Pachypanchax omalonotus (DUMÉRIL, 1861)

Powder-blue Panchax

SynonymsTop ↑

Poecilia omalonota Duméril, 1861; Poecilia nuchimaculata Guichenot, 1866; Pachypanchax homalonotus (Duméril, 1861)

Etymology

Pachpanchax: from the Greek pachy, meaning ‘thick’, and the generic name Panchax, in reference to member species’ appearing rather like a ‘chubby’ Panchax.

Classification

Order: Cyprinodontiformes Family: Aplocheilidae

Distribution

Type locality is given as ‘Nosy Be [Nossi-Bé], northern Madagascar, 12°42’S, 48°16’E’, and this species is restricted to the island of Nosy Be off northwestern Madagascar plus the Sambirano River system on the adjacent mainland and some small, first-order streams of the Ankify peninsula.

Adult males are polymorphic both within and between populations. For example the Djabala Creek population contains yellow and red morphs, the Ambatozavavy River (Nosy Be) population blue and red.

Red males from Djabala Creek have a greater number of metallic gold scale rows on the flanks while some Ambatozavavy males of both morphs uniquely possess dark submarginal bands in the dorsal and anal fins, though this is more common in the blue form.

The Sambirano River population also contains blue and red morphs which usually have a thin dark edge to the anal-fin, a feature also present in some red males from the Ambatozavavy system.

Although restricted P. omalonotusis abundant throughout its range and unlike some congeners is not threatened by non-native competitors nor predators, therefore it’s currently classified as ‘Vulnerable’ under criteria established by the IUCN (Loiselle, 2006).

Habitat

On Nosy Be it inhabits middle and upper reaches of small freshwater streams with substrates of rocks, cobbles or sand which tend to flow under forest cover.

It is absent from the island’s crater lakes and brackish coastal environments and appears to occur in the absence of competitors since no sympatric fishes or larger invertebrates have been recorded.

On the mainland it’s been collected from shaded streams plus marshy habitats in floodplain lakes of the Sambirano River.

Water parameters recorded at four habitats on both Nosy be and the mainland ranged as follows: pH 7.0-7.5; GH 2-15°; KH 2-11°;  conductivity 34.0-173.0 μS/cm2; temperature 25.2-27.8°C/77.4-82.0°F.

Symaptric fish species include Teramulus waterloti, Ambassis natalensis, Paretroplus damii, Paratilapia polleni, Ptychochromis oligacanthus, Awaous aenofuscus, Glossogobius giuris, Anguilla spp., mnon-native Oreochromis niloticus plus Macrobrachium spp. shrimp and some small atyids.

The majority of habitats are located less than 100 m AMSL but one population inhabiting the Ramena River, a tributary of the Sambirano, exists at around 700 m AMSL and exhibits some morphological differences compared with lowland populations including a relatively larger size, more rows of scales on the cheeks and more transverse scale rows anterior to the anal-fin origin and around the caudal peduncle.

The scales on the lower two-thirds of the flanks have dark, rather than light centres, and some males possess a dusky submarginal band in the anal-fin.

This population is provisionally assigned to P. omalonotus pending further study as per Loiselle, 2006.

Maximum Standard Length

70 – 90 mm.

Aquarium SizeTop ↑

An aquarium with base dimensions of 90 ∗ 30 cm should be the smallest considered.

Water Conditions

Temperature: 20 – 28 °C

pH: 6.0 – 7.5

Hardness: 36 – 268 ppm

Diet

The faeces of wild fish has been observed to contain both aquatic and terrestrial insects, suggesting it feeds both at the water surface and from the substrate.

In the aquarium it’s an unfussy feeder and will accept quality dried products as well as live and frozen fare, including earthworms of its own body length as well as bloodworm, Tubifex, Daphnia, etc.

Wingless fruit flies of the genus Drosophila and tiny crickets are also excellent foods if gut-loaded prior to use.

Behaviour and CompatibilityTop ↑

Relatively peaceful although much smaller fishes are likely to be predated on, and given its conservation status it’s perhaps best maintained alone.

It can be maintained in a group provided the aquarium contains sufficient refuges.

Rival males will regularly fight amongst themselves but serious physical damage is rare.

Sexual Dimorphism

Males grow larger than females and develop more-extended fins as they mature, plus they are far more colourful than females.

Nuptial males develop a narrow dark stripe extending laterally and posteriorly from the snout to beneath the dorsal-fin origin.

