Taxonomy & naming
Mastacembelus dayi was described by the Belgian-British ichthyologist George Albert Boulenger in 1912, based on specimens from the Irrawaddy River, Myanmar — the type locality. The Catalog of Fishes (Eschmeyer, the authority for valid names) places it in the genus Mastacembelus, the type genus of the family Mastacembelidae within the order Synbranchiformes. This taxonomy is the key fact about this fish: it is an "eel" only by shape and common name. The true eels belong to the order Anguilliformes and are not closely related; the spiny eels arrived at their elongate, eel-like form independently, through convergence on a burrowing lifestyle.
The genus Mastacembelus spans both tropical Africa and Asia. The African members of the group were for many years divided among separate genera such as Aethiomastacembelus and Caecomastacembelus before being consolidated back into Mastacembelus. Mastacembelus dayi has remained stable in the genus and is not part of that African taxonomic churn. The species epithet dayi honours the British zoologist Francis Day, a prolific describer of South and Southeast Asian freshwater fishes in the nineteenth century.
Morphology
Mastacembelus dayi is a medium-sized spiny eel, reaching approximately 12 in total length according to FishBase records. The body is greatly elongate, laterally compressed toward the rear, and effectively scaleless to the naked eye — the scales are tiny and deeply embedded in a thick mucus coat. The snout is drawn out into the fleshy, trunk-like proboscis characteristic of the genus, tipped with tubular nostrils used to probe the substrate for hidden prey. A series of short, isolated dorsal spines runs along the back ahead of the soft dorsal fin, which is continuous with the anal fin around the tapering tail; the caudal fin carries 25–27 rays.
The most striking feature is the pattern: the body is marked with pale spots that have conspicuous dark edges, and the spots on the body often take on a vertically elongate shape, producing a leopard- or chain-like impression. This patterning distinguishes it from some of the larger, more uniformly marked Asian Mastacembelus and gives rise to both of its common names. Sexes are not externally distinct in this species; ripe females may become more rounded in the abdomen.
Habitat
Mastacembelus dayi is native to the Irrawaddy and Chindwin river drainages of Myanmar, with records suggesting possible occurrence in adjacent areas of eastern Thailand. FishBase describes it as a freshwater, demersal (bottom-dwelling) tropical species. It inhabits rivers and streams with moving water, dense aquatic vegetation, and substrates of sand, pebbles, or a mixture of the two.
Like other spiny eels of the Asian lowlands, it is strongly associated with benthic microhabitats — buried in soft substrate or sheltering among root masses, leaf litter, and submerged vegetation during the day. Seasonal migration into deeper canals and floodplain areas during the dry season has been reported, a common pattern in species adapted to the monsoon hydrology of mainland Southeast Asia. Water chemistry in these riverine habitats ranges broadly: temperature 73–82 °F (74–82 °F), hardness up to 25 dGH, and pH from 6.0 to 8.0 — tolerant conditions for an Asian river fish.
Feeding
Mastacembelus dayi is a nocturnal carnivore. In the wild it forages after dark along the riverbed, using the sensitive fleshy proboscis to probe sand, pebbles and root tangles for insect larvae, worms, crustaceans, and small fishes. The probing snout and the elongate body are adaptations for working along and within the substrate rather than hunting in the water column.
In the aquarium it should be offered meaty foods in the evening, after lights go out. Bloodworms, tubifex, earthworms, and other live or frozen invertebrates are accepted readily. Newly imported individuals may fast for a week or more as they settle, refusing food in bright light or while observed; feeding strictly after dark encourages shy fish to accept food. Many specimens can eventually be trained onto sinking prepared foods, though live and frozen invertebrates should remain the dietary staple.
Mating
As a spiny eel of the family Mastacembelidae, Mastacembelus dayi is a substrate egg-scatterer in fresh water. It is not a true eel and does not undertake any marine spawning migration — it breeds entirely in freshwater rivers and streams, without the catadromous life history of the Anguilla true eels. Spawning is believed to be triggered by seasonal cues associated with rainfall and rising water levels, the flood-and-retreat cycle of monsoon hydrology that drives reproduction across many Southeast Asian freshwater fishes.
In captivity, courtship has not been well documented in this species. The limited accounts available suggest that a conditioning pair, stimulated by large water changes with cooler, well-oxygenated water to mimic seasonal rains, will circle and entwine among floating plants before eggs are scattered. There is no pair bond, no nest, and no site selection beyond a loose association with vegetation or fine roots. The adults do not guard or tend the eggs once spawning is complete.
Breeding
Mastacembelus dayi has not been successfully bred in the home aquarium in any documented or widely reported case. Like most spiny eels, it is considered extremely difficult to breed in captivity, and virtually all specimens in the aquarium trade are wild-collected. The species lays adhesive eggs — reported to hatch in approximately three to four days at tropical temperatures — but reliable rearing through the larval stage has not been achieved outside specialist settings.
Where captive spawning attempts have been described for closely related Asian Mastacembelus, the approach involves a large, mature aquarium with soft substrate, overhead floating plants, and triggers of significant water changes using cooler, well-oxygenated water that simulate the onset of monsoon rains. Eggs hatch into tiny larvae that require infusoria or other microscopic foods before they can accept newly hatched brine shrimp. The combination of large tank requirements, difficulty in sexing a compatible pair, and the demanding larval stage means that captive breeding is the exception rather than the rule, and parental care is absent throughout.
In the aquarium
Mastacembelus dayi is an occasionally available aquarium fish suited to specialist spiny-eel keepers rather than the community tank. At up to 12 in it is a moderate-sized member of the family and can be maintained in tanks considerably smaller than those needed for the fire eel or tire-track eel, but it still requires a well-established, mature aquarium with a deep bed of fine sand for burrowing — this is not optional, as spiny eels deprived of a substrate into which they can fully disappear become chronically stressed. Caves, PVC pipes, and dense plant cover complete the set-up.
The fish is scaleless and therefore sensitive to copper-based medications, which must be avoided entirely; water quality must be high, with consistent temperature (73–82 °F), moderate flow, and good oxygenation. Like all spiny eels it is an accomplished escape artist, capable of forcing through the smallest gap in a lid or climbing equipment cables; a tight-fitting, fully sealed cover is essential. It is nocturnal and secretive, typically spending daylight hours buried or hidden, and it should be fed meaty foods after lights out. It is generally peaceful with fish too large to swallow and can cohabit with robust, medium-to-large species, though it will prey on anything small enough to fit in its mouth.
Conservation
Mastacembelus dayi was assessed as Least Concern on the IUCN Red List in 2010. The assessment notes that the species is widely distributed across the Irrawaddy and Chindwin river drainages and is utilized at low levels by local fishers; there was no evidence at the time of assessment of a significant range-wide population decline.
The rivers it inhabits — the Irrawaddy and Chindwin systems of Myanmar — are subject to increasing pressure from hydropower development, sediment extraction, agricultural runoff, and lowland habitat modification. As a demersal species dependent on well-oxygenated flowing water with intact substrate and bankside vegetation, it may be more sensitive to these changes than its Least Concern status implies for any particular local population. The species is known from relatively few specimens historically (FishBase notes the type description from three specimens), which may reflect genuine rarity or simply under-collection in remote drainages. Continued survey work would clarify whether the low-impact assessment holds as development intensifies across Myanmar's river basins.