Taxonomy & naming
Paretroplus tsimoly was described in 2001 by Melanie L. J. Stiassny, Prosanta Chakrabarty and Paul V. Loiselle, in a paper in Ichthyological Explorations of Freshwaters (12(1): 29–40) that reviewed the relationships of the Malagasy genus and erected this new species from the Betsiboka River drainage of northwestern Madagascar. It has never been reclassified to another genus and carries no junior synonyms; the original combination stands. The genus Paretroplus itself was established by Pieter Bleeker in 1868, its name signalling kinship to the Asian genus Etroplus — and that kinship is the heart of the group's interest. Etroplinae is a Gondwanan relict lineage: molecular work, including John Sparks' 2008 monograph in the Bulletin of the American Museum of Natural History, recovered the subfamily (the Malagasy Paretroplus plus the Indo–Sri Lankan Etroplus and Pseudetroplus) as the sister group to all remaining cichlids, a split rooted in the breakup of the southern supercontinent rather than in any recent dispersal. Within Paretroplus, Sparks placed tsimoly among the elongate, river-adapted species, recovering it and the blood-finned Paretroplus nourissati as a sister pair, with the big red P. lamenabe nearby in the same 'lamena' clade. The species epithet has a tangled history. FishBase repeats the original authors' statement that 'tsimoly' is simply the fish's name in the Sakalava dialect of the Akalimilotrabe region, to be treated as a noun in apposition. The ETYFish Project, however, reads the description more closely and reports that the name in fact honours the limnologist Karen J. Riseng, who collected the holotype with the senior author on their first Madagascar expedition in 1994; by that account 'tsimoly' is a Latinization that would more properly have been spelled risengae had the Code permitted the correction. Either way the name is to be carried as written.
Morphology
This is a moderately deep-bodied, laterally compressed cichlid — fusiform and normal in profile rather than the disc-like silhouette of the lake dambas — that reaches about 10 in standard length in the largest (male or unsexed) specimens, with females recorded to roughly 5.5 in SL; aquarium accounts often cite a more typical adult length around 6.5 in. The fin counts run to fifteen to seventeen dorsal spines and thirteen to fourteen soft rays, seven to eight anal spines and eleven to twelve soft rays, over thirty-one or thirty-two vertebrae. The single most distinctive feature, and the source of the English name 'blue-lipped Madagascar cichlid,' is the lips: in adults they become markedly enlarged and lobed, slate- to blackish-blue in life and dark bluish-grey in preserved fish, a trait that separates tsimoly from every congener. The flank pattern is the second diagnostic. Paretroplus tsimoly carries two wide, dark brown-to-black midlateral bands that converge below the lateral midline — these represent the second and third (or third and fourth) bars in the body's barring series. It shares this convergent-band motif with only P. lamenabe and Paretroplus nourissati, but is set apart from both by the rest of the pattern: in tsimoly the posterior four or five bars on the flank are all about equally prominent (the bar on the caudal peduncle is sometimes pale), whereas in its two relatives only the central two bars stand out strongly. Sexual dimorphism is modest and size-related — males grow distinctly larger than females — with no dramatic difference in colour reported between the sexes.
Habitat
Paretroplus tsimoly is endemic to Madagascar and, within it, to the Betsiboka River system of the northwest — a large basin of roughly 55,0 mi² that drains westward to the sea near Mahajanga, placing the species squarely in the Northwestern Madagascar freshwater ecoregion. It is known from the type locality in the Betsiboka and from the neighbouring Boinakely River. Unlike the lake-bound members of the genus, tsimoly is a riverine, rheophilic fish: it is found in flowing water broken by isolated pools, over a rocky bottom interspersed with patches of cobble and coarse gravel, in shallow water down to about two metres. These are warm tropical rivers; published in-situ chemistry for the species is scant, but the genus as a whole occupies the warm, hard-water, near-neutral-to-alkaline rivers and floodplain lakes of northwestern Madagascar, broadly in the range of 72–82 °F and pH around 7.0–8.5. The defining feature of the habitat in conservation terms is its instability — the Betsiboka is one of the most heavily eroded river basins on Earth, its waters stained brick-red by soil washing off a deforested catchment, a backdrop that frames everything about the fish's status.
Feeding
Like all Paretroplus, Paretroplus tsimoly is an omnivore rather than a specialist, and its gut contents bear this out. Examined stomachs held mainly crushed insects — most of them the nymphs of odonates (dragonflies and damselflies) and hemipterans (true bugs) — together with significant amounts of fibrous plant material. FishBase places the species at a trophic level of about 2.8, the level of a low-order omnivore that takes both small invertebrate prey and vegetable matter. The robust, lobed lips and the bicuspid, hooked dentition that Sparks documented across the river-dwelling dambas suit a fish that works hard surfaces — picking and scraping aquatic insects and plant material from the rock, cobble and gravel of its flowing-water habitat. In this it differs sharply from the Asian Etroplus suratensis, a dedicated herbivore; the whole Malagasy radiation, by contrast, is omnivorous.
