Taxonomy & naming
Nanochromis transvestitus was described in 1984 by Donald J. Stewart and Tyson R. Roberts in the journal Copeia, from material the two collected during a National Geographic-funded ichthyological survey of the Zaïre (Congo) basin in 1973. The holotype is a 1 in female (Field Museum of Natural History, FMNH 93584), taken near the village of Ipeke at the southern end of Lake Mai-Ndombe (formerly Lac Inongo or Lac Léopold II) on 14 September 1973; a large paratype series of 118 specimens was deposited across the Field Museum, the Smithsonian (USNM), the California Academy of Sciences (CAS), the Natural History Museum in London, and the Musée Royal de l'Afrique Centrale in Tervuren. The species has remained monotypic in synonymy — no junior names have been erected. The genus Nanochromis Pellegrin is a group of small dwarf cichlids endemic to the Congo River basin, distinguished from all other African cichlids by the upper lateral line running contiguous with the dorsal-fin base along more than half its length. Stewart and Roberts placed transvestitus within the 'nudiceps group' of Nanochromis sensu stricto — alongside Nanochromis nudiceps, Nanochromis parilus, Nanochromis splendens, Nanochromis consortus and Nanochromis minor — a clade sharing a distinctive infraorbital bone arrangement, a scaleless cheek and nape, and an absence of predorsal bones. Within that group, transvestitus is diagnosed by its striking female coloration, a subterminal mouth in which the upper jaw projects slightly beyond the lower, and reduced meristic counts. Lake Mai-Ndombe later proved to hold a second, larger congener: Nanochromis wickleri Schliewen & Stiassny, 2006, described from the northern end of the same lake.
Morphology
This is one of the smallest cichlids in the world. In the type series the largest specimen — a male — measured just 1.5 in in standard length, and sexually mature females ranged from 1–1.5 in; well-fed aquarium fish run a little larger, with females commonly exceeding 2 in total length and males reaching 2.5 in. The body is elongate for a dwarf cichlid, with a rounded caudal fin typical of the genus. Both sexes share an olive-brown head, nape and caudal peduncle, a network of light and dark scale margins producing a faint reticulation, and a well-defined dark mark on the upper rear corner of the gill cover set off by a pale ring. The famous difference appears in mature fish. The male is comparatively plain — drab olive brown overall, his roughly seven broken dark vertical bars barely visible, his median fins largely unmarked except for dark edging on the dorsal and upper caudal, and his dorsal and anal fins drawn out into slight filamentous tips, the caudal ending in a point. The female is the spectacle: her abdomen and lower flanks flush a deep reddish-maroon to violet, and her caudal, anal and soft dorsal fins turn dark gray to nearly black, crossed by three or four bold, irregular white bands. Remarkably, this reversed pattern is already developed in the smallest specimens examined, fish of only about 0.5 in. The white tail bars begin to show on aquarium juveniles as young as four months, making the sexes easy to tell apart well before maturity.
Habitat
Nanochromis transvestitus is endemic to Lake Mai-Ndombe and its satellite waters in the Cuvette Centrale, the saucer-shaped lowland depression at the interior heart of the Congo basin, entirely within the Democratic Republic of the Congo. Mai-Ndombe is a large but shallow, dendritic blackwater lake — roughly 2,300 km2, about 84 mi long, with an average depth of only about 10 ft — that drains from its southern end into the Lukenie-Fimi River and onward to the Kasai and the Congo. Its water is the colour of dark tea: highly acidic and rich in dissolved humic compounds, so deeply stained that a hand thrust 8–12 in below the surface vanishes from sight, leaving the bottom in near-total darkness even under midday sun. Where Stewart and Roberts collected the type series, the water measured pH 4.0, dissolved oxygen 6 mg/l and 30.5 degrees C, with hobbyist measurements at the lakeshore of pH 4.0 to 4.5 and a hardness near 1 to 2 dGH. The fish lives along the shoreline in shallow water, from the edge down to about 3 ft, among large porous rocks and waterlogged timber over a hard-packed fine sediment littered with decaying plant debris; nearby rocks and submerged trees are often crusted with a hard, sharp freshwater sponge (Spongilla). The dark, stained water itself is the species' chief shelter from predators overhead. FishBase records the species at pH 5.0 to 6.0 and 24 to 26 degrees C, but those figures sit well above the extreme acidity and warmth of the type locality.
Feeding
Nanochromis transvestitus is a small benthic invertivore that works the leaf litter and soft sediment of the lake margin. Examination of stomach contents from wild specimens in the original description found benthic Cladocera (water fleas), midge (chironomid) larvae and mites, taken among much larger quantities of amorphous brown detritus — possibly derived from the abundant Spongilla sponge of its habitat. The short gut (slightly less than the standard length), the conical jaw teeth set in two or three rows, and the subterminal mouth are all consistent with a diet of small bottom invertebrates picked and winnowed from the substrate rather than scraped or sifted in bulk. FishBase places the species at a trophic level near 3.1. In its blackwater community much of the food base is allochthonous — terrestrial material and floating vegetation washed in during the flood season — so a litter-picking invertivore of this kind is well matched to the lake's seasonally pulsed productivity.
