Amazonspinther dalmata

Bührnheim, Cristina M., Carvalho, Tiago P., Malabarba, Luiz R. & Weitzman, Stanley H., 2008, A new genus and species of characid fish from the Amazon basin - the recognition of a relictual lineage of characid fishes (Ostariophysi: Cheirodontinae: Cheirodontini), Neotropical Ichthyology 6 (4), pp. 663-678 : 669-673

Monophyly of Amazonspinther dalmata + Spintherobolus

A close relationship between Amazonspinther dalmata and Spintherobolus is strongly supported by fifteen unam-

biguous synapomorphies, described below. The first ten characters herein analyzed were more extensively discussed by Malabarba (1998) and Weitzman & Malabarba (1999) as synapomorphies for Spintherobolus , and are herein expanded as synapomorphies of A. dalmata + Spintherobolus .

A complex, patterned series of exposed neuromasts is distributed on the head and body (ch. 69 in Malabarba, 1998: 216, see discussion under ch. 15 in Weitzman & Malabarba, 1999: 12-13, figs. 4, 10 and 11). This is a character shared by A. dalmata and Spintherobolus species. Patterns of exposed neuromasts found in A. dalmata are similar and homologous to those of Spintherobolus (compare Fig. 4 here with Weitzman & Malabarba, 1999: 124, 10).

The dentary has a large anterior fenestra ( Fig. 5 ), associated with a large epidermal, papilla-like structure surrounded by a deep groove that has its deep internal portion lodged in the dentary fenestra. The external surface of this papilla bears several exposed neuromasts. The ventral face of the dentary bone, posterior to the fenestra, is concave (ch. 5 in Malabarba, 1998: 216, and ch. 16 in Weitzman & Malabarba, 1999: 128, fig. 12).

Infraorbital bones are reduced in number and possibly fused. Amazonspinther dalmata has the first and second infraorbitals fused and slightly bifurcated anteriorly, and the third infraorbital reduced ( Fig. 11 ). Fourth to sixth infraorbitals are absent, as observed for Spintherobolus (see discussion under ch. 9 in Malabarba, 1998: 216, and ch. 18 in Weitzman & Malabarba, 1999: 128, fig. 13).

There is a small number of pelvic-fin rays, not exceeding six branched rays (see ch. 14 in Malabarba, 1998: 216, and discussion under ch. 19 in Weitzman & Malabarba, 1999: 129, fig. 14). Amazonspinther dalmata has i,5,i and Spintherobolus species i,4,i; i,5; i,5,i or i,6, while remaining cheirodontines show i,7-8 branched rays.

The anal fin has a small number of 9-16 branched rays. This number varies among the species of this clade, with the highest range observed in S. broccae (13-16 branched rays; state 1), an intermediate range in S. ankoseion and S. leptoura (11-14; state 2), and the smallest number found in A. dalmata and S. papilliferus (8-9 and 9-10, respectively; state 3). State 0 corresponds to 16 to 24 branched rays, following Weitzman & Malabarba (1999: character 20; figs. 9 and 15). Reduction in the number of branched anal-fin rays is a synapomorphy of A. dalmata + Spintherobolus . The remarkably reduced number of branched anal-fin rays in A. dalmata and S. papilliferus is the lowest observed in Cheirodontinae but it is most parsimoniously accepted as basal in the clade Amazonspinther + Spintherobolus .

An anterior pseudotympanum lies anterior to the first pleural rib ( Fig. 1 ) (ch. 2 in Malabarba, 1998: 216, and ch. 21 in Weitzman & Malabarba, 1999: 130).

The symphyseal dentary joint surfaces are smooth oval articulations lacking the intercalated and folded bony surfaces found in outgroup characids. The articulation is supported by tough ligamentous tissue (ch. 4 in Malabarba, 1998: 216 and ch. 24 in Weitzman & Malabarba, 1999: 131).

Lateral line is reduced to 2-6 perforated scales (see ch. 60 in Malabarba, 1998: 216, and discussion under ch. 25 in Weitzman & Malabarba, 1999: 131). Amazonspinther dalmata has 4-5 scales, averaging 4.8, similar to that of S. papilliferus (4.6).