Mating
Detailed field observations of courtship in Paretroplus tsimoly are lacking, but captive accounts and the reproductive biology of the genus are consistent and well enough understood to describe with confidence. As in most Madagascan cichlids, it is the male that initiates spawning: he selects a cave-like spawning site within the pair's territory and "invites" a receptive female into it. Spawning in Madagascar is seasonal, reported by local fishermen and aquarists to fall in the latter part of the dry season, roughly August through October, as conditions concentrate fish and clarify water. Both partners defend the chosen cave through the pre-spawning period, and breeding coloration in this species is described as spectacular. As is usual in substrate-spawning cichlids, territoriality intensifies sharply once a pair commits to a site, the partners turning aggressive toward conspecifics and other intruders that approach it. Compared with its close relative Paretroplus nourissati, however, Paretroplus tsimoly is notably laid-back — even while breeding — and tolerates conspecifics well enough to be raised in a species group or a mixed Madagascan community aquarium.
Breeding
Paretroplus tsimoly is a biparental cave spawner that practises prolonged care of its young; breeding pairs have been observed in the wild guarding free-swimming fry. Eggs are laid on the roof of a cave — a rock crevice or hollow in nature, with flowerpots, slate caves, ceramic tubes and artificial caves pressed into service in captivity — and care of them follows an unusual division of labour: the male tends the eggs directly while the female guards the surrounding territory, a complete reversal of the more familiar cichlid arrangement in which the female does the close brood care. Once the fry are free-swimming, both parents extend guard duty over the wriggler and juvenile stages. No precise wild clutch count has been published for tsimoly specifically; congeners in the genus produce moderate clutches on the order of a few hundred eggs, and the long-term, two-parent brood defence is the genus norm rather than the exception. Konings rates tsimoly as usually an excellent aquarium parent — a marked contrast with the more egg-eating-prone Paretroplus nourissati — but breeders still often hatch the eggs artificially as a precaution against losses among the vulnerable fry, the same safeguard used across the genus's rarer species. Paretroplus tsimoly itself remains very little kept and has not figured prominently in the captive-breeding programmes that have propagated several of its lake-dwelling relatives.
In the aquarium
Paretroplus tsimoly is essentially an advanced specialist's fish rather than a commercial aquarium subject. It is rarely imported, and what is known about keeping it comes largely from the broader experience of the Madagascar-cichlid community, which has worked hardest with the lake species. The husbandry implications of its biology are nonetheless clear. This is a sizeable, river-adapted cichlid — adults can approach 10 in — that needs a large tank with a long, open footprint, brisk water movement and high oxygenation to echo its flowing, rocky home; a substrate of cobble, coarse gravel and water-worn rock with a few robust hiding places suits it far better than a planted aquascape. Water should be warm (roughly 75–82 °F) and moderately hard and near-neutral to slightly alkaline, in keeping with northwestern Madagascar's mineral-rich rivers rather than the soft, acidic water of South American or West African biotopes. Diet is straightforward for an omnivore: a varied menu of meaty foods (insect larvae, small crustaceans) alongside vegetable matter and quality prepared foods keeps it in condition. Temperament is where tsimoly stands out from its genus-mates: it is described as notably laid-back, even during breeding, and can be kept successfully in either a species tank or a mixed Madagascan community aquarium — a real contrast with its more combative relative Paretroplus nourissati, which it does not tolerate well as a tankmate. Some genus-typical caution still applies as fish mature and pair off, so tankmates should still be chosen for size and robustness and the tank scaled to give subordinate fish room to retreat. Anyone fortunate enough to obtain the species should treat it as a conservation responsibility: this is a threatened, narrow-range endemic, and the goal with such fish is a maintained, breeding line, not a curiosity.
Conservation
The IUCN Red List assesses Paretroplus tsimoly as Endangered (EN) under criterion B1ab(iii,v) — a small, declining range with continuing decline in habitat quality and in the number of mature individuals. The assessment was carried out on 2 July 2016 by J. S. Sparks and published in the 2020 update, with the population trend recorded as decreasing. The threats are the familiar ones bearing down on Madagascar's freshwater fish: catastrophic deforestation and the erosion and siltation that follow, which have turned the Betsiboka into one of the most sediment-choked rivers on the planet, together with the spread of introduced species — tilapia and snakeheads brought in as food fish — through the lower drainage. Against that backdrop, tsimoly is in a relatively fortunate position among its kin. The describers noted that it remains reasonably abundant in both its type locality and the neighbouring Boinakely River, and attributed this in large part to a quirk of its ecology: unlike the deep-bodied lake dambas, whose flesh is prized and which are intensively gill-netted, this river species has never been a target of local fishermen and so escapes the direct fishing pressure that has driven congeners such as Paretroplus menarambo to the brink of extinction in the wild. It carries no CITES or CMS listing. Its long-term security depends less on harvest control than on whether the forests of its watershed survive.