Bluespine Unicornfish (Naso unicornis): Breeding Guide
Naso unicornis pair-spawns in open reef water and produces pelagic larvae. Its huge adult size and long planktonic stage make breeding it in home aquaria impossible.
Overview
The Bluespine Unicornfish, Naso unicornis, is widely distributed across the Indo-Pacific from the Red Sea and East Africa to the Hawaiian, Marquesas and Tuamotu islands, north to southern Japan and south to Lord Howe and Rapa. FishBase gives a maximum fork length of 70 cm and a remarkably wide depth range of 0 to 180 m. Mature fish develop a forehead horn that first appears as a bump at about 12 cm in length.
It is a powerful open-water swimmer that grazes leafy macroalgae and reaches a very large adult size. No captive breeding has been documented.
Sexing
According to FishBase, males apparently have a longer horn, larger peduncular keels and longer caudal filaments than females of the same size. These secondary sexual differences develop with maturity, but they overlap enough that reliable sexing of individuals is difficult outside of large adults.
Spawning Behavior & Trigger
FishBase reports that N. unicornis spawns in pairs, and that pair-spawning has been observed on the reef. This matches the surgeonfish pattern of brief pelagic ascents: a pair rises off the bottom to release eggs and sperm into open water, typically near the reef edge and often around lunar phases when currents disperse the gametes.
Fertilization is external and there is no nest or parental care. The floating eggs are carried away on reef currents immediately after release.
Egg & Fry Care
Eggs are pelagic, hatching into transparent acronurus larvae adapted to open-ocean drift. As in other surgeonfishes, this planktonic stage can exceed 39 days before the