The coracoid bone of the pectoral girdle ( Fig. 12 ) is reduced in length, and more or less discoid in shape (ch. 13 in Malabarba, 1998: 216, and ch. 26 in Weitzman & Malabarba, 1999: 131, fig. 18).

The pectoral-fin is relatively short. Weitzman & Malabarba (1999: 132; ch. 28) found this character to be ambiguous, supporting two equally parsimonious hypotheses: the acquisition in a common ancestor to Spintherobolus and a reversal in S. ankoseion , or the independent acquisition in S. papilliferus and in the clade S. broccae + S. leptoura . The presence of a short pectoral fin in A. dalmata (13.82-16.30 % of SL, mean 15.19) supports this character as a synapomorphy of Amazonspinther + Spintherobolus , and the longer pectoral fin of S. ankoseion as autapomorphic and a reversal.

New characters added herein are as follows:

The teeth are conical to tricuspid (ch. 36, Fig. 5 ). We treat the conical or tricuspid teeth separately and independent of teeth pedunculation (ch. 3). The conical to tricuspid teeth are proposed as a synapomorphy of A. dalmata + Spintherobolus (vs. multicuspid teeth of remaining cheirodontines - except the compsurin Macropsobrycon uruguayanae ).

The antorbital of A. dalmata and Spintherobolus species is short and rounded to oval (ch. 38, Fig. 11 ), instead of elongate, slender and ventrally expanded, as observed in the remaining Cheirodontinae . It resembles that of Carnegiella , Gasteropelecidae (adnasal in Weitzman, 1954). Not checked in S. leptoura .

The gill rakers are short and conical, instead of elongate and lanceolate (ch. 40, Fig. 6 ).

The gill rakers on the lower branch of the first gill arch are absent or only the posteriormost gill raker at the junction of the ceratobranchial and the epibranchial is present (ch. 41, Fig. 6 ). Gill rakers on the lower branch of the first gill arch are always present on remaining cheirodontines. First branchial arches of the four known Spintherobolus species were checked for comparisons with A. dalmata . All the species lack gill rakers or have only one gill raker (the posteriormost) on lower branchial branch, and the gill rakers are conical and the shortest among cheirodontines ( Figs. 6 a-d). Amazonspinther dalmata has 3 upper gill rakers and none on the lower branch (two c&s specimens) ( Fig. 6a ). Spintherobolus papilliferus has none (1), 5(4), 6(6), 7(2) upper gill rakers, and one (13) lower gill raker (the posteriormost). The anteriormost gill rakers on upper branch are very short in the specimen photographed ( Fig. 6b ). Spintherobolus broccae has 1(2), 2(4) on upper gill rakers, and none (2) or 1(4) lower gill rakers ( Fig. 6c ). Spintherobolus ankoseion has 1(4), 2(2), 3(2), 4(1), 5(2) upper gill rakers and one (11) lower gill raker (the posteriormost, in photographed specimen not visible, damaged) ( Fig. 6d ). In contrast, Spintherobolus leptoura has 2-3 upper gill rakers, and one lower gill raker (two alcohol specimens examined). For comparison, Cheirodon ibicuhiensis has 7 gill rakers on upper branch and 11 on lower branch, and Serrapinnus heterodon has 5 on upper branch and 13 on lower branch ( Fig. 6 e-f).

The maxillary shape is irregular, not bearing a smooth dorsal border. Instead, there are two concave sections in the dorsal border of the maxilla separated by a dorsal short projection in the bearing tooth region of the bone (ch. 42, Fig. 5 ). The anterior arm of the maxilla that articulates to the premaxilla is thicker than usual in the Characidae . Remaining cheirodontines have a smooth dorsal border in the maxilla.

