Celebes Rainbowfish

The Celebes Rainbowfish (Marosatherina ladigesi) is a freshwater species endemic to the island of Sulawesi (formerly Celebes) in Indonesia, and is the sole member of its genus. Though commonly grouped with true rainbowfishes in the aquarium trade, it is classified in the family Telmatherinidae, a group of sailfin silversides found mainly in Sulawesi's unique freshwater systems. The species is best known for its tall, elegant sail-like fins, which have made it a long-standing favorite in the aquarium hobby. Its restricted island distribution, combined with ongoing habitat pressures from agriculture and development in Sulawesi, has raised conservation concern, and it is assessed as vulnerable in some regional reviews. It belongs to the order Atheriniformes.

Celebes Rainbowfish are slender, streamlined fish typically 6-8 cm long.

• Body: silvery with a subtle bluish iridescent sheen along the flanks
• Fins: tall, sail-like second dorsal and anal fins, translucent with black and yellow trim, especially pronounced in males
• Tail: deeply forked
• Head: small and pointed with a large eye The elevated, sail-like fin shape combined with black-and-yellow fin edging is the clearest identifying feature, distinguishing this species from true Melanotaenia rainbowfishes, which have shorter, less dramatically elongated fins.

Celebes Rainbowfish are found only in clear, flowing freshwater streams and rivers on the island of Sulawesi, Indonesia, often in areas with sandy or rocky substrate and moderate current. Water is typically warm (24-28°C), well-oxygenated, and relatively clear, supporting the species' preference for visually oriented courtship and shoaling behavior. It favors habitats with some aquatic vegetation or overhanging bank cover for shelter, while still frequenting more open, current-swept stretches for feeding. Because Sulawesi's freshwater systems are geologically isolated, the species' distribution is naturally confined to specific island drainages, and habitat degradation from surrounding land use poses an ongoing concern for wild populations.

Celebes Rainbowfish form loose, active shoals in open and moderately flowing stream sections, feeding during the day on small aquatic invertebrates, insect larvae, and algae. Males raise and display their tall sail-like fins in courtship rituals directed at females and in mild competitive posturing with rival males, generally without serious aggression. Spawning occurs repeatedly under favorable conditions, with females scattering adhesive eggs among fine vegetation or root structures; adults provide no parental care afterward. Eggs hatch in roughly a week, and fry begin feeding on microscopic organisms soon after hatching. As an endemic omnivore, the species contributes to invertebrate population regulation within Sulawesi's distinctive freshwater ecosystems.