Mating
Like all Nanochromis, this is a territorial, monogamous cave spawner rather than a mouthbrooder, and pairing is the central drama of keeping it. The species is intensely aggressive toward its own kind. Dominant individuals — usually but not always males — will viciously attack rivals and unreceptive partners, and if the largest fish in a group is a female she too will drive off others, males included; removing a dominant fish simply promotes the next in line, so the aggression cannot be eliminated, only managed. In spacious aquaria a single male will hold a zone within which two or three females maintain smaller, hierarchically arranged sub-territories on the periphery, without forming stable bonds outside the breeding cycle. When a female ripens she takes the initiative in courtship: she darkens, intensifies the red-violet of her belly and the white-on-black of her fins, bends her body into an S or U shape and shimmies before the male to flaunt her colours, then leads him toward a chosen shelter for bouts of joint digging that can run two to five days. The male, if interested, darkens in turn and makes his own vertical bars more obvious. The reversed dichromatism almost certainly reflects this female-forward courtship and the female's leading role in defending the nest; Stewart and Roberts noted the genus would make an interesting subject for a comparative study of how degrees of sexual colour difference map onto each sex's share of territorial and parental duty.
Breeding
Spawning takes place inside a cave — a rock hollow, a hidden recess, or a ceramic shelter — with the female preferring a small, secretive opening she can clean obsessively for days beforehand. As spawning nears she develops a short ovipositor; she then turns upside down and glides across the cave ceiling, laying row after row of elliptical, yellow-greenish eggs roughly 0.5 in across, while the male enters periodically (or sometimes fertilizes from just outside) to release milt. Clutches are small, matching the fish's size: two ripe wild females in the type series each carried about 70 eggs, and aquarium spawns typically run 30 to 70 depending on the female's size and age. Care is biparental but divided: the female tends the eggs and larvae inside the cave, displaying her full breeding colours, while the male patrols and defends the wider territory, the two behaving toward each other much as they did during courtship. At 27 to 28 degrees C the eggs hatch in about five days, and the wrigglers — moved between shelters by the parents, especially the female — go free-swimming around day nine, by then about 0.5 in long and patterned like leopard skin. Both parents then lead the fry shoal, the female signalling danger with a body shudder that freezes the young in place. Successful reproduction is genuinely conditional on water chemistry: keepers report that eggs simply fail to develop above about pH 5.7 or in hard water, the optimum being roughly pH 4 to 5, very soft (GH about 4 to 9), and 27 to 28 degrees C — a reflection of how finely the egg is tuned to the extreme blackwater of Mai-Ndombe.
In the aquarium
Nanochromis transvestitus has been a quietly prized fish among West African dwarf-cichlid specialists since the mid-1980s, but it is firmly an advanced keeper's species, demanding on two fronts: water chemistry and temperament. It is rarely seen in shops and is still largely wild-caught or the work of dedicated breeders.
The water is the first hurdle. These fish come from water with almost no hardness and a pH around 4.0 to 4.5, and while they may survive and even look healthy in harder, less acidic conditions, soft and acidic water is required for long-term health and for any realistic chance of raising fry. The practical recipe is reverse-osmosis or otherwise very soft water, acidified and darkened with peat filtration and tannin- and humic-rich additives (Indian almond leaves, alder cones, blackwater extract) to mimic the stained lake. A pH of 5.0 or lower and near-zero hardness is the target for breeding; maintenance can sit a little higher but not by much. Temperature is kept warm, around 80 to 82 degrees F (27 to 28 degrees C). Mature, stable filtration is important, as is gentle handling of that low-buffered water — heavily acidic, soft water swings quickly and must be watched.
Temperament is the second hurdle and the more common cause of failure. Intraspecific aggression is severe; a settled pair can turn on each other without warning. A pair is best kept in a well-structured tank with multiple dense, sight-breaking hiding places — caves, wood, leaf litter, thick planting or even spawning mops — so the harassed fish can vanish completely from the other's view. Some keepers run large groups of 20-plus in barer tanks, where aggression is diluted across the crowd, but that is a high-stress, short-term arrangement that rarely yields pairs. Either way the substrate should be sand or fine gravel, since the fish dig, and several small caves with narrow entrances should be provided for the female to choose among.
Feeding is straightforward: they take most prepared and frozen foods readily and relish live foods such as mysis, Artemia, Daphnia and bloodworm, which also condition them for spawning. As bottom-oriented fish they leave the upper water column to others. Tankmates should be chosen for tolerance of soft, acidic, warm water and a preference for the mid-to-upper levels — small soft-water tetras or barbs are the usual choice; other substrate-bound fish, most catfish, and Corydoras are best avoided as they compete for the floor. Other cichlids are generally a poor idea, though the species' water requirements are close enough to those of discus that the two have at least been speculatively paired. Wild-type colour and breeding readiness both depend heavily on getting the blackwater chemistry right; keepers who skimp on it tend to keep the fish alive but never see them spawn.
Conservation
The IUCN Red List assesses Nanochromis transvestitus as Endangered (EN B1ab(v)), assessed on 16 February 2009 by T. Moelants and published in 2010. The listing rests on the species' very restricted range — it is endemic to Lake Mai-Ndombe near Ipeke and a few satellite lakes in the central Congo system, one of three fishes considered endemic to the Mai Ndombe freshwater ecoregion (with Amphilius opisthophthalmus and Hemichromis cerasogaster). The principal threat identified is fishing practice: on Mai-Ndombe, fine-mesh mosquito netting is used as so-called 'caterpillar nets' that indiscriminately remove every fish in the water, small species included. A second, looming pressure is the prospect of exploiting the methane reservoirs discovered beneath the lake. Population trend is recorded as unknown, and the lake's ichthyofauna remains poorly surveyed, so endemism and richness are likely underestimated. The species carries no CITES listing. As a narrow-range blackwater endemic with exacting water requirements, it is far more exposed to local habitat change than the wide-ranging cichlids of the Congo mainstem, and its long-term outlook is tied to the fate of this single shallow lake.