Monophyly of Spintherobolus

Most characters previously used to diagnose Spintherobolus by either Malabarba (1998) or Weitzman & Malabarba (1999) are herein proposed as synapomorphies of Amazonspinther + Spintherobolus . Two characters remain synapomorphic for Spintherobolus : the lack of an adipose fin (Weitzman & Malabarba, 1999: 131, ch. 17), and the relatively small eye. The eyes of Spintherobolus species are remarkably small compared to that of remaining cheirodontines (see discussion in Weitzman & Malabarba, 1999: 131, ch. 27). Amazonspinther dalmata has large horizontal eye diameter 33.0-39.4% of head length compared to 21.3-28.7% observed in Spintherobolus .

The hypothesis of relationships among Spintherobolus species proposed herein ( Fig. 10 ) agrees with that proposed by Weitzman & Malabarba (1999), in which S. papilliferus is the sister species of one clade formed by the other three species of the genus. Additionally, the polytomy formed by S. broccae , S. leptoura , and S. ankoseion (Weitzman & Malabarba, 1999) was resolved in the present study. The low number of gill rakers on the upper branch of the first gill arch (1-3, ch. 39) supports a hypothesis of sister group relationship between S. broccae and S. leptoura ( Fig. 10 ), a character that independently appears in A. dalmata . The higher number of gill rakers observed in S. ankoseion and S. papilliferus is suggested as plesiomorphic.

Remarks on Amazonspinther apomorphic characters

A high number of ventral procurrent caudal-fin rays (11- 28) is a synapomorphy for the Cheirodontini , and the small number (7-9) observed in Amazonspinther ( Fig. 3 ) is a reversal (Malabarba, 1998; Bührnheim, 2006). Similarly, the ventral procurrent caudal-fin rays articulating with the hemal spines of at least the four posterior caudal vertebrae is a synapomorphy of the tribe Cheirodontini , and the reduced articulation observed in Amazonspinther ( Fig. 3 ) is a reversal (Weitzman & Malabarba, 1999: 9, ch. 9; fig. 8).

The phylogenetic position of † Megacheirodon unicus

† Megacheirodon unicus (Travassos & Santos, 1955) is an extinct cheirodontine, previously proposed as sister-group to Spintherobolus by M. C. Malabarba (1998a,b). The species is known only through two fossilized specimens, a female and a highly sexually dimorphic male. Support for its inclusion in the tribe Cheirodontini is given by the apomorphic traits found in both anal and caudal fins of the male specimen.

The inclusion of the fossil in the analysis and the most parsimonious resulting tree allowed the recognition of † Megacheirodon as sister group to Spintherobolus + Amazonspinther ( Fig. 13 ), but we have reservations about these results. Three apomorphic traits are shared by † M. unicus and Spintherobolus species and are absent in Amazonspinther , and six synapomorphies are shared by Spintherobolus species and Amazonspinther , and are absent in † M. unicus . Sixteen characters, however, are missing in the fossilized remains of this fish (1, 2, 15, 16, 17, 21, 24, 28, 31, 34, 35, 38, 39, 40, 41, and 43). Further complications are related to the lack of mature males of Amazonspinther dalmata that resulted in coding 13 characters as missing in this species (6, 7, 8, 9, 10, 11, 12, 13, 14, 22, 23, 32 and 37). Although monophyly of the Clade † Megacheirodon + Spintherobolus + Amazonspinther , seems to be strongly supported, the internal relationships among the three genera may change with the addition of the missing information.

So far, characters supporting a close relationship between Spintherobolus and Amazonspinther and that are absent in † Megacheirodon unicus (see M. C. Malabarba, 1998 for characters description in † M. unicus ) are, infraorbital bones reduced or fused (ch. 18); one unbranched and five or six branched pelvic-fin rays (ch. 19); anal fin with iii unbranched and 8-16 branched rays (ch. 20); lateral line with 2 to 6 perforated scales (ch. 25), the coracoid bone of the pectoral girdle discoid in shape (ch. 26), and the teeth conical or with three cusps (ch. 36).

