Pachypanchax omalonotus
Creators: name: Paul V. Loiselle
Publisher: name: Zenodo
Year: 2006
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Cyprinodontiformes
Aplocheilidae
Pachypanchax
Pachypanchax omalonotus

description: Pachypanchax omalonotus (Duméril, 1861)
Poecilia omalonota Duméril, 1861: 257, pl. 22, fig. 7 (type locality: Nosy Be [Nossi-Bé], off northern Madagascar, 12°42’S, 48°16’E. Lectotype: MNHN 2935).
Diagnosis
Pachypanchax omalonotus males differ from male P. playfairii in lacking raised dorsolateral squamation and in having a color pattern that does not feature rows of discrete red spots. The dorsal fin of female P. omalonotus lacks the ocellated black basal blotch present in female P. playfairii. P. omalonotus has the shortest (14.0 ± 0.9 % SL) and deepest (15.2 ± 0.9% SL) caudal peduncle of any Malagasy Pachypanchax, and also differs from P. sakaramyi and P. sparksorum sp. nov. in having the scales of the chest the same size as those of the flanks and in its marked color polymorphism. The presence of rows of discrete metallic gold spots on the flanks and the absence of iridescent white edging along the upper and lower margins of the caudal fin distinguishes living male P. omalonotus from all remaining Malagasy congeners.
Description
Morphometric characters appear in Table 2. A Pachypanchax of moderate size, with a distinctly pointed snout. Mouth wide, cleft directed upward. A single row of slightly recurved, conical teeth present in each jaw. Eight to eleven (mode=10) branchiospines on first gill arch. Two scale rows present on cheeks. Frontal squamation of E-type, with H scales present. Cephalic neuromast pattern open in 18 of 21 specimens examined, closed in the remainder. Scales cycloid, 31-34 (mode=32) along midlateral line. Fourteen transverse scale rows immediately anterior to origin of anal fin; 18 scale rows around caudal peduncle. Scales on chest the same size as those on flanks. Vertebrae 14 pre-caudal + 16 caudal.
Dorsal-fin origin above midpoint between origins of tenth and eleventh anal-fin rays. Dorsal-fin rays ii,9 (2); iii,9 (4); i,10 (2); ii,10 (24); iii,10 (5); ii,11 (9). Ninth or tenth dorsal ray longest in males, fifth or sixth dorsal ray longest in females. Anal fin rays ii,14 (9); ii,15 (10); ii,16 (14); iii,16 (7); ii,17 (4); iii,17 (5). Sixteenth or seventeenth anal ray longest in males, seventh or eighth ray longest in females. Longest ray in both dorsal and anal fins elongated, forming a short filamentous extension in males. Base of dorsal and anal fins scaled. Caudal fin rounded truncate, basal half to two-thirds heavily scaled. Pelvic-fin rays i,5. Pectoral-fin rays 12-16 (mode=14).
Coloration
Live individuals (males). Males of Pachypanchax omalonotus are characterized by color polymorphism. Figure 1 depicts a sexually quiescent yellow-morph male from the Djabala Creek population, which also occurs in a red morph. Figure 2 depicts a sexually quiescent blue morph male from the Ambatozavavy population. Red morph males of the latter population differ from those of the Djabala population in having fewer rows of golden spangles forming a discrete band along the midlateral region. Figure 3 depicts a red morph male of the Sambirano basin population, which also occurs in a blue morph. Black submarginal banding in the dorsal and anal fins has not been observed, to date, in Sambirano or Djabala males, and is encountered much less often in red than in blue morph Ambatozavavy males. A narrow black anal-fin margin is variably present in red morph Ambatozavavy males, but appears to be a consistent element of the color pattern of Sambirano males. Courting males of all populations develop a narrow black midlateral stripe that extends from the snout to a point immediately beneath the dorsal fin origin. Figure 4 depicts a female of the Djabala Creek population.
Preserved specimens (males). Upper half of head, lips and dorsum pale brown, shading to beige on the cheeks, operculum and flanks. Throat and venter yellowish white. Scale centers paler in large males, forming lines of light spots on the flanks. Vertical fins and ventrals clear beige. A reticulate pattern of small light dots variably present basally in dorsal and caudal fins. Vertical fins of some males in the both the Djabala Creek and Ambatozavavy populations may be marked with darker submarginal bands of varying intensity. The pectorals are hyaline. (Females): Similar to that of males, but lacking regular lines of lighter spots on flanks. Fins uniformly hyaline.