Reproduction

If conditions are suitable this species is not difficult to breed and will deposit its eggs among live plants, aquatic mosses, synthetic mops, etc.

It’s perhaps best spawned in a group which can be left in situ and in a well-decorated set-up some fry may escape predation by the adults, otherwise eggs or medium can be removed to a separate container for incubation.

NotesTop ↑

For a number of decades this species was thought to occur throughout much of Madagsacar’s western slope but populations immediately to the north and south of the range described above (see ‘Distribution’) are currently considered to represent unidentified species, while a number of populations from the Mahavavy du Sud and Betsiboka river systems, streams inbetween their mouths and the Lake Kinkony basin have been described as P. arnoulti Loiselle, 2006.

Many fish in the aquarium hobby previously identified as P. omalonotus are in fact P. arnoulti and as a result a degree of hybridisation has undoubtedly occurred.

Should you wish to keep either species as a long-term breeding project it’s therefore advisable to work only with fish of guaranteed origin, and it’s also important to note that those appearing in much older aquarium literature as P. omalonotus are P. arnoulti.

P. omalonotus can be told apart from P. arnoulti by the presence of metallic gold spots on the flanks (vs. absence), absence of iridescent white edges to the anal-fin and lower caudal-fin lobe (vs. presence) and less-rounded dorsal and anal fins.

It differs from P. playfairii in that males lack raised scales on the dorsal surface of the body and red spots on the flanks, while females lack a black ocellus in the dorsal-fin.

It has the shortest (14.0 ± 0.9 % SL) and deepest caudal peduncle among all congeners occurring on Madagascar and differs from P. sakaramyi and P. sparksorum  in that the pectoral (chest) scales are not smaller than those on the flanks, plus males are polymorphic.

The characters separating it from P. arnoulti are also applicable to other Malagasy Pachypanchax spp. although some additionally possess a white edge to the upper caudal-fin lobe.

The genus is endemic to Madagascar and the granitic Seychelles with one recently translocated population known from the island of Zanzibar.

Following Loiselle (2006) it is diagnosed as follows: maxilla 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; presence of a single pair of tubular nares; refelective pineal spot absent; squamation in anterior portion of body of the ‘E-type’ (sensu Hoedemann, 1958) with prominent ‘H-type’ scales; lateral line not present although some large specimens possess a shallow pit in the centre of some scales; papillae on scales and fin rays absent; haemal arches not expanded and haemal spines without pleural ribs; in adults hypural plates fused to form a hypural fan, with the join lines visible in juveniles; caudal-fin rounded to truncate with the central rays never extended; basal third to three-quarters of caudal-fin heavily-scaled, the scales in straight rows one scale wide, each series on the interspace between two rays; caudal-fin without a median lobe; filamentous extensions of the dorsal and caudal-fins variably present in males; dark gular bar variably present; no cross bars on the body; no dark spot at base of caudal-fin in males, variably present in females.

The position of the family Aplocheilidae within the Order Cyprinodontiformes and its constituent members appears to be in question.

It’s generally been considered a natural, monophyletic lineage comprising species from India, South East Asia, The Seychelles and Madagascar but the results of a phylogentic analysis by Hertwig (2008) suggest that the genus Aplocheilus represents the basal sister group to all other Cyprinodontiformes with Pachypanchax forming a separate, less-ancient evolutionary lineage.

The author stopped short of suggesting new family groups for the two genera, however, and both remain in Aplocheilidae at time of writing.

References

1. Loiselle, P. V., 2006 - Zootaxa 1366: 1-44
   A review of the Malagasy Pachypanchax (Teleostei: Cyprinodontiformes, Aplocheilidae), with descriptions of four new species.
2. Hertwig, S. T., 2008 - Zoologica Scripta 37(2): 141-174
   Phylogeny of the Cyprinodontiformes (Teleostei, Atherinomorpha): the contribution of cranial soft tissue characters.
3. Murphy, W. J. and G. E. Collier, 1997 - Molecular Biology and Evolution 14(8): 790-799
   A molecular phylogeny for aplocheiloid fishes (Atherinomorpha, Cyprinodontiformes): the role of vicariance and the origins of annualism.
4. Myers, G. S., 1933 - American Museum Novitates No. 592: 1 p.
   Pachypanchax, a New Genus of Cyprinodont Fishes from The Seychelles Islands and Madagascar.