Taxonomy & naming
Macrognathus siamensis was described by the German-British ichthyologist Albert Günther in 1861, the parenthetical authority indicating that the species was later moved from its original genus. The Catalog of Fishes (Eschmeyer, the authority for valid names) places it in Macrognathus, one of the two main Asian spiny-eel genera recognised alongside Mastacembelus. Macrognathus species are characterised by a shorter, rounder snout (the name means 'long-jawed' in Greek) and a spotted-fin pattern, and tend to be smaller and more slender than the larger Mastacembelus of the same region.
The family Mastacembelidae is placed in the order Synbranchiformes alongside the swamp eels (Synbranchidae). This placement is the essential taxonomic fact about the peacock eel: it is not an eel in the fish-order sense. The true eels belong to the order Anguilliformes, which is a completely separate lineage of largely marine and catadromous fishes. The spiny eels' elongate, sinuous body is a case of convergent evolution — a shape independently arrived at by unrelated animals that share a burrowing lifestyle — and not shared ancestry with Anguilla. The common name 'spiny eel' remains in wide use and is well established in the hobby, provided one understands its limitations.
The specific epithet siamensis means 'of Siam', the former name for Thailand, reflecting the country from which the type specimen was obtained.
Morphology
Macrognathus siamensis is a slender, elongate fish reaching a recorded maximum of 12 in standard length, though aquarium specimens more often reach 8–10 in. The body tapers gradually toward a pointed tail, and the skin is scaleless in appearance — the scales are tiny and deeply embedded — and covered in a layer of protective mucus. Like all spiny eels, the snout is drawn out into a fleshy, mobile proboscis with a pair of tubular nostrils at its tip, used to probe soft substrate in search of invertebrates. Along the dorsal midline runs a row of short isolated spines ahead of the soft dorsal fin — the feature that gives the family its name and distinguishes it from the unrelated true eels.
The colour is typically pale to medium brown or olive-tan along the back, fading to cream or pale yellow below, with a dark lateral stripe running from the snout to the tail. The defining feature is the series of circular, black-centred, pale-edged ocelli — peacock-eye spots — arranged along the bases of the dorsal and anal fins, which inspired both the common name and the scientific usage of the pattern as an identification feature. Females are generally more robust through the belly when gravid; otherwise the sexes are difficult to distinguish externally.
Habitat
The peacock eel is a fish of lowland tropical fresh water across mainland Southeast Asia. Its confirmed native range spans the Mekong, Chao Phraya and Maeklong river systems, placing it across Thailand, Cambodia, Laos and Vietnam; FishBase records it as benthopelagic in tropical freshwater environments. Within those drainages it occupies the slower reaches: backwaters, floodplain pools, swamps, canals, ponds and marginal shallows with sandy or silty mud bottoms into which it can burrow.
It favours warm conditions — the species is tropical in the strict sense — and aquarium husbandry guidelines drawn from its native climate converge on a temperature range of about 73–79 °F (73–79 °F) and a pH of 6.5–7.5, reflecting the soft-to-neutral, slightly acidic character of the lowland river water it inhabits. The species is not a fish of fast, clear, oxygen-rich streams; it is adapted to still or slow-moving, often turbid water that floods seasonally with the monsoon rains. Like all spiny eels it is scaleless and therefore more sensitive to dissolved copper and chemical insults than a scaled fish of comparable size.
Feeding
Macrognathus siamensis is a nocturnal, bottom-dwelling carnivore. In the wild it hunts after dark along soft-bottomed substrate, using its probing, sensory snout to locate benthic prey: aquatic insect larvae, small crustaceans, annelid worms and other invertebrates that live in or on the silt. FishBase records its diet as benthic insects, crustaceans and worms; it may also take small fish, though its modest size makes it less of a piscivore than the larger spiny eels.
In captivity it accepts a range of meaty foods appropriate to its size: frozen bloodworm, brine shrimp, daphnia and Mysis shrimp are all well received, and live foods such as earthworm pieces and small crickets are taken eagerly. Many specimens will eventually accept sinking pellets once established, though live or frozen invertebrates should remain the dietary backbone. Feeding should be offered after lights-out to match the species' nocturnal habits, and it should not be housed with faster or more aggressive feeders that will consume food before it can locate it.
