Cheirodontinae

Weitzman, Stanley H. & Malabarba, Luiz R., 1999, Systematics of Spintherobolus (Teleostei: Characidae: Cheirodontinae) from eastern Brazil, Ichthyol. Explor. Freshwaters 10 (1), pp. 1-43 : 6-8

Subfamily Cheirodontinae , Clade A

Spintherobolus species belong to a restricted Cheirodontinae as diagnosed phylogenetically by Malabarba (1994; in press). For purposes of hypothesizing the phylogenetic relationships of Spintherobolus we summarize his diagnosis using the first four characters listed below. The Cheirodontinae as diagnosed by these features is equivalent to Clade A in our cladogram.

(1) A pigmented humeral spot is absent. Preserved Cheirodontines often appear to have a humeral spot but the darkened area in the humeral region is due to a muscle hiatus just lateral to the anterior portion of the swimbladder ( Fig. 4 ). The swimbladder may have dark pigment on it, but this is not the same pigment that forms the humeral spot of the epidermis of other characiforms.

Absence of a humeral spot occurs in relatively few other Characiform species, but none of these have all four of the characters listed here. This statement suggests that the lack of humeral spots in some other characiforms is homoplastic with regard to their absence in the Cheirodontinae . At this time we are unable to provide an analysis of the cladistic significance of the humeral spot and its absence in other characiforms because the phylogenetic significance of the distribution of its presence or absence is unstudied and unknown in nearly all of them. We can only suggest that the loss of this character occurred independently in certain outgroup characids.

(2) A characteristic hiatus occurs in musculature of the body wall in the area of the swimbladder between the first and second pleural ribs, exposing the swimbladder ( Figs. 4-6 ). This hiatus is not a simple derived absence of musculature tissue, but a derived and organized feature of the body wall presumably making it more efficient in transmitting sound from the environment to the swimbladder, perhaps by avoiding interference from active muscle tissue. This hiatus is an organ called the pseudotympanum because the membranous wall of the swimbladder is exposed to fluid and some fat in the muscular hiatus. The absence of interference with sound transmission from active muscle tissue may increase the efficiency of that transmission from water external to the skin and scales to the wall of the swimbladder. From there sound is transmitted to the inner ear through the intermediate contact between the swimbladder wall and the Weberian apparatus. Also the absence of muscle tissue against the membranous swimbladder wall may allow that wall more freedom to respond to sound waves. Note: The species of Spintherobolus have two rather than one pseudotympanum and their posterior pseudotympanum (between the first and second pleural ribs) is homologous to that in all other species of the Cheirodontinae ; see more detailed description in synapomorphy 21 below for Spintherobolus .

Among other characiforms, a pseudotympanum of different structure and presumably not homologous with that found in the Cheirodontinae is also found in the species of the Characinae. The implications of this are discussed by Malabarba (in press).

(3) Cheirodontine teeth have a proximal peduncle or pedicle and an expanded and compressed distal region bordered by a few to many cusps ( Fig. 7 ). Note: In the case of the species of Spintherobolus the modification of this flattened, expanded distal portion into an essentially conical tooth is a secondary alteration according to the hypothesis of relationships presented here; see also apomorphy 3R below. Note that not all of these teeth in Spintherobolus are conical.

Somewhat similar teeth occur in Piabucus and Iguanodectes , genera referred to the tribe Iguanodectini of the Tetragonopterinae by Géry (1977). Even when these genera are entered in our analysis and their teeth are coded the same as those of the Cheirodontinae , these two genera are ex ­ cluded from that subfamily in the most parsimonious cladogram.

(4) All Cheirodontines have a single series of teeth on the premaxilla. This feature is discussed by Malabarba (in press) but it is not restricted to the Cheirodontinae and has appeared convergently many times in the Characidae and some other characiforms.

The Cheirodontinae consists of several major clades ( Malabarba, 1994; in press). We treat only one of these, Clade B containing Spintherobolus , in detail in our cladogram ( Fig. 3 ). We defer treatment of the other cheirodontine clades to Malabarba (in press). The species of Spintherobolus form a subgroup of clade B and share several derived features of the ventral procurrent caudal-fin rays not found in other characiforms, including other clade A Cheirodontines. Other Clade A Cheirodontines are represented in Figure 3 by Odontostilbe fugitiva Cope , the type species of Odontostilbe .

In addition to the species of Spintherobolus Clade B Cheirodontines include the species of Cheirodon (sensu Casciotta et al., 1992) and are here labeled as Clade C in Figure 3 . Some other species at present placed in the polyphyletic and paraphyletic Odontostilbe are here placed in Clade E in our cladogram and the earliest described species of this clade, Odontostilbe piaba, represents the included species our Figure 3 .

Figures

Fig. 4. Spintherobolus broccae, USNM 287324, adult male, 17.9 mm SL. Head and anterior body region, right side. Two pseudotympanums, black areas are located posterior to the dorsal portion of the opercular opening. The posterior pseudotympanum is homologous with the single pseudotympanum of other Cheirodontines while the anterior pseudotympanum is found only in Spintherobolus. The small dark dots arranged in rows arer the neuromast pattern over the face and opercular region. In the region ventral to the pectoral fin a few neuromasts can be seen partly bordering the scales. Such neuromasts are also present in the dorsal or back region. However, these are sometimes hard to distinguish from the somewhat larger dark chromatophores that often have somewhat indistinct borders. The rows of neuromasts are especially clear in the infraorbital region and on the dentary.

Fig. 5. Spintherobolus ankoseion, paratype, MCP 19253, adult female, 25.2 mm SL. Anterior and posterior pseudotympanal region of left side, anterior is to the left. The skin and scales were removed, exposing the muscles of the lateral body wall and the pleural ribs of the fifth and sixth vertebrae (first and second pleural ribs). The black, pigmented swimbladder walls (the actual pseudotympanums) of both the anterior and posterior pseudotympanal regions have been removed, showing the black interior of the swimbladder.

Fig. 6. Spintherobolus leptoura, paratype, MCP 19254, adult female, 25.4 mm SL. Anterior and posterior pseudotympanums of left side. The skin and scales were removed, exposing the muscles of the lateral body wall, pleural ribs of the fifth and sixth vertebrae (first and second pleural ribs), and the black, pigmented wall of the swimbladder. The larger black area to the right represents the pseudotympanum present in all Cheirodontines, while the black area to the left is the area of the pseudotympanum characteristic of the species of Spintherobolus.

Fig. 7. Odontostilbe sp., MCP 11136, adult male 37.0 mm SL. SEM photograph of a frontal view of the anterior three premaxillary teeth of the right side. Illustrated for each tooth are a pedicles and a flattened, expanded distal region bordered by approximately 10 tooth cusps. Scale marker = 100 μm.

Fig. 3. A phylogeny of Spintherobolus and related cheirodontine taxa based on consensus tree of 3 equally parsimonious cladograms. Tree statistics: Tree Length = 49, Consistency Index = 0.796, Retention Index = 0.867.