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

Taxonomy & naming

Diplotaxodon macrops was described by George Turner and Jay Stauffer in 1998, in a paper that introduced three new deep-water Diplotaxodon at once and redescribed a fourth (Ichthyological Exploration of Freshwaters 8(3): 239–252). The holotype, a specimen now held at the Natural History Museum in London, was trawled off Monkey Bay at the southern end of the lake from a depth of about 100 m. Catalog of Fishes and FishBase both list the name as valid and unchanged since.

The genus Diplotaxodon was erected by Ethelwynn Trewavas in 1935 for the silvery offshore haplochromines of what was then called Lake Nyasa; its name combines Greek roots meaning roughly "double-row teeth," and the type species is Diplotaxodon argenteus. The species epithet macrops is the easy part — it simply means "large eyes," a fitting label for a fish whose eye is the largest, proportionally, of any in the genus. Diplotaxodon is a small but genuinely confusing group: roughly ten described species plus a long list of undescribed forms catalogued by specialists. Complicating matters, what researchers call macrops appears to be a complex of closely related forms — population-genetic work distinguishes at least three (informally 'macrops offshore', 'macrops black dorsal', and 'macrops ngulube') that look nearly identical in the hand but differ in male breeding dress and are genetically distinct.

Appearance

This is a small fish. The original description gives a maximum standard length of about 12.5 cm (roughly 5 in), and most individuals are smaller; first maturity is reported at around 11 cm. The body is fusiform — a clean, slightly elongate torpedo shape built for cruising open water rather than threading through rocks — with a fairly long dorsal-fin base.

The standout feature is in the name. Diplotaxodon macrops has a notably large eye, with horizontal eye diameter measured at roughly 35–38% of head length, the highest figure in the genus; its close relative D. longimaxilla, by comparison, sits nearer 27–32%. That oversized eye is exactly what you would expect of an animal gathering scarce photons in deep, dim water. Out of breeding condition the fish is plainly silvery, with the countershaded, herring-like look shared across the genus. The diagnostic color appears in ripe males, which turn dark — described as black — with a white margin to the dorsal fin. Those subtle, almost monochromatic male patterns matter more than they look: across the macrops complex, the presence, absence, and shape of black-and-white dorsal markings are among the few visible features that separate otherwise look-alike forms.

Range & habitat

Diplotaxodon macrops is endemic to Lake Malawi (also called Lake Nyasa and Lake Niassa), found nowhere else on Earth. Unlike the brightly colored mbuna of the rocky littoral, it is an offshore, benthopelagic fish — living near the bottom and in the water column over the open shelf rather than along the shore.

It is a creature of depth. FishBase records the species between about 76 and 128 m, while survey and genomic studies place the broader macrops complex deeper still: typically 100–220 m during the day, with peak abundance recorded near 220 m. That puts it squarely in what biologists call Lake Malawi's "twilight zone," roughly 50–220 m down, where light is reduced to a narrow band of blue and the long-wavelength (red) end of the spectrum has been filtered out entirely. This habitat is constrained from below by chemistry: Lake Malawi is meromictic and permanently stratified, and its deep water is anoxic, with oxygenated water reaching only to about 170–220 m. Diplotaxodon macrops therefore lives close to the lowest depth a fish can occupy at all — at the dim, cool, oxygen-limited edge of the habitable lake.

Ecology & diet

Diplotaxodon macrops is a zooplanktivore — a plankton-picker rather than a predator of other fish. The original authors and FishBase describe it feeding on insect larvae (the lake's vast clouds of Chaoborus phantom-midge larvae are a key offshore food), small crustaceans, and diatoms, consistent with its low estimated trophic level of around 2.8. The large eye and fine feeding apparatus suit the task of spotting and selecting individual planktonic prey in low light.

Ecologically, the offshore Diplotaxodon are anything but marginal. A two-year trawl-and-gillnet survey of Lake Malawi's pelagic zone found that cichlids made up roughly 88% of the offshore fish biomass, and that two zooplanktivorous Diplotaxodon species alone accounted for about 71% of it. In other words, this unassuming silvery genus is one of the dominant components of the open-water food web — the link between the lake's plankton and its larger offshore predators, chiefly the elongate, pike-like Rhamphochromis. Diplotaxodon macrops is one strand of that abundant but easily overlooked deep-water community.

