Pachypanchax Myers

Paul V. Loiselle, 2006, A review of the Malagasy Pachypanchax (Teleostei: Cyprinodontiformes, Aplocheilidae), with descriptions of four new species., Zootaxa 1366, pp. 1-44 : 4-7

[[ Genus Pachypanchax Myers ]]

Generic placement of the Malagasy aplocheilids

Myers (1933) based the genus Pachypanchax upon four diagnostic characters: (1) limited mobility of the maxilla, resulting from its attachment posteriorly by the skin to the postorbital region of the skull; (2) presence of unexpanded haemal arches; and (3) a rounded caudal fin whose (4) basal half is heavily scaled, the scales in straight rows, one scale wide, each series covering the interspace between two caudal rays. Myers placed particular emphasis on the caudal squamation of Pachypanchax , which he found to be unique among Old World aplocheilid cyprinodonts examined by him, and matched only by that of the Neotropical genus Austrofundulus .

Myers’ conclusions regarding the diagnostic value of the caudal squamation of Pachypanchax are supported by the examination of specimens of several species of the aplocheiloid genera Aplocheilus , Epiplatys , Aphyosemion , (subgenera Chromaphyosemion , Fundulopanchax , Mesaphyosemion and Scriptaphyosemion ), and Nothobranchius ; the rivulid genera Austrofundulus , Austrolebias , Rachovia and Rivulus ; the cyprinodontid genera Cyprinodon , Cubanichthys and Orestias ; the fundulid genus Fundulus ; and the profundulid genus Profundulus . Aplocheilus lineatus and A. panchax both possess one or two basal rows of small scales on the caudal fin that overlap the fin rays, a condition also present in Epiplatys fasciolatus and Nothobranchius melanospilus . In specimens of the remaining aplocheiloid genera examined, the caudal is unscaled. Caudal squamation comparable to that seen in Aplocheilus has been observed in Austrolebias bellottii , Rachovia pyropunctata , Cubanichthys cubensis , and Fundulus lima . A narrow triangular patch of scales extending from the base of the caudal half way to its margin was observed in large specimens of Rivulus hartii , but a degree of caudal squamation comparable to that seen in Pachypanchax is present only in the distantly related Neotropical cyprinodontiform genera Austrofundulus and Profundulus .

Scheel (1968) proposed two additional morphological features that served to differentiate Pachypanchax from the aplocheiloid genera Aplocheilus and Epiplatys . He found the central rays of the caudal fin to be prolonged in the latter two genera, producing a median lobe, whereas, in the absence of such a feature, the caudal fin is rounded in Pachypanchax . He also noted that, in Pachypanchax , the frontal squamation was of the E- type sensu Hoedemann (1958), usually with prominent H scales. In Aplocheilus and Epiplatys , to the contrary, the frontal squamation was of the G type, without H scales. The presence of a rounded or truncate caudal fin characterizes all of the Malagasy aplocheilids known to date. However, examination of larger series of Malagasy material suggests that the E type of frontal squamation does not unequivocally characterize these killifish. While Scheel’s observation regarding this character is generally correct, occasional aberrant individuals with the G pattern have been found to occur in several Malagasy species.

Following a more detailed morphological investigation of Pachypanchax , Parenti (1981) discovered six additional apomorphic characters, of which two were deemed to have particular diagnostic value: a) the squamation of adult males, in which the posterior edge of the scales of the dorsum and flanks is angled away from the body; and b) the fusion of the five hypural bones into a single fan-shaped plate. As previously noted, in the absence of Malagasy material, Parenti was unable to determine whether P. omalonotus shared derived skeletal characters with P. playfairii .

The absence of a reflective pineal (frontal) spot in living individuals is a further feature that unites P. playfairii and the Malagasy taxa. A reflective pineal spot is a conspicuous feature of the color pattern of the aplocheiloid genera Aplocheilus , Adamas and Epiplatys . However, this feature is lacking in the color pattern of Pachypanchax and in the remaining aplocheiloid genera, although it occurs sporadically within a number of New World cyprinodontiform lineages.