Two synapomorphies are shared by all Spintherobolus species and † Megacheirodon unicus , but these are observed only in mature males, and mature males of A. dalmata were unavailable for comparisons. Both characters, however, are related to the fusion and reduced proximal portion of the anteriormost ventral procurrent caudal-fin rays. Since A. dalmata has a small number of ventral procurrent caudal-fin rays, located posteriorly and in the usual position found in other characids ( Fig. 3 ), we expect the following characters are unlikely to occur in A. dalmata : the anterior ventral procurrent caudal-fin rays in adult males, those that have their proximal ends inserted anterior to the hemal spine of the antepenultimate vertebrae, are proximally fused to one another (Weitzman & Malabarba, 1999: 130, ch. 22; fig. 8); and the anterior ventral procurrent caudal-fin rays of males have reduced proximal portions, not rising above the area of fusion between the rays, while the posterior dorsal portions of these rays are fused into a flat compressed plate that inserts between the hemal spine of the antepenultimate vertebra and the hemal spines of the anterior vertebrae (Weitzman & Malabarba, 1999: 131, ch. 23; fig. 8).

One additional apomorphic trait present in † Megacheirodon unicus , clearly discernible in the male specimen (M. C. Malabarba, 1998: 195, fig. 3), is the pterygiophore of the sixth branched anal-fin ray, directed dorsally, away from the fifth which is directed anteriorly and in parallel with the pterygiophores anterior to it ( Sarraf, 1997: Fig. 6 ; Weitzman & Malabarba, 1999: character 33; fig. 9). Such character is shared with S. ankoseion , S. leptoura and S. broccae , again suggesting a close relationship of † Megacheirodon unicus to Spintherobolus .

Figures

Fig. 4. Lateral view (left side) of the head of the holotype of Amazonspinther dalmata showing the pattern of neuromasts distribution (MCP 38623, 19.63 mm SL).

Fig. 5. Premaxilla, maxilla, and dentary (right side) of Amazonspinther dalmata (MCP 37572, unsexed 14.98 mm SL c&s).

Fig. 11. Infraorbitals and antorbital of Amazonspinther dalmata (MCP 37572, CP: 14.98 mm, right side).

Fig. 1. Holotype of Amazonspinther dalmata, new species, MCP 38623, undetermined sex, 19.63 mm SL.

Fig. 12. Pectoral girdle of Amazonspinther dalmata showing the discoidal coracoid. Left side, external view (MCP 37572, 14.98 mm SL, c&s).

Fig. 6a-f. First branchial arches, left side, external view: (a) Amazonspinther dalmata, MCP 37571, female 14.05 mm SL, (b) Spintherobolus papilliferus, MZUSP 51022, unsexed 23,79 mm SL, (c) Spintherobolus broccae, MCP 19196, unsexed 17.43 mm SL, and (d) Spintherobolus ankoseion, MCP 19280, female 23.10 mm SL. Below cheirodontines: (e) Cheirodon ibicuhiensis, MCP 13663, male 36.10 mm SL, and (f) Serrapinnus heterodon, MZUSP 16740, male 30.20 mm SL.

Fig. 10. Phylogeny of Amazonspinther dalmata and related cheirodontine taxa based on the single resulting most parcimonious tree (tree length = 53, Consistency Index = 0.88, Retention Index = 0.92). Characters 6, 7, 8, 9, 10, 11, 12, 13, 14, 22, 23, 28, 32 and 37 were coded as missing in the data matrix for A. dalmata. Characters 10, 11, 12, 22, 23, 27 are ambiguous and were optimized using ACCTRAN (above) or DELTRAN (below) optimizations.

Fig. 3. Posterior caudal vertebrae and associated bone elements of Amazonspinther dalmata (left side view) (MCP 37572, 14.98 mm SL, c&s).

Fig. 13. Phylogeny of Amazonspinther dalmata and related cheirodontine taxa, including †Megacheirodon unicus (tree length = 55, Consistency Index = 0.85, Retention Index = 0.91). Characters 6, 7, 8, 9, 10, 11, 12, 13, 14, 22, 23, 28, 32 and 37 were coded as missing in the data matrix for A. dalmata, and characters 1, 2, 15, 16, 17, 21, 24, 28, 31, 34, 35, 38, 39, 40, 41 and 43 were coded as missing in the data matrix for †M. unicus. Ambiguous optimizations in both A. dalmata and †M. unicus are shown according to ACCTRAN (above) or DELTRAN (below) optimizations.