Taxonomy & naming
Haplochromis chromogynos was described by the British ichthyologist Peter Humphry Greenwood in 1959, in the third part of his monumental multi-decade revision of the Lake Victoria Haplochromis (Bulletin of the British Museum (Natural History), Zoology 5(7): 179–218). Greenwood erected it for piebald-female fish from the lake and singled the species out, even in the original description, as 'a rare example of polychromatism' in which all females belong to the mottled morph while all males show the normal plain coloration. The species sits in the Haplochromini, the tribe that contains the vast endemic radiations of Lakes Victoria, Malawi and the surrounding rivers. Greenwood later placed it in his subgenus Paralabidochromis, and much of the modern hobby and several authors (e.g. Seehausen 1996) write it as Haplochromis (Paralabidochromis) chromogynos or simply Paralabidochromis chromogynos; FishBase and the Catalog of Fishes retain the conservative, broad genus Haplochromis, which is the heading used here. The tangled nomenclature reflects a real problem: the Lake Victoria flock diversified so recently and so explosively that genus limits within it remain unsettled, and chromogynos has at times been confused under the names of the lake's snail-eating cichlids (it appears in synonymy lists alongside misapplied uses of bicolor and sauvagei). Within Paralabidochromis, Seehausen grouped it as the nucleus of a 'chromogynos complex' of long-headed, chequer-patterned rock cichlids — the so-called 'Chessboard Mbipi' — closely allied to the lobe-lipped chilotes complex.
Morphology
This is a small cichlid: FishBase lists a maximum of 4.5 in standard length, and most individuals are smaller. The body is the typical moderately deep, laterally compressed haplochromine shape, with a relatively long head (head length about a third or more of the standard length) that places it among the long-headed members of its complex. The dentition is that of an invertebrate picker rather than a crusher — Greenwood and later Seehausen describe slender, fine, regularly spaced, acutely pointed and slightly recurved unicuspid teeth in the outer rows, with unicuspid teeth in the inner rows as well. The species' defining feature is colour, and specifically the dramatic difference between the sexes. Males are a plain, fairly unobtrusive blue-grey to greenish-grey. Females — and this is the trait that gives the fish its name — are piebald: bold irregular black blotches on a clear white to silvery ground. In aquarium strains the mottling is even more varied, ranging from clean black-and-white to busy mixtures of red, green, black and orange, set off by fins flecked with iridescent blue, red and yellow. This is a true sex-limited polychromatism: the mottled pattern is normally restricted to females, although Greenwood noted black-male variants from unknown localities, and the trait is part of a wider phenomenon of orange-blotched (OB) and piebald female morphs that recurs across many Lake Victoria rock cichlids.
Habitat
Haplochromis chromogynos is endemic to Lake Victoria, the vast, shallow East African Rift lake shared by Tanzania, Uganda and Kenya, and is part of the Lake Victoria Basin freshwater ecoregion. It is a rock-dweller — one of the lake's 'Mbipi', the community of haplochromines tied to rocky islands and shores. Unlike many of its rock-bound relatives, which are essentially never found off rock, chromogynos and its close relative chilotes are caught somewhat more often over sand, but according to Seehausen hardly ever more than about 50 metres from rocks; the species' typical habitat is the small-rock-and-pebble interface where boulders meet sand. It is a shallow-water, littoral fish: FishBase gives a depth range to about 23 ft, and Seehausen recorded chromogynos-complex fish at roughly 13 ft over gently sloping rock-and-pebble substratum. Lake Victoria's surface waters are warm and alkaline, broadly pH 7.2–9.0 (commonly around 8) and roughly 73–81 °F in the inshore zone the species occupies. The lake's defining recent change is its loss of water clarity: eutrophication and turbidity since the mid-twentieth century have collapsed the clear-water light environment under which the flock's colour-based mate choice evolved.
Feeding
Despite the prominence of snail-eating specialists in the Lake Victoria flock, Haplochromis chromogynos is not one of them — its fine, pointed, recurved unicuspid teeth are those of an insectivore and invertebrate picker, not the molarised crushers or the leverage-jaws of the lake's oral and pharyngeal molluscivores (the separate Macropleurodus, Platytaeniodus, Hoplotilapia and Labrochromis lineages). The gut contents of examined specimens consisted mainly of caddisfly (Trichoptera) larvae together with the sand-grain cases they build, plus a few midge (chironomid) larvae and mayfly (baetid) nymphs. FishBase places the species at a trophic level of about 3.2. In life it works the rock surfaces and the adjacent sand interface, picking small bottom-living insect larvae and other invertebrates from among the boulders — an epilithic and benthic insectivore of the rocky littoral, foraging in the same chequerboard-rock zone it shares with algae scrapers, pickers and small piscivores of the Mbipi community.
