Taxonomy & naming
Theriodes sandakanensis was described by Robert Inger and Ph. K. Chin in 1962, with a type locality near Sandakan in northern Borneo — the source of its species epithet. Its combination has changed several times since. It was originally placed in Acanthophthalmus, one of the old "eel loach" genera used for slender, elongate Southeast Asian Cobitidae; Kottelat and colleagues later moved it to Lepidocephalichthys (1993), and it was subsequently transferred again to Pangio (Havird & Page, 2010) as those catch-all genera were progressively revised.
More recent taxonomic work has split it out once more, and it is now recognised as the only species in the monotypic genus Theriodes, within the family Cobitidae — the true spined loaches. As with much of this corner of Cobitoidea, readers may still encounter the fish under an older name (Acanthophthalmus sandakanensis, Lepidocephalichthys sandakanensis, or Pangio sandakanensis) in older literature; Theriodes sandakanensis is the current valid combination.
Morphology
This is a small, elongate, eel-like loach, reaching only about 1.5 in in total length — one of the smallest loaches recorded from Borneo. Like other Cobitidae it carries the family's diagnostic erectile suborbital spine, a small defensive spike housed in a groove beneath the eye, and its body is covered in tiny, deeply embedded scales that leave the skin functionally naked — a trait that matters for how sensitive it is to medication.
Detailed morphological description beyond the type material is sparse in the literature available for this profile; body form and colouration follow the slender, banded pattern typical of small Bornean cobitids, but specific pattern details are not well documented outside the original taxonomic works.
Habitat
Theriodes sandakanensis is endemic to northern Borneo, where it is known from stream habitats in the vicinity of Sandakan. It is a tropical species — this is warm-water Borneo, not one of the temperate Eurasian spined loaches — living in freshwater stream environments typical of the region's lowland and foothill drainages.
Specific data on flow, substrate and water chemistry at collection sites is not documented in the sources available here; as a demersal Cobitidae it is reasonable to expect it favours a soft, sandy or fine substrate in slow-to-moderate flowing water, consistent with its relatives in the family, but this profile does not assert habitat detail beyond what is sourced.
Feeding
No species-specific feeding studies are documented for Theriodes sandakanensis. Based on its placement in Cobitidae, it is reasonable to expect it forages as a benthic omnivore, sifting a soft substrate for worms, small insect larvae, tiny crustaceans and detritus in the manner typical of small spined loaches.
This is inference from family-level biology rather than a direct observation for this species, and should be read as such — data on its actual wild or captive diet is sparse to non-existent.
Mating
No courtship or spawning behaviour has been documented for Theriodes sandakanensis. Nothing in the sources available for this profile describes sexual dimorphism, pairing behaviour, or breeding triggers for the species, and given how little-known the fish is even taxonomically, this is not surprising.
Breeding
There is no record of Theriodes sandakanensis being bred in the aquarium, and it does not appear to be established in the hobby at all. Spined and pond loaches (Cobitidae) as a family are egg-scatterers with no parental care, typically dropping adhesive eggs among plants; a handful of temperate species — the Eurasian spined loaches (Cobitis, Sabanejewia) and the dojo or weather loach (Misgurnus) — have been pond- or tank-spawned by hobbyists, but the tropical Cobitidae, the group this species belongs to, generally have not. There is no reason to expect Theriodes sandakanensis to be an exception; it should be assumed rarely or never bred in captivity.
In the aquarium
Theriodes sandakanensis is not established in the aquarium trade — it does not appear in the hobby literature or stocklists consulted for this profile, and there is no meaningful care record to draw on. What can be said follows from its family: as a near-scaleless Cobitidae it would be expected to be sensitive to copper and full-strength medications, and as a small tropical sand-associated species it would need warm water and a soft substrate to behave naturally.
Given the almost total absence of husbandry data, this profile does not attempt to prescribe tank parameters beyond noting it is a tropical, not coldwater, species. Anyone encountering this fish should treat it as a data-sparse rarity rather than a known quantity.
Conservation
Theriodes sandakanensis is assessed by the IUCN Red List as Near Threatened, with the most recent assessment dated 2019. As a narrow-range endemic restricted to streams around Sandakan in northern Borneo, it is exposed to the general pressures facing Bornean freshwater habitats — deforestation, agricultural conversion and stream degradation — though this profile does not have source-specific detail on population trend or the precise threats cited in its assessment.
The species is essentially unknown in the aquarium trade, so there is no evidence of collection pressure; its conservation concern, such as it is, traces to habitat rather than the hobby.