. com.), but which were not preserved for study, whereas Plesiolebias xavantei reaches about 30 mm SL (Costa et al, 1988; Costa, 1998a). In addition, the geographic range of P. filamentosus is within the Amazonian forest, altitude about 100-130 m, whereas the habitat of P. xavantei is in the Cerrado, a savannah region located at an altitude about 210-250 m.

Plesiolebias canabravensis , from the middle rio Tocantins basin, was first identified as P. aruana (Costa, 1998c), a species known only from the middle rio Araguaia basin (Fig. 13). Although the geographic range of both species are in adjacent regions, they are physically separated by hills bordering the Araguaia and Tocantins river basins. Plesiolebias canabravensis differs from P. aruana in having an anal-fin base with white elongate spots in males (vs. long curved bars, ventral tips anteriorly directed, often converging to a point on anterior margin of fin); 1 + 27-31 + 1 infra-orbital neuromasts (vs. 1 + 16-22 + 1), and a dorsal-fin origin between the neural spines of vertebrae 13 and 14 (vs. between neural spines of vertebrae 12 and 13).