Taxonomy & naming
Haplochromis plagiodon was described by Charles Tate Regan and Ethelwynn Trewavas in 1928, in their paper 'Four new cichlid fishes from Lake Victoria' (Annals and Magazine of Natural History, series 10, vol. 2, pp. 224–226). The type locality is the Sesse Islands (Ssese Islands) in the Ugandan part of Lake Victoria, and the species rests on a single specimen: the unique holotype, BMNH 1909.5.4.29, in the Natural History Museum, London. The name has stood unchanged since, validated by P. H. Greenwood's foundational 1959 revision of the Lake Victoria Haplochromis and by the CLOFFA checklist treatment of van Oijen and colleagues (1991); the Catalog of Fishes lists no junior synonyms. Regan and Trewavas placed the fish in Hilgendorf's genus Haplochromis (1888), the catch-all that for most of the twentieth century held the entire Lake Victoria flock. Modern workers have since split that flock across many smaller genera (Astatotilapia, Pundamilia, Mbipia, Ptyochromis, Yssichromis and others), and several authors would treat plagiodon under a restricted Haplochromis or in an allied genus; because no formal recombination is in universal use and FishBase, GBIF and the Catalog of Fishes all retain the original binomial, we keep the species under the Haplochromis heading here. Greenwood regarded plagiodon as part of a small dental lineage alongside the algae-rasping Haplochromis obliquidens: in describing Haplochromis annectidens (Trewavas, 1933), the trio was framed as having diverged in separate directions from a common toothed stock — plagiodon and obliquidens being the two Lake Victoria endpoints.
Morphology
This is a small cichlid: the maximum recorded length is 3.5 in standard length, the figure carried by FishBase from the CLOFFA checklist, and the holotype and the handful of examined specimens are all in that modest range. In overall build it is a typical generalized haplochromine — a moderately deep, laterally compressed body with the single continuous dorsal fin and the rounded-to-slightly-pointed caudal of the group. The diagnostic feature is in the mouth, and it gave the fish its name: the teeth of the outer series carry a long, oblique anterior cusp, an obliquely slanting crown form (plagio-odon, 'slanting tooth') that Regan and Trewavas singled out in the description. This dentition sits the species near the obliquidens dental type, the cropping/scraping tooth shape of the lake, rather than the stout crushing teeth of mollusc-eaters or the long caniniform fangs of the piscivores. Live coloration is poorly documented in the literature — the original description worked from preserved material — though aquarium photographs of Mwanza Gulf males show the blue-bodied, dark-barred, red-finned livery typical of breeding haplochromine males. As in essentially all the lake's haplochromines, the sexes differ: males develop the brighter nuptial colours and bear egg-spots (ocelli) on the anal fin, while females and non-breeding fish are a plainer silvery-brown; precise dimorphism data specific to plagiodon were not published with the type.
Habitat
Haplochromis plagiodon is endemic to Lake Victoria, the great shallow equatorial lake shared by Tanzania, Uganda and Kenya and the heart of the Lake Victoria Basin freshwater ecoregion. Within the lake the species is a fish of the shallows: FishBase records it as occurring in shallow water over an exposed sandy beach, and the type series came from the Sesse Islands. Lake Victoria is geologically young, broad and shallow (mean depth around 131 ft, maximum near 262 ft), and its inshore sand-beach habitat — where this species lives — is a sunlit, wave-washed zone only a few metres deep, so a working depth band of roughly 0–16 ft fits the documented biotope rather than the deep open water. The lake's water is warm and hard-alkaline by the standards of the African rifts: surface temperatures typically run about 73–81 °F, and pH in the open lake is mildly to moderately alkaline, on the order of 7.2 up toward 9.0, with moderate conductivity. No in-situ chemistry was measured specifically for the plagiodon collection sites, so those figures are the lake-wide envelope rather than a reading at the fish's beach. The species' precise modern range within the lake is not well mapped; the IUCN assessment records confirmed occurrence in the Tanzanian and Ugandan waters.
Feeding
Haplochromis plagiodon is an insectivore that forages over sandy bottoms. The most concrete dietary evidence comes from gut examination reported in the early literature: the intestines of the two specimens examined contained the remains of Diptera (true-fly, e.g. chironomid midge) and Ephemeroptera (mayfly) larvae, together with many fine sand grains — the signature of a fish that picks insect larvae from and just below the sand surface and ingests sediment in the process. FishBase places its trophic level at about 3.2, consistent with a small bottom-feeding invertivore rather than a plant-scraper or a fish-eater. The slanting outer teeth that name the species suit a cropping, picking bite. In the structure of the Lake Victoria flock — which Greenwood, Witte and others famously partitioned into trophic guilds (algae-scrapers, insectivores, zooplanktivores, mollusc-crushers, paedophages, piscivores and more, each guild containing many look-alike species) — plagiodon belongs to the insect-eating, sand-dwelling fraction of the radiation.
