Cichlid Atlas — the world's cichlid waters, in depth

Taxonomy & naming

Trematocara variabile was described by the Belgian ichthyologist Max Poll in 1952, in the fourth installment of his series on new cichlids collected by the Belgian hydrobiological mission to Lake Tanganyika in 1946–1947 (Bulletin de l'Institut Royal des Sciences Naturelles de Belgique 28(49):1–20). The type locality is Moba Bay, about a kilometer offshore south of M'toto on the Congolese coast. The genus name is a useful clue to the animal: from the Greek trematos, "hole," and kara, "head" or "face," a reference to the conspicuously enlarged sensory pores that perforate the skull of these fishes.

The species sits in the subfamily Pseudocrenilabrinae and the tribe Trematocarini, a small endemic radiation of roughly nine Trematocara species plus a couple of close allies. Within that group T. variabile is one of the harder fish to pin down by eye; live coloration is muted and variable, which is presumably what earned it the epithet variabile. Catalog of Fishes (Eschmeyer's catalog) and FishBase both treat the name as valid with Poll, 1952 as the authority, and the original description carries a registered ZooBank nomenclatural act. The fish has no established English common name; the IUCN assessment lists the Afrikaans name "Sakasaka," a label aquarists never use.

Appearance

This is a small, slim cichlid. The largest recorded specimen measured 8.7 cm (3.4 in) total length, and most are smaller; the body is elongate rather than the deep, slab-sided shape many lake cichlids show. Meristics are unremarkable for the family: roughly 9–11 dorsal spines and 10–13 soft rays, three anal spines with 9–11 soft rays, and 29–30 vertebrae. The most distinctive features are not in the fins but in the head, where the enlarged cephalic pores of the lateral-line system give the genus its name and, almost certainly, much of its ability to hunt in low light.

Color is subdued and sexually dimorphic. According to the Cichlid Room Companion's account and field photographs from tanganyika.si, ripe males develop a species-specific melanin pattern of two to three broad dark longitudinal bands along the flanks together with darkened fins, while females stay largely silvery and show a black spot at the front of the dorsal fin. The sexes are similar in size. A practical complication, noted by divers, is that this nuptial patterning fades at night when the fish move into shallower water, so live animals are genuinely difficult to identify in the field and easily confused with congeners.

Range & habitat

Trematocara variabile is endemic to Lake Tanganyika and is distributed lake-wide, with records from all four riparian nations — Burundi, the Democratic Republic of the Congo, Tanzania, and Zambia — and from the lower reaches of the Malagarasi River, the lake's largest affluent. It is a benthopelagic fish of soft bottoms: it is most often associated with muddy substrate near river deltas, typically at depths on the order of 20–60 m (about 65–200 ft), though there are older records from very shallow water during the day. The IUCN assessment notes that, among the Trematocara, T. variabile occupies the shallowest habitats of the genus.

That places it in what aquarists and the lake's habitat guides sometimes call the "unknown depths" — the dim, sediment-covered offshore floor that lies beyond the reach of recreational divers and well below the colorful rocky shore most people picture. Lake Tanganyika is enormous, ancient, and clear, with hard, alkaline water (pH roughly 8.3–9.2) and a stable upper-layer temperature near 24–27 °C (75–81 °F). Critically, oxygen is confined to the upper water column — to roughly 240 m in the south and only about 100 m in the north — so even a "deep" cichlid like this one lives within a relatively thin oxygenated skin over a vast anoxic abyss.

Ecology & diet

Trematocara variabile is a micro-predator on small invertebrates. Stomach contents and assessments describe a diet of easily digested prey: swimming crustaceans — the lake's abundant zooplankton — together with chironomid (midge) larvae and other small invertebrates taken from or near the bottom. FishBase places it at a trophic level of about 3.3, squarely in the small-carnivore range. The enlarged head pores are the key to how it makes a living: in turbid water and in darkness, a well-developed mechanosensory lateral line lets a fish detect the faint water movements of prey it cannot see, a sense other cichlids have been shown to use for feeding in the dark.

Its ecological role is best understood at the genus level. Trematocara are light-shy, schooling fishes that hold over deep water by day and undertake a nightly vertical migration toward the surface to feed, tracking the same zooplankton that rises after dark. In doing so they act as a link between the lake's planktonic production and its bottom community, and they form part of the prey base for larger offshore predators such as Bathybates. Some members of the genus have been recorded by night divers descending to extraordinary depths during the day, which makes Trematocara collectively among the deepest-living cichlids known.