Behavior & breeding

Like other haplochromine cichlids of Lake Malawi, Diplotaxodon are maternal mouthbrooders: the female takes the fertilized eggs into her mouth and broods the developing young there, a strategy that frees the fish from needing a fixed nest site and suits a mobile, open-water existence. Detailed in-situ observation is difficult at these depths, so much of what is known comes from trawl samples, gonad cycles, and genetics rather than direct watching. Offshore surveys did estimate seasonal breeding cycles for the dominant Diplotaxodon from gonad activity, indicating organized rather than year-round, haphazard spawning.

The most interesting behavioral story here is about how these fish stay distinct. In the blue-only twilight zone, the bright nuptial colors that keep shallow-water cichlid species apart simply do not work — there is no red or yellow light to reflect. A study of sympatric Diplotaxodon (sampled between 40 and 200 m) found that males instead advertise in 'monochromatic' black, white, and silver patterning, and that even forms within the macrops complex that overlap in space remain reproductively isolated and genetically differentiated. It is a neat demonstration that sexual selection can drive and maintain species boundaries using pattern and brightness alone, where color is unavailable.

In the aquarium

Honestly, this is not an aquarium fish, and any care advice should start there. Diplotaxodon macrops is a deep, offshore, schooling species captured almost exclusively by commercial trawls and research nets, not by collectors working the shore. It is essentially absent from the ornamental trade; hobbyists who have encountered any Diplotaxodon at all describe it as a rare curiosity rather than a stocked species, and macrops specifically is far less seen than its commercially fished relative D. limnothrissa.

If one were ever kept, its biology argues against easy success. A fish adapted to cool, dim, oxygen-limited water at 100 m or more, with a large light-gathering eye and a habit of open-water schooling, has little in common with the bright, aggressive rock-dwellers that dominate Malawi tanks. It would want a long, dark, well-oxygenated tank with swimming room rather than rockwork, would likely be stressed by intense lighting, and would need to be kept in a group of its own kind rather than mixed with boisterous mbuna. The practical takeaway is the kind of thing a seasoned keeper would say plainly: admire this species for what it tells us about Lake Malawi's depths, not as a candidate for the fish room.

Conservation

The IUCN Red List assesses Diplotaxodon macrops as Least Concern (assessed 22 June 2018, by J. Kazembe and A. Konings). As a lake-wide endemic with no aquarium-trade pressure and a wide offshore range, it currently faces no species-specific threat severe enough to warrant a higher category — and it is important not to overstate the case: the species itself is judged secure.

That said, the water body it depends on is under real strain. A 2023 basin-wide review of Lake Malawi/Niassa/Nyasa (Chavula et al., Journal of Great Lakes Research 49(6): 102241) ranked fishery health, invasive species, and climate change as the catchment's top concerns. The lake's flagship chambo (native tilapia) fishery has declined and stayed low since around 2010, a signal of how heavily the lake is fished. Deforestation around the catchment drives sediment and nutrient loading into the shallows, and long-term monitoring shows the shallow water has warmed by roughly 0.7 °C (against about 0.18 °C in the deep), strengthening the lake's already-permanent stratification.

For an offshore, deep-water plankton-feeder like Diplotaxodon macrops, the relevant risks are not shoreline development or rock-habitat loss but the open-water ones. Stronger stratification and surface warming reduce the mixing that returns nutrients to the sunlit layer, which can cut the lake's primary productivity and, with it, the zooplankton this fish eats. Its deep habitat is also hemmed in by the lake's anoxic bottom water, so any expansion of that oxygen-poor zone would compress the band it can occupy. And as the abundant pelagic Diplotaxodon are a backbone of the offshore food web and a target of the mid-water trawl fishery, the species shares in the broader question of how sustainably Lake Malawi's open water is being exploited. The fish is secure today; the system it belongs to is not unstressed.

Diplotaxodon macrops

About this species