Mating
No courtship or social behaviour has been described specifically for Haplochromis plagiodon, and the species has been so little studied that any account must be drawn by analogy to the well-documented Lake Victoria haplochromines. In that flock, males are the showy, territorial sex: a ripe male takes up and defends a small patch of substrate or a sand bower, advertises in nuptial colour, and courts passing females with lateral displays and quivering, the anal-fin egg-spots serving as a lure during spawning. There is no lasting pair bond — mating is polygynous and promiscuous, the male's role ending at fertilization. Mate choice by females on the basis of male colour is, in several Victoria genera, a central driver of the explosive speciation for which the lake is famous. Whether plagiodon forms discrete spawning territories on its sand-beach habitat, or how its males display, has not been recorded.
Breeding
Haplochromis plagiodon is a maternal mouthbrooder, the reproductive mode shared by the entire Lake Victoria haplochromine flock; FishBase records 'mouthbrooding by females.' In the typical pattern for these fishes, the female lays a small clutch of large yolky eggs, takes them immediately into her mouth, and — in the classic egg-dummy sequence — the male's anal-fin spots draw her to mouth at his vent so the clutch is fertilized inside her buccal cavity. She then carries the developing eggs and larvae in her mouth for roughly two to three weeks, not feeding during the brood, and releases free-swimming fry that may continue to shelter in her mouth when threatened. Clutch sizes have not been published for plagiodon itself; congeneric Lake Victoria haplochromines of this small size typically produce broods on the order of a few dozen eggs (commonly ~20–60), which is the genus-typical figure to expect here. There is no male parental care. No spawning season, fecundity count or fry-development data specific to this species appear in the literature.
In the aquarium
Haplochromis plagiodon is essentially not an aquarium fish. It is a small, scientifically obscure Lake Victoria endemic with no established presence in the ornamental trade; unlike the brilliantly coloured 'Victoria haps' that hobbyists do keep (Pundamilia nyererei, the various Astatotilapia and so on), plagiodon is known to aquarists mainly from a handful of photographs of wild-caught Mwanza Gulf fish kept by specialists. There is no body of husbandry experience to report, and these notes are by analogy to its better-known relatives rather than a record of keeping this species.
Were it maintained, the requirements would be those of the Lake Victoria insectivorous haplochromines generally: hard, alkaline water (pH around 7.5–8.5, moderate hardness) held warm at roughly 75–81 °F, with a sand bottom that matches its natural beach habitat and lets it sift for food. Like its relatives it would be a maternal mouthbrooder best kept as one male with several females to spread his attention, in a tank long enough (on the order of a metre / 200+ litres) to let subordinate fish escape — Victoria haps are vigorous, and males harass females and rivals hard during breeding. Diet would be small invertebrate fare (frozen and live chironomid larvae, daphnia, brine shrimp) reflecting its wild insectivory. The realistic assessment, though, is that this is a fish of conservation and scientific interest, not a hobby subject, and it is best left to the field and the museum.
Conservation
The IUCN Red List assesses Haplochromis plagiodon as Vulnerable, under criterion D2, assessed on 31 March 2010 and published in 2010 by F. Witte, M. P. de Zeeuw and E. Brooks; the listing records occurrence in Tanzania and Uganda and, at assessment, an increasing population trend. The D2 criterion flags a species with a very restricted area of occupancy or very few locations, such that a plausible future threat could drive it to Critically Endangered or Extinct in a short time — an apt description for a narrow-range Lake Victoria endemic. The backdrop is the lake's twentieth-century ecological collapse: the deliberate introduction and 1980s population explosion of the predatory Nile perch (Lates niloticus), compounded by eutrophication, deoxygenation of deeper water, and the spread of water hyacinth, drove what is widely regarded as the largest vertebrate mass extinction of modern times, with an estimated 200 or more endemic haplochromine species lost or reduced to remnants. Haplochromis plagiodon is among the survivors; that it persists, and that its trend was judged to be increasing as the perch fishery declined and some haplochromines partially recovered, is the cautiously hopeful note in an otherwise grim chapter. It carries no CITES listing. Because the species is so poorly documented, its conservation status should be read with the caveat that basic population and range data are thin.