Behavior & breeding

By temperament T. variabile is a timid, non-aggressive, schooling fish — a far cry from the territorial brawlers of the rocky shore. Its days are spent over open soft bottom and its nights in the water column, a rhythm that shapes everything about it.

Reproduction has not been observed directly in the wild, and this is an honest gap rather than a settled fact. Based on its close relatives, it is assumed to be a maternal (female) mouthbrooder with a polygynous mating system, the female carrying and incubating the eggs in her buccal cavity. That inference is reasonable — mouthbrooding is the norm across the Trematocarini and much of the lake's cichlid fauna — but no detailed account of spawning behavior, clutch size, or brood care in this species appears in the accessible literature. Anyone who tells you precisely how T. variabile breeds is extrapolating from the genus.

In the aquarium

This is a connoisseur's fish, not a beginner's. Trematocara variabile is only sporadically available — it has to be wild-collected from deep water, so it appears in the trade in small numbers and at a premium, and there is no meaningful captive-breeding pipeline behind it. The most detailed keeping account in print comes from hobbyist Sumer Tiwari, writing for Amazonas Magazine, who kept a group in a roughly 50-gallon (about 190 L) Tanganyika biotope at around 76 °F (24 °C), pH near 8.1. His most useful observation is about feeding: the fish readily took food drifting in the water column but lost interest the instant it settled on the substrate, so foods that stay suspended — quality flakes, frozen mysis and brine shrimp — worked far better than sinking pellets. That matches a nocturnal, water-column micro-predator rather than a bottom-grubbing sifter.

Keep it as a peaceful, soft-bottom species: a sand floor, dim lighting, plenty of open swimming room, hard alkaline water, and a group rather than a lone individual. The honest cautions are that it is shy and easily out-competed — in Tiwari's tank, more assertive Paracyprichromis harassed the Trematocara over food and space — so tankmates should be calm and similarly sized, and the usual roster of boisterous Tanganyikan rock-dwellers is a poor match. Given its rarity, its specialized feeding, and the lack of breeding know-how, it is a fish for experienced Tanganyika keepers who want something genuinely unusual, not a showpiece for a community tank.

Conservation

On paper Trematocara variabile is secure: the IUCN Red List assessed it as Least Concern in 2006 (assessor G. Ntakimazi), describing it as a common, widespread species with an area of occupancy likely exceeding 2,000 km². The assessment is candid about its limits, though — the population trend is listed as Unknown, the entry is flagged as needing updating, and the named threats are beach-seine fishing in the inshore zone and rising siltation and turbidity from catchment soil erosion. The assessors explicitly warned that the species could decline if those inshore pressures worsen. A pointed counterpoint comes from Ad Konings via the Cichlid Room Companion, who notes that the fish has not been seen by daytime divers in shallow water where it occurred decades ago and suggests it may have been extirpated from areas reachable by beach seines — arguing the species would be better classed as Data Deficient than Least Concern. That disagreement is worth taking seriously: "Least Concern" here reflects a thin, twenty-year-old evidence base, not a clean bill of health.

The wider state of Lake Tanganyika sharpens the concern. Climate warming has strengthened the lake's stratification and weakened the mixing that once lifted nutrients into the sunlit surface, and O'Reilly and colleagues (Nature, 2003; doi:10.1038/nature01833) estimated this has cut primary productivity by roughly 20%, with an implied drop of around 30% in fish yields. Paleoecological work by Cohen and colleagues (PNAS, 2016; doi:10.1073/pnas.1603237113) found that warming has shrunk the oxygenated benthic habitat by about 38% in their study areas, compounding declines in fishes and endemic molluscs — a direct squeeze on exactly the kind of soft-bottom, near-floor zone T. variabile depends on. Layered on top are decades of shoreline deforestation and sedimentation that smother littoral and deltaic bottoms (Cohen et al., 1993), and an intense pelagic fishery built on the clupeids Stolothrissa and Limnothrissa and the predatory Lates, feeding millions across the four nations that share the lake and now coordinated, in principle, through the Lake Tanganyika Authority. For a zooplankton-feeding fish tied to the inshore muddy floor near river mouths, those basin-scale pressures — less plankton, less oxygenated bottom, more silt, and seines working the shallows — bear squarely on its habitat. The species itself is not formally threatened, but the lake it depends on is unmistakably under strain, and the data needed to know how this particular cichlid is faring simply have not been collected.

Trematocara variabile

About this species