Large Spiny Eel, Belted Spi (Mastacembelus circumcinctus) — 6-8cm

Overview

The Mastacembelus circumcinctus, commonly known as the Large Spiny Eel or Belted Spiny Eel, is a sleek, burrowing species for freshwater aquarium fish enthusiasts seeking an unusual centerpiece. Native to South and Southeast Asia, this striking eel grows to around 35 cm in captivity and displays distinctive banded patterning. Its nocturnal habits and fascinating burrowing behavior make it a captivating addition to peaceful large aquariums, though careful tankmate selection is essential. At Tropical Fish Co, we supply healthy juveniles that adapt well to aquarium life and reward attentive care with years of engaging, active behavior.

Appearance and Coloration

Mastacembelus circumcinctus displays a robust, elongated body adorned with bold dark brown or black vertical bands or belts running across a golden-brown or tan base color. The banding is crisp and consistent, earning the species its "belted" common name. The head is slightly compressed with a pointed snout and small eyes. The dorsal and anal fins run almost the full length of the body, creating a ribbon-like appearance. The body is muscular and capable of powerful burrowing movements. Under subdued lighting, the banded pattern is particularly striking and provides camouflage against natural substrates.

Natural Habitat and Distribution

This species is associated with freshwater habitats in the Indian subcontinent and nearby regions of South and Southeast Asia. It favors slow to moderate-flowing rivers, lakes, and floodplains with soft substrate, submerged logs, and abundant hiding places. Water conditions are typically warm (24–27 °C), neutral to slightly acidic, and well-oxygenated. Mastacembelus circumcinctus is primarily nocturnal, hiding during the day in burrows, root tangles, or under rocks. At dusk, it emerges to hunt for small invertebrates, worms, and organic debris. The natural environment is dimly lit with a soft mud or sand substrate.

Aquarium Setup and Décor

A 250-litre aquarium is suitable for a single adult Belted Spiny Eel. Provide a tightly fitted lid with no gaps, soft sand or fine substrate for burrowing, and plenty of hiding places such as PVC pipes, caves, and driftwood. Subdued lighting is essential to reduce stress and encourage natural behavior. Avoid sharp-edged décor or coarse gravel, as the eel's skin is sensitive and can be damaged. This species is an escape artist and can injure itself on sharp décor or equipment. Secure all equipment and décor, as the eel may dislodge items while burrowing. Maintain excellent water quality with regular water changes.

Feeding and Nutrition

Mastacembelus circumcinctus is carnivorous. Offer meaty sinking foods such as bloodworms, earthworms, blackworms, chopped shrimp, insect larvae, and quality carnivore pellets. Feed after lights out if necessary, as this species is primarily nocturnal. Feed once daily in the evening, ensuring food reaches the substrate. This species is a predatory feeder and will consume any tankmate small enough to be swallowed. Avoid overfeeding, as uneaten food can degrade water quality. Once acclimated, some individuals will accept high-quality sinking pellets.

Behaviour and Social Structure

The Belted Spiny Eel is primarily nocturnal and semi-aggressive. It hides during the day, often buried in the substrate with only the head visible, and emerges at dusk to hunt. This species is solitary and can be territorial, especially toward conspecifics or other bottom-dwellers. It may prey on very small tankmates and can be assertive at feeding time. Despite its predatory nature, it is not overly aggressive toward tankmates of similar size. The eel is a powerful burrower and will constantly rearrange substrate and décor. Handle with care; the spines can inflict painful wounds.

Compatible Tank Mates

Ideal companions include robust fish of similar adult size that will not fit in its mouth, non-aggressive midwater fish too large to be eaten, and large, peaceful bottom-dwellers only if they do not compete aggressively for space. Avoid housing with very small fish, shrimp and other small invertebrates, fin-nipping species, aggressive territorial fish, or fish small enough to be eaten. Fast-moving, mid-water species that will not be harassed are best.

Breeding in the Aquarium

Captive breeding is uncommon and not well documented; spawning likely requires excellent water quality, mature fish, and seasonal environmental cues such as temperature fluctuations and increased rainfall simulation. Sexual dimorphism is subtle and not well documented. If breeding is attempted, provide a very large aquarium, excellent water quality, and seasonal conditioning. Fry, if produced, require specialized care and high-protein micro-foods.

Why Buy from Tropical Fish Co

At Tropical Fish Co, every Belted Spiny Eel is quarantined, health-checked, and acclimated to UK water conditions before dispatch. We source our fish from reputable suppliers who prioritize ethical collection and sustainable practices. Our livestock arrives in insulated packaging with heat packs during colder months, ensuring safe transit. With decades of combined fishkeeping experience, our team is available to answer questions about care, compatibility, and tank setup. Whether you're building a large oddball aquarium or expanding an established collection, we're here to support your success. Order with confidence and enjoy premium freshwater fish delivered to your door.