Taxonomy & naming
Mastacembelus favus was described by the Indian ichthyologist Sunder Lal Hora in 1924. The Catalog of Fishes (Eschmeyer, CAS) places it in the genus Mastacembelus, the type genus of the family Mastacembelidae, within the order Synbranchiformes. That order is the crucial fact: it unites the spiny eels with the swamp eels but has no close relationship to the true eels of the order Anguilliformes. The eel-like body of M. favus is convergent — an independent adaptation to a burrowing, elongate lifestyle, not a mark of common ancestry with Anguilla or Conger.
The genus Mastacembelus is large, spanning tropical Africa and Asia, and its African members were historically parcelled into satellite genera such as Aethiomastacembelus, Afromastacembelus and Caecomastacembelus. Those names are now treated as junior synonyms and all are folded back into Mastacembelus. The Asian species M. favus has always sat in the core genus and is unaffected by that churn. It is most easily confused with M. armatus (the zig-zag eel), which shares overlapping range and similar patterning but grows substantially larger — to around 35.5 in — against the 27.5 in maximum recorded for M. favus. Wet Web Media and FishBase both note ventral markings as a useful separating character for M. favus.
Morphology
The tire-track eel is a large, elongate fish, reaching a recorded maximum of 27.5 in standard length. The body is greatly elongate and laterally compressed toward the tail, scaleless in appearance — the tiny embedded scales are hidden under a thick coat of mucus. The snout is drawn out into a fleshy, tubular proboscis tipped with paired tubular nostrils, a sensitive organ used to probe gravel and organic debris for invertebrate prey. A row of short, isolated dorsal spines runs ahead of the soft dorsal fin; the soft dorsal continues seamlessly into the anal fin around the tapered tail, a characteristic spiny-eel body plan.
The colour pattern is the species' defining feature: a pale to tan base overlaid with a reticulate, net-like tracery of dark brown lines and blotches that suggests the irregular tread of a tire, giving rise to the common name. Ventral markings are present and help distinguish M. favus from M. armatus. Sexual dimorphism is not pronounced externally, though ripe females become fuller-bodied as they approach spawning condition.
Habitat
Mastacembelus favus is native to flowing freshwaters of Thailand and the Malay Peninsula. FishBase records its distribution across this region, and it is a fish of lotic habitats — rivers, streams and channels — with gravel or mixed substrates, where it shelters among rocks, submerged wood and vegetative debris during daylight hours. It inhabits the shallow to moderate depths of flowing water rather than the sluggish floodplain backwaters more typical of some other spiny-eel congeners.
Water parameters from the aquarium literature (Aqualog) place it in warm, soft to slightly hard conditions: 72–81 °F (72–81 °F), pH 6.5–7.5, and a hardness range of roughly 1–15 °N. These figures reflect the generally soft to neutral water of its native river systems in Southeast Asia. It is not a rift-lake species and does not require hard or alkaline water. Like all Mastacembelidae it is scaleless and particularly sensitive to copper-based medications and to deteriorating water quality.
Feeding
The tire-track eel is a nocturnal, benthic carnivore. FishBase records its natural diet as benthic insect larvae and worms, taken by probing gravel and organic debris with the sensitive fleshy snout after dark. The down-turned mouth and elongate body are well suited to hunting along and within the substrate rather than in open water.
In the aquarium it should be offered meaty foods in the evening: earthworms, bloodworm, tubifex, and other frozen or live invertebrates are readily accepted. Newly imported fish are often shy and will refuse food in bright light or while observed; feeding after lights-out with live or frozen worms usually prompts a response and helps the fish settle. Many specimens eventually accept sinking carnivore pellets once established, though live and frozen foods remain the most reliable trigger.
Mating
Spiny eels are egg-scattering spawners that give no parental care, and M. favus follows this pattern. It is not a true eel and undergoes no oceanic migration to spawn; breeding takes place entirely in fresh water. In the wild, spawning appears to be triggered by the flood season: Aqualog records that M. favus breeds during periods of seasonal rain and rising water, conditions that mimic the onset of monsoon floods across its range.
Courting pairs circle and entwine among vegetation, eventually scattering adhesive eggs across plants, roots and substrate. There is no nest construction and no pair bond beyond the spawning act itself; the adults play no further role once eggs are released. Precise field observations are scarce, as with most large nocturnal burrowers, and most of what is known derives from occasional aquarium accounts rather than systematic study in the wild.
Breeding
No successful breeding of M. favus in captivity has been recorded in the available literature — FishBase and Aqualog both note that the species has not been bred in captivity. This is typical for the large spiny eels: their adult size, the difficulty of sexing and conditioning a compatible pair, and the requirement for seasonal flood-cue triggers all present significant obstacles in the home aquarium.
Where spawning might eventually be attempted, the general spiny-eel approach applies: a large, mature, soft-substrate aquarium with fine-leaved plants; conditioning with live and frozen foods over an extended period; and simulation of the monsoon onset through large water changes with slightly cooler, softer water. Eggs are adhesive and scattered among vegetation; any resulting larvae would be minute and require infusoria-grade first foods. Until reliable captive breeding is reported, all tire-track eels in the trade should be assumed to be wild-caught or pond-raised rather than tank-bred.
In the aquarium
The tire-track eel is a striking but demanding fish that grows to 27.5 in and is long-lived, meaning the tank it will ultimately need is substantially larger than the juvenile usually offered for sale implies. A spacious aquarium of at least 59–71 in in length is appropriate for an adult, fitted with a deep bed of soft sand or fine gravel for burrowing, multiple caves, pipe sections and shaded retreats, and robust biological filtration able to handle the output of a large carnivore. It is scaleless and acutely sensitive to copper-based medications and poor water quality; a stable, mature system and careful quarantine practice are essential.
Escape prevention is non-negotiable: like all spiny eels it is an accomplished climber that will locate and exploit any gap in the lid, breathe at the surface, and exit the tank given the opportunity. Every opening must be sealed. It is nocturnal and will spend daylight hours buried or wedged into cover, emerging after dark to feed. It can be kept with robust, peaceful fish too large to be swallowed, but will take small tankmates at night. Compatible spiny-eel companions are possible in a very large system with sufficient space and hiding places to reduce territorial conflict.
Conservation
Mastacembelus favus is assessed as Least Concern on the IUCN Red List, with the assessment dated December 2019. Its range across Thailand and the Malay Peninsula encompasses several major river systems, and there is no evidence of a range-wide decline at the time of assessment.
That picture may become more complicated over time. The lowland rivers and floodplain habitats on which it depends across Southeast Asia are under sustained pressure from damming, water extraction, sand and gravel mining, agricultural run-off and pollution. As a large, slow-maturing predator requiring clean, well-oxygenated flowing water, M. favus would be expected to show sensitivity to habitat degradation, even if current population data do not indicate a crisis. The ornamental trade draws on wild-caught stock. The species remains secure region-wide for now, but its dependence on flowing, unpolluted river systems makes it worth monitoring as development pressures intensify across the Mekong and peninsular watersheds.