Mating
Spiny eels are egg-scattering spawners that give no parental care, and Macrognathus siamensis follows that pattern. It is not a true eel and does not undertake any form of catadromous migration to spawn in salt water; breeding takes place in fresh water. In the wild, spawning is linked to the annual monsoon cycle: the rising water levels, warming temperatures and increased food availability of the wet season appear to trigger reproductive activity, and pairs will move into flooded marginal vegetation to court and spawn.
Captive accounts describe a chasing and circling courtship in which the male pursues the female before the pair entwine briefly among fine-leaved plants or floating vegetation, scattering small, adhesive eggs across plant surfaces and leaf litter. There is no nest, no pair bond beyond the act of spawning, and no further interaction between the adults and the eggs once laid. Wild courtship has not been studied in detail, since the fish is nocturnal and lives in turbid, vegetated water that makes direct observation difficult.
Breeding
The peacock eel is regarded as very difficult to breed in the home aquarium and no routine captive-breeding programme exists; the great majority of fish in the ornamental trade are wild-caught, with some commercial pond production in Southeast Asia. Where spawning has been reported in captivity, it has typically been stimulated by simulating the onset of the rains — large water changes with slightly cooler, softer water, combined with a raised water level and increased live or frozen food — after which a compatible pair may spawn spontaneously among fine-leaved plants.
The eggs are small and adhesive, and hatch within two to three days at tropical temperatures. The larvae are extremely tiny and require infusoria or similarly fine first foods before they can take newly hatched brine shrimp nauplii; raising them to a size where they can feed reliably is the main bottleneck. No parental care is given at any stage; the adults should be removed after spawning to protect the eggs. The practical rarity of successful home breeding means that captive-bred peacock eels are an unusual offering and wild fish predominate in the trade.
In the aquarium
The peacock eel is one of the more manageable spiny eels for the home aquarium, partly by virtue of its modest adult size — 8–10 in is typical in captivity — but it shares all the care requirements that define the family. A fine sand substrate at least 2 in deep is essential: this is an animal built to burrow, and a gravel or bare bottom denies it a core natural behaviour and causes chronic stress. Hiding places in the form of caves, pipes, or dense plant roots satisfy its daytime retreat needs, and subdued lighting encourages natural activity. A mature, well-filtered tank with stable chemistry is important because the scaleless skin is sensitive to ammonia spikes and poor water quality.
The paramount requirement is a fully sealed, escape-proof lid. Macrognathus siamensis, like all spiny eels, is an adept escape artist that can locate and exploit any gap in a cover, navigate the smallest openings at the waterline, and breathe briefly in moist air — meaning that escaped fish are often found alive but dying on the floor some distance from the tank. Every potential exit must be sealed. The peacock eel is generally peaceful toward fish too large to be swallowed, but will consume small shrimp, nano fish and any invertebrate it can fit in its mouth, all under cover of darkness. Tankmates should be chosen accordingly, and copper-based medications must never be used in the same system.
The species is also significant in the food fishery of its native region, and is commercially important in Thailand and neighbouring countries both for local consumption and for the international aquarium trade. Wild collection pressure is real, and buying healthy, well-established captive specimens where available reduces demand on wild populations.
Conservation
Macrognathus siamensis is assessed as Least Concern on the IUCN Red List, assessed in 2011. It is a widespread and adaptable species across the major lowland river systems of mainland Southeast Asia, tolerant of the slow, turbid and seasonally fluctuating conditions that characterise much of its range, and FishBase records it as commercially important both for food and the ornamental trade — the latter fact implying a large and accessible population. There is no evidence of a range-wide population decline at present.
The pressures on the waters it inhabits are nevertheless real and increasing: damming for irrigation and hydropower across the Mekong and Chao Phraya basins alters flow regimes and disrupts the seasonal flood pulses on which the species' breeding appears to depend; agricultural runoff degrades lowland water quality; and sand extraction from river beds removes the soft substrate the fish needs for burrowing and foraging. Aquarium and food-trade collection add a further layer of harvest pressure. The current Least Concern listing reflects the species' resilience and wide distribution rather than an absence of stressors, and ongoing monitoring in the context of major Mekong development projects will be important to confirm its continued security.