Mating
Like the rest of the Lake Victoria haplochromine flock, Haplochromis chromogynos is a polygynous maternal mouthbrooder in which the sexes lead separate lives outside of spawning and males do not help raise young. Males hold and defend small territories among the rocks and display to attract females; in the wild, mate choice in these cichlids is strongly visual, driven by male nuptial coloration and the female's assessment of it under the lake's light. The species' striking sexual dichromatism — drab males, mottled females — is itself bound up with this system: female-limited polychromatism of this kind is thought to be maintained partly because the conspicuous mottled pattern carries different costs and benefits for the two sexes, and such colour polymorphisms are implicated in the very speciation processes that built the flock. Aquarium observation matches the wild picture: males can be rough with one another over territory and status, so a single male with several females is the stable social arrangement, whereas multiple males in a small space leads to bullying.
Breeding
Reproduction follows the standard haplochromine pattern: Haplochromis chromogynos is a maternal mouthbrooder. The female spawns a relatively small clutch, takes the eggs into her mouth, and broods the eggs and then the larvae in her buccal cavity, releasing free-swimming fry only after they have absorbed the yolk; the male contributes nothing beyond fertilisation. Lake Victoria haplochromines of this size typically lay only a few dozen eggs per spawning (commonly on the order of 20–60), trading fecundity for the safety of oral incubation — exact clutch counts for chromogynos itself are not well documented, so this is the genus-typical figure. Many species use 'egg-spot' markings on the male's anal fin during spawning to coordinate fertilisation, a hallmark of the haplochromine breeding repertoire. In aquaria the species is reported to be hardy and to breed readily: keepers find that a harem of one male and several females in a rock-decorated tank will spawn without special inducement, with brooding females best left undisturbed or moved to a separate tank to carry their fry to release. The combination of a small clutch and full maternal care gives modest but reliable broods.
In the aquarium
Haplochromis chromogynos is genuinely rare in the hobby — it is kept by a small circle of Lake Victoria specialists and conservation-minded keepers rather than being a trade staple, and most aquarium stock traces to a few collections (the well-known 'Zue Island' line among them). Where it is kept, it has a reputation as a hardy, undemanding and easily bred Victorian, which is part of why it matters as an ark species for a fish that is imperilled in the wild.
A tank in the region of 47 in / four feet (roughly 55 US gal or more) suits a harem, decorated with piles of rock to create territories and broken sightlines over a sand or fine-pebble substrate that recalls the rock-and-sand interface the species favours in the lake. Water should be hard and alkaline to mirror Lake Victoria: pH around 7.5–8.5 and a temperature of roughly 75–81 °F. As with most haplochromines, clean, well-filtered water and regular water changes keep them in condition; they are not fussy about chemistry within the alkaline range.
Stock them as one male with several females — the single most important rule, since males are rough with rival males but are not especially aggressive toward other species, so two or more males crowded together is the usual cause of trouble. They mix well with other robust, similarly sized Lake Victoria haplochromines and make a colourful community of Victorians; the one serious caution is hybridisation. Because the flock's species are so closely related and interfertile, different Lake Victoria (and Lake Malawi) haplochromines should not be mixed if any breeding is intended, and chromogynos in particular should be kept as a clean, single-species or carefully curated group to preserve the line. Feeding is straightforward: a quality cichlid flake or small pellet, supplemented with frozen or live invertebrate foods (bloodworm, brine shrimp, daphnia) in keeping with its natural insect-picking diet.
Conservation
The IUCN Red List assesses Haplochromis chromogynos as Vulnerable under criterion D2 (assessed 31 March 2010 by Witte, de Zeeuw & Brooks; published 2010), with the range given across Kenya, Tanzania and Uganda. The listed threats are the ones that reshaped the entire Lake Victoria fish community: predation by the introduced Nile perch (Lates niloticus), whose population exploded in the 1980s and drove a mass decline of the haplochromine flock; fishing pressure; and — uniquely damaging for a fish whose biology turns on female choice of male colour — hybridisation driven by the loss of water transparency, as eutrophication-clouded water blurs the visual signals that keep closely related species reproductively isolated. Greenwood- and Witte-era surveys document the species disappearing from most known northern localities and not being seen in the south after about 1982, and the population that was later found at Zue Island appears to be a remnant. Cichlid Room Companion's account argues the fish may in reality be closer to Critically Endangered than the 2010 listing suggests, noting that falling lake levels add further pressure on a species tied to specific near-shore rock micro-habitats. It carries no CITES listing. As with so many of Lake Victoria's endemic cichlids, the strongest insurance for the species now lies as much in captive 'ark' populations as in the lake itself.