Opsarius signicaudus

Overview

The western Thai headwater fish Opsarius signicaudus has an elongated dark tail-base blotch extending onto the fin. Fast swimming and vigorous feeding require clean oxygenated water and ample space for a group.

Distribution & habitat

Known records are restricted to headwater tributaries of the Khwae Yai and Khwae Noi within the Mae Klong basin. The type locality lies in Sangklaburi, Kanchanaburi.

Moderate to fast streams and rivers over gravel, cobbles, boulders, and bedrock are occupied. Adults tend to use deeper flowing sections, while juveniles shelter in shallows or slower pockets behind large rocks.

Aquarium setup

Use sand, fine gravel, and rounded stones for a stream layout, adding roots without sacrificing open water. Sturdy attached ferns or Anubias can be included.

A mature stable system is essential. Prevent organic waste accumulation and maintain abundant oxygen with moderate circulation, extra aeration, or pumps as needed. Change roughly 30–50% weekly and secure the cover tightly.

Diet & feeding

The genus mainly feeds near the surface on aquatic and terrestrial insects, possibly also taking small fishes and bottom invertebrates. That account should not be treated as a confirmed species-specific classification.

Supplement quality dry food with live or frozen bloodworms, Artemia, and chopped earthworms. Gut-loaded fruit flies and small crickets offer additional variety.

Behaviour & compatibility

Its speed and strong feeding response can overwhelm slow or timid companions, and very small fishes face possible predation. Robust open-water cyprinids of similar size and suitable bottom dwellers sharing its needs are better options.

Keep at least five to form a loose social hierarchy. Two or three may leave subordinates under constant pressure, while a solitary fish may target similar-looking companions. Group living still involves competition.

Sexual dimorphism

Males develop a fuller chest, more head tubercles, and stronger color in breeding condition. Females may become slightly larger and are especially thick-bodied when ripe.

Breeding

No established aquarium method is available.

Notes

The tail blotch combines the last flank bar with the large mark at the tail base, extending lengthwise onto basal rays. Approximately 36–39 lateral-line scales and short barbels, with rostral barbels absent in some fish, assist separation from other large-spotted relatives.

Exports may have occurred under incorrect names. Tail-mark orientation, barbel length, and scale rows are more dependable clues than a trade label alone.