Taxonomy & naming
Macrognathus keithi was described by the American ichthyologist Albert William Herre in 1940 and is listed in the Catalog of Fishes (Eschmeyer, the authority for valid names) under the genus Macrognathus, one of the principal genera of the family Mastacembelidae. Macrognathus is distinguished from its close relative Mastacembelus primarily by the arrangement of its dorsal spines and the position of the snout. Both genera sit within the order Synbranchiformes — the same order as the swamp eels — and both are entirely distinct from the true eels of the order Anguilliformes. The eel-like shape of Macrognathus keithi is a classic case of convergent evolution: unrelated lineages arriving at the same elongate, burrowing body plan independently.
The species epithet keithi honours an individual named Keith, though the precise referent is not detailed in the original description. The species has no widely accepted common name in the aquarium trade, where it is occasionally listed by its scientific name alone or under generic labels such as 'Borneo spiny eel'. Its placement in Macrognathus has remained stable, and no synonyms or subsequent combinations are recorded in the literature available for this account.
Morphology
Macrognathus keithi is a small mastacembelid, reaching a maximum recorded standard length of approximately 8.5 in — modest compared to the large, metre-long species of the hobby but typical for the smaller Macrognathus of Southeast Asia. The body is long, slender, and laterally compressed toward the tail, with the scaleless, mucus-coated skin characteristic of the family. A row of short, isolated dorsal spines runs ahead of the soft dorsal fin, which continues around the tip of the tail and merges with the anal fin in the mastacembelid fashion.
The snout is drawn out into a fleshy, trunk-like rostral process tipped with paired tubular nostrils, used to probe soft substrate and leaf litter for prey. The down-turned mouth opens beneath this snout. Coloration details for this species are not well documented in the literature, but Macrognathus species of Borneo generally show brown to olive ground colours with variable lateral patterning. External sex differences are not described; ripe females of related species become broader across the body.
Habitat
Macrognathus keithi is known from northern Borneo, where it inhabits fresh waters of the lowland zone. FishBase records it as a benthopelagic freshwater species found in leaf banks, under log debris, and among rocks — the sheltered, structured microhabitats that mastacembelids routinely exploit as refugia and foraging grounds. This is a fish of warm, shallow, lowland streams and river margins rather than the open channel, its presence associated with accumulated organic material on the bed.
The waters of lowland Borneo are generally warm and soft to slightly acidic, draining through heavily forested peat-swamp and hill-stream catchments. Temperatures in such habitats typically range from about 75–82 °F, with pH values that can run below neutral in blackwater and peat-influenced streams. As with other scaleless spiny eels, Macrognathus keithi is expected to be sensitive to deteriorating water quality and copper contamination. Precise pH and hardness tolerances have not been formally measured for this species.
Feeding
Macrognathus keithi is a benthic carnivore, its probing rostral snout adapted to locate invertebrates buried or concealed in leaf litter, soft mud, and substrate debris. In common with other small Macrognathus species, it is expected to take insect larvae, worms, and small crustaceans under the cover of darkness. The fleshy, probing snout and subterminal mouth are the anatomical signatures of this foraging style across the mastacembelid family.
In an aquarium setting, small spiny eels of this size can be offered bloodworm, tubifex, small earthworm pieces, and other meaty invertebrate foods in the evening. Feeding after lights-out respects the species' nocturnal habits and reduces competition with diurnal tankmates. Species-specific feeding observations in the scientific or hobbyist literature are extremely limited for Macrognathus keithi.
Mating
Macrognathus keithi is a spiny eel and, as such, belongs to a family of substrate egg-scatterers — it is emphatically not a true eel and does not undertake any marine spawning migration. Spawning in mastacembelids is typically triggered by the seasonal rise and warming of water associated with the monsoon or wet season, when fish move onto flooded margins and scatter adhesive eggs among fine-leaved vegetation, roots, and leaf litter. No nest is built and no pair bond persists beyond the brief spawning act.
No species-specific courtship observations are on record for Macrognathus keithi, but the mating pattern can be inferred from the broader mastacembelid literature: courting individuals entwine and circle among vegetation before releasing eggs in small batches, with the adhesive eggs clinging to plant surfaces or settling into the substrate. There is no parental care at any stage.
Breeding
Spiny eels are rarely bred in the home aquarium, and Macrognathus keithi is no exception. The family's eggs are adhesive and scattered among vegetation or debris in fresh water, with no parental care and hatching occurring within a few days under warm conditions. Captive spawnings of mastacembelids in general require a stable, mature, well-planted soft-substrate setup, conditioning on live and frozen invertebrates, and a triggering water change that mimics the arrival of the wet season — cooler, softer water introduced in volume.
Published accounts of captive breeding specifically for M. keithi are absent from the available literature. Its modest adult size, relative to the large display species more commonly attempted, makes it a candidate for breeding in a smaller dedicated tank than those species require, but the broader challenges of the family — sexing fish, achieving condition, and successfully rearing the tiny larvae through their earliest stages — apply equally here. First foods for newly hatched spiny-eel larvae are infusoria-grade, with newly hatched brine shrimp suitable only after the larvae have grown.
In the aquarium
Macrognathus keithi is a small, secretive spiny eel that is rarely encountered in the mainstream hobby and is more likely to appear in specialist collections focused on the smaller mastacembelids of Southeast Asia. At approximately 8.5 in maximum standard length it is manageable in a modestly sized aquarium, but it shares the care demands of all scaleless, burrowing spiny eels: a deep bed of fine, soft sand it can vanish into, abundant cover in the form of smooth caves, pipes, and leaf litter, stable warm water, and a diet of meaty foods offered after dark.
Like all mastacembelids it is a determined escape artist that will locate and exploit any gap in a tank cover; a fully sealed, weighted lid is not optional. It is scaleless and therefore highly sensitive to copper-based medications and to deteriorating water quality, both of which can kill it rapidly without the warning signs that scaled fish display. Tankmates should be peaceful and too large to be eaten, and it should not be housed with other spiny eels of similar size without close observation for territorial conflict. In a well-managed, sheltered setup it should prove a long-lived, if largely nocturnal and hidden, resident.
Conservation
Macrognathus keithi is assessed as Least Concern on the IUCN Red List, with the most recent evaluation recorded in December 2019. The species is known from northern Borneo, a region that retains substantial freshwater biodiversity but faces ongoing pressure from deforestation, agricultural conversion (particularly palm oil), peat drainage, and the attendant siltation and pollution of lowland streams.
At the scale of the Red List assessment the species is not considered at immediate risk, but the lowland stream and leaf-bank habitats it relies on are among the most heavily affected by land-use change in the Bornean lowlands. Narrow-range species of this type, with limited documented occurrence data, can be vulnerable to localised population loss even when their formal conservation status appears secure. Continued monitoring of the state of Borneo's lowland freshwater systems is relevant to the long-term security of this and many other specialist fish.