Range
Contrary to the widely held view that P. omalonotus is broadly distributed on the western versant of Madagascar (Arnoult, 1959; Kiener, 1963; Scheel, 1968), this species is restricted to the island of Nosy Be, to the basin of the Sambirano River on the immediately-adjacent mainland, and to several small independent coastal streams on the Ankify peninsula (O. Lucanus, pers. com.), immediately to the south of the mouth of the Sambirano (Figure 5). Pachypanchax omalonotus is replaced in river drainages to the north and south by as yet undescribed congeners.
Natural history
On Nosy Be, P. omalonotus is restricted to the middle and upper reaches of small streams flowing over rocky or cobble/sand bottoms under some sort of forest cover. It is absent from the island’s numerous crater lakes and has never been collected in brackish water. In this regard, P. omalonotus and its Malagasy congeners differ significantly from P. playfairii, which moves freely between brackish and fresh water (Hartig-Beecken, 1980). On the mainland, it inhabits both shaded streams and the marshy shallows of lakes in the Sambirano River flood plain. Table 3 presents data on the water chemistry of P. omalonotus habitats on both Nosy Be and on the mainland. According to local informants, stream populations are stable, but the size of lacustrine populations varies both seasonally and from one year to the next, depending upon rainfall. No other fish or macrocrustaceans were collected syntopically with P. omalonotus on Nosy Be. Sambirano drainage populations have been found to occur syntopically with the following fish species: Teramulus waterloti, Ambassis natalensis, Paretroplus damii, Ptychochromis oligacanthus, Oreochromis niloticus, Awaous aenofuscus, and Glossogobius giuris; as well as several crustaceans, including Macrobrachium spp. and several small atyid shrimp species.
Feces of freshly captured specimens contained recognizable remains of both the imagos of terrestrial insects and the nymphs and larvae of aquatic insects. This suggests that, in the wild, P. omalonotus feeds both at the surface and from the bottom. On Nosy Be, Glossogobius giuris is the only fish large enough to pose a threat to adult P. omalonotus. On the mainland, it is also at risk from the three Anguilla species reported from northwestern Madagascar, as well as from the native cichlid Paratilapia polleni. Its preference for shallow water renders P. omalonotus vulnerable to both herons and Corythornis vintsioides, the Malagasy malachite kingfisher.
Captive specimens taken from Djabala Creek, on Nosy Be, in mid-October began spawning within a week of their arrival in the United States in early November. Based upon observations of the growth rate of juveniles in captivity, the size distribution of captured specimens suggests a protracted breeding season lasting through the austral summer, which coincides with the rainy season in northwestern Madagascar.
Conservation status
Pachypanchax omalonotus is abundant throughout its range and to date must contend with neither exotic predators nor competitors. It is classified as vulnerable following the criteria established by the World Conservation Union (Raminosoa et al., 2002).
Discussion
Most localities from which P. omalonotus has been collected are less than 100 m a.s.l. The exception to this pattern is a single lot of fish (AMNH 232475), collected from the shallows of the main channel of the Ramena River, the principal tributary of the Sambirano, at an altitude of 700 m a.s.l. These six specimens are 54.8-70.2 mm SL, large for wild-caught P. omalonotus. They differ from low-altitude populations in having three, rather than two rows of scales on the cheeks, a greater number of transverse scale rows immediately anterior to the origin of the anal fin (15 vs. 14) and around the caudal peduncle (22 vs. 18). The scales on the lower two-thirds of the flanks are marked with dark, rather than lighter centers, and a dusky submarginal band comparable to that present in some Nosy Be populations of P. omalonotus is evident in the anal fin of some males. Regrettably, no data on life colors were recorded when the specimens were collected (C.J. Raxworthy, pers. com.). Pending the collection of additional Pachypanchax material from other high-altitude localities in the Sambirano-Ramena drainage, there is no way to determine the relative importance of genetic and environmental factors in producing these unusual phenotypes. These specimens are thus provisionally assigned to Duméril ’s species. However, due to their uncertain status, neither meristic nor morphological data from this series were used in the redescription of P. omalonotus.
description: Published as part of Paul V. Loiselle, 2006, A review of the Malagasy Pachypanchax (Teleostei: Cyprinodontiformes, Aplocheilidae), with descriptions of four new species., pp. 1-44 in Zootaxa 1366 on pages 7-14