The distribution of this suite of diagnostic characters within the genus Pachypanchax is presented in Table 1. It is clear that neither the absence of dorsal and anal fin filaments in males nor the presence of dark markings in the gular region unambiguously defines the genus. Furthermore, the raised dorsolateral scales on the body of male P. playfairii are not found in any Malagasy representatives of the genus, nor in all populations of nominal P. playfairii (P. de Rham, pers. com.), and thus represent an autapomorphy of the topotypical population of this Seychellois endemic. The ocellated black spot in the dorsal of juvenile and female P. playfairii is also characteristic of all known species of Aplocheilus . It is not present in any Malagasy aplocheilid, although a diffuse dusky blotch is present in some females of P. sakaramyi . This color pattern is most parsimoniously interpreted as pleisomorphic within the Aplocheilidae . However, P. playfairii shares diagnostic skeletal characters: a heavily scaled, rounded caudal fin; and lack of a reflective pineal spot with all save one of the presently known Malagasy aplocheilids. These data support both Myers’ and Parenti’s inclusion of Poecilia omalonota and Panchax sakaramyi within the genus Pachypanchax , and no further ambiguity should attend their generic placement. With the exception of Poecilia nuchimaculata Guichenot, 1866 , the same holds true for Madagascar’s remaining aplocheilids.

Guichenot (1866) described Poecilia nuchimaculata from a single specimen collected by J. P. Goudot, a French resident of Madagascar from 1835 until his death in the early 1860's. The specimen lacks precise locality data and as Goudot’s journals were lost when he disappeared while in the field (Dorr, 1997), subsequent efforts to determine its provenance have proved fruitless. Published data (Huber, 1998) indicate that with regard to both morphometric and meristic characters the type falls within the range that defines the Malagasy representatives of the genus Pachypanchax . He noted that the prefrontal scales of this specimen are smaller than those of other Pachypanchax species, while the lateralis system consists of open rather than enclosed pores. The caudal squamation is rather light, with scales present only on the basal third of the fin (de Soutter, pers. com.), a feature shared with only one other Malagasy congener. Radiographic examination of the caudal skeleton confirms Huber’s observation that a shallow notch separates the upper plate, formed by the fusion of third, fourth and fifth hypural bones and the lower plate, formed by the fusion of the first and second bones. This condition is more reminiscent of that found in many representatives of the Nothobranchidae (upper and lower plates separated by a deep groove), than that seen in Aplocheilus (hypural skeleton made up of three independent elements, the upper- and lowermost comprising the fused fourth and fifth and first and second hypural bones respectively).

In the absence of additional material of P. nuchimaculatus , there is no way to determine whether the observed anomalies in squamation and caudal skeleton of the type specimen are idiosyncratic or reflect significant population-level differences. Pending the acquisition of additional material of this species, it seems advisable to follow Parenti (1981) and provisionally assign Poecilia nuchimaculata to the genus Pachypanchax .

Pachypanchax Myers, 1933 may thus be diagnosed as follows: Maxillary relatively immobile, bound at its posterior end to the preorbital by a fold of skin. Premaxillary ascending processes flat and broad, tapered posteriorly and not overlapping in the midline. A single pair of tubular nares present. Reflective pineal spot absent. Frontal squamation typically of the E-type, with prominent H scales. In very large specimens, a shallow pit may be present in the center of some scales along the midlateral line, but there is no evidence of any connection to an underlying neuromast. Scales and fin rays lacking papillae. Haemal arches unexpanded, no pleural ribs on haermal spines. Hypural plates fused to form a hypural fan in adults, joint lines visible in juveniles. Caudal fin rounded or rounded-truncate, the middle rays never extended. Basal third to three quarters of caudal fin heavily scaled, the scales in straight rows, one scale wide, each series covering the interspace between two caudal rays. Caudal fin lacking a median lobe. Filamentous extensions of the dorsal and anal fins of males variably present. Dark gular bar variably present. Pigmentation pattern does not include cross bars on body. Basal dorsal-fin spot absent in males, variably present in females.

Type species: Pachypanchax playfairii (Gunther 1866) .

The genus is endemic to Madagascar and the granitic Seychelles. The Zanzibari population of Pachypanchax playfairii represents a recent translocation and (Parenti [1981] notwithstanding) there are no confirmed records of this species from the East African mainland (Seegers, 1980). Apart from two species native to eastward- and northward flowing drainages in the extreme north, Malagasy Pachypanchax are restricted to the island’s western versant, from the Ambohitra Massif (Massif d’Ambre) south to the basin of the Tsiribihina River.