Taxonomy & naming
Haplochromis tridens was described by Charles Tate Regan and Ethelwynn Trewavas in 1928, in the Annals and Magazine of Natural History, on material trawled from Lake Victoria off the Tanzanian shore (Victoria Nyanza, roughly 1°04'S, 32°13'E) at a depth of over 100 feet (about 98 ft). The name was published in the original combination Haplochromis tridens; Greenwood (1967) later designated a lectotype (BMNH 1928.6.2.41, with one paralectotype) held in the Natural History Museum, London. The species has been treated as valid ever since, and the Catalog of Fishes retains it as Haplochromis tridens Regan & Trewavas, 1928.
Lake Victoria's haplochromines are a textbook adaptive radiation — several hundred closely related, recently evolved species packed into a single young lake — and their generic classification has long been unstable. Many were at various times split into a constellation of small genera erected on trophic or dental specializations; Haplochromis tridens was assigned to the genus Tridontochromis, a name built on the same three-toothed feature as its species epithet. Most modern workers, however, treat these segregate genera as synonyms and retain the broad, inclusive Haplochromis for the Victoria flock, which is the usage followed here. In the ecological revision of the flock by van Oijen, Witte and colleagues, Haplochromis tridens is placed among the 'prawn-eaters,' a group united by tricuspid outer teeth, large eyes and a narrow interorbital — features the original describers had already singled out.
Morphology
Haplochromis tridens is a small cichlid: FishBase gives a maximum of 4.5 in standard length, and the prawn-eater group as a whole runs to adults of about 3.5–4.5 in SL. The body is moderately slender (body depth roughly 29–35% of standard length). The most consistent features of the group, and the ones the species was named for, are its dentition and its eye: the outer rows of the oral jaws carry mostly tricuspid (three-cusped) teeth — the 'three-toothed' character behind both tridens and the segregate-genus name Tridontochromis — and the eye is conspicuously large, on the order of a quarter to a third of head length, with the interorbital region correspondingly narrow (in tridens specifically, narrower than the eye is long). Large eyes are typical of fishes that forage in the dim light of deeper water. Live coloration of this particular species is poorly documented; like most Victoria haplochromines, breeding males would have been the more colourful sex, and females and non-breeding fish drabber, but no detailed colour description is reliably available, and sexual dimorphism beyond the usual mouthbrooder pattern is not well characterized.
Habitat
Haplochromis tridens is endemic to Lake Victoria, the great equatorial lake shared by Tanzania, Uganda and Kenya, and is known only from that basin. It is a fish of the open, offshore lake rather than the shoreline: the type series came from water more than 100 feet deep, and FishBase records the species occurring some distance offshore over a soft (mud or sand) bottom in deep water, citing a depth around 98 ft. Within the prawn-eater group the species are described as mostly benthic, living on or near the bottom in deeper water, with some possibly entering mid-water at times. Lake Victoria is a warm, relatively shallow, alkaline lake; its surface waters typically sit around 73–81 °F with a pH from roughly neutral to strongly alkaline (about 7.2–9.0), and these are the broad conditions across this species' offshore range. It does not occur in the rocky shoreline habitats favoured by many of the more familiar 'Victoria cichlids' of the aquarium hobby.
Feeding
Haplochromis tridens belongs to the prawn-eaters, a trophic guild whose principal food is Caridina nilotica, the small caridean shrimp that is abundant over Lake Victoria's offshore bottoms; zooplankton can also form part of the diet, and stomach examinations of this species have additionally turned up blue-green algae, empty diatom frustules, crustacean fragments and unidentifiable insect remains, consistent with a generalist invertebrate-and-plankton feeder rather than a strict specialist. Its estimated trophic level is around 2.8 — a low-level predator on small invertebrates. The tricuspid teeth and large eyes fit this role: prey is taken in the dim, deeper water, and the modestly cusped dentition handles soft-bodied shrimp and plankton rather than hard shells or fish. Ecologically the prawn-eating haplochromines were important: before the Nile perch upsurge they were, in the words of the van Oijen/Witte revision, 'the only fishes in the lake which consumed prawns in large quantities,' making them a significant link between the invertebrate base and larger predators of the deep lake.
Mating
Detailed courtship and social behaviour have not been documented for Haplochromis tridens specifically. It is a haplochromine, and the Lake Victoria flock follows a well-known reproductive template: females are maternal mouthbrooders and males court and spawn without forming pair bonds. In the lake's better-studied haplochromines, males develop bright nuptial colours and display to females over a small spawning site, the female takes the eggs into her mouth almost immediately, and fertilization is effected as she mouths the male's anal-fin egg-spots (or vent) — but how much of this elaborate display applied to a drab deepwater prawn-eater living offshore is unknown. What is reasonably inferred is that, as in its relatives, the male's role ended at spawning and all subsequent care fell to the female.
Breeding
Like all Lake Victoria haplochromines, Haplochromis tridens is a maternal (female) mouthbrooder: the female incubates the fertilized eggs and the resulting larvae inside her buccal cavity, releasing free-swimming fry only after they have largely absorbed the yolk, and continues to shelter them in her mouth when threatened. Specific reproductive data for this species are scarce — the spawning season of the prawn-eaters is described in the source literature only as 'probably continuous,' and fecundity, egg size and growth were recorded as unknown. As a guide to scale, Victoria haplochromines are small-clutch brooders: congeners typically carry only a few dozen to perhaps a couple of hundred relatively large eggs, the trade-off of mouthbrooding being few, well-provisioned young rather than many small ones. No figure has been published for tridens itself, so any clutch estimate is genus-typical rather than measured.
In the aquarium
Haplochromis tridens is essentially never kept in the aquarium and has no place in the trade. It is a small, drab, deepwater offshore fish with no documented colour appeal, it was never a fishery target, and it is not bred for the hobby; the colourful 'Lake Victoria cichlids' that hobbyists actually keep are rock-dwelling and shoreline species, not offshore prawn-eaters like this one. There is no body of captive husbandry experience to draw on. Were it ever maintained, the general requirements of Victoria haplochromines would apply — warm water (roughly 75–81 °F), hard, alkaline conditions (pH well above 7), pristine water quality, and a maternal-mouthbrooder breeding setup with enough space and cover to let a brooding female retreat — but with the important caveat that this is a deep-living species adapted to dim, cool-for-the-tropics offshore conditions rather than a sunlit reef. In honest terms, this is a fish of scientific and conservation interest, not an aquarium subject.
Conservation
The IUCN Red List assesses Haplochromis tridens as Data Deficient (assessed 31 March 2010 by Witte, F. & de Zeeuw, M.P.), reflecting how little is known of its current population and distribution. The single threat the assessment names is predation by introduced Nile perch (Lates niloticus) — and it notes this as a potentially reversible pressure. That threat is the central event in the recent history of the entire Lake Victoria flock: the Nile perch, established in the lake by the 1950s and exploding in the 1980s, is implicated in the disappearance of an estimated 200 or more endemic haplochromine species, one of the most severe vertebrate extinction episodes of modern times. Deepwater bottom-dwellers such as the prawn-eaters were among the hardest hit — the literature explicitly contrasts them with shallow rocky-habitat cichlids that fared comparatively better — and the Catalog of Fishes flags this species' Lake Victoria distribution as 'extinct.' Whether Haplochromis tridens survives in any refuge is genuinely uncertain, which is precisely why it sits in the Data Deficient category rather than a definite threat or extinction listing. The partial rebound of some haplochromine populations as Nile perch was fished down in the 1990s offers a thread of hope, but there is no confirmation that it extends to this species.