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Pachypanchax arnoulti LOISELLE, 2006


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

arnoulti: named for Jacques Arnoult ‘in recognition of his many contributions to Malagasy ichthyology, and who first introduced this and other fishes from Madagascar to science and to the aquarium hobby in the 1950’s.


Order: Cyprinodontiformes Family: Aplocheilidae


Current known range includes the Mahavavy du Sud and Betsiboka river systems in western Madagascar, some first-order streams entering the Mozambique Channel between their mouths and Lake Kinkony plus its satellite lakes in the Mahavavy du Sud delta region.

Additional localities include a hill stream flowing from the Tampoketsa highlands and the upper Kamoro River basin, and may also occur in the Mahajamba and Soahany watersheds.

Type locality is ‘swamp draining into a tributary stream of the Ikopa River, flowing parallel to RN-4 at Antanimbary Village (17°10’79”S, 46°50’97”E), 246 m a.s.l. (Betsiboka River drainage)’ with the type series F2 descendants of fish collected there.

This species’ population have declined dramatically since the 1950s and when last surveyed it was abundant only in the upper Kamoro River, the small streams between the estuaries of the Betsiboka and Mahavavy du Sud rivers and satellite lakes of the Kinkony basin.

At other localities where it was reported abundant in the past it has been replaced by non-native swordtails, Xiphophorus hellerii, and mosquitofish, Gambusia holbrooki.

Many habitats have also been devastated by removal of natural forest for conversion to cultivation and pastureland, a practise which has led to an enormous increase in soil erosion.

As a result it’s currently classified as a species of special concern by the World Conservation Union, i.e., its status needs to be monitored on a regular basis.


Appears to be something of a habitat generalist and has been collected from a range of environments.

The type locality consists of a swamp with vegetation dominated by Pandanus and Dracaena spp. containing clear, tannin-stained, soft (total hardness < 17.1 ppm), slightly acidic (pH 6.2) water with low conductivity (16 μS/cm²).

Microfauna included various aquatic Coleoptera and Hemiptera while Xiphophorus hellerii, was collected in the outflowing stream but not the swamp itself. There were no aquatic plants or filamentous algae.

In a creek flowing into the Akalimolitra River, a tributary within the Betsiboka system, it was collected from the water was clear, transparent, neutral (pH 7.0), moderately hard (total hardness 68.4 ppm) and weakly conductive (34 μS/cm²) with no aquatic plants or filamentous algae.

Although only dragonfly nymphs were observed in the creek sympatric fishes in the main river channel included Paretroplus kieneri, P. tsimoly, Glossogobius giuris, Awaous macrorhynchus and non-native Oreochromis mossambicus.

Maximum Standard Length

60 – 80 mm.

Aquarium SizeTop ↑

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

Water Conditions

Temperature20 – 28 °C

pH6.0 – 7.5

Hardness36 – 179 ppm


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, TubifexDaphnia, 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.


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 ↑

P. arnoulti has been in the aquarium hobby for many years but was identified as the congener P. omalonotus prior to its description.

It appears in the majority of older aquarium literature under that name 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.

P. arnoulti can be told apart from P. omalontotus by the absence of metallic gold spots on the flanks (vs. presence), presence of iridescent white edges to the anal-fin and lower caudal-fin lobe (vs. absence), possession of more rounded dorsal and anal-fins and a longer caudal peduncle (17.0 ± 1.9% SL vs. 14.7 ± 1.6% SL).

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, and can be distinguished from P. sakaramyi and P. sparksorum  in that the pectoral (chest) scales are not smaller than those on the flanks.

It also has the longest head (30.2 ± 1.8% SL) among all congeners occurring on Madagascar and lacks red pigmentation on the body and fins, this separating it from P. patriciae and P. varatraza.

The genus Pachypanchax 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.


  1. Hertwig, S. T., 2008 - Zoologica Scripta 37(2): 141-174
    Phylogeny of the Cyprinodontiformes (Teleostei, Atherinomorpha): the contribution of cranial soft tissue characters.
  2. Loiselle, P. V., 2006 - Zootaxa 1366: 1-44
    A review of the Malagasy Pachypanchax (Teleostei: Cyprinodontiformes, Aplocheilidae), with descriptions of four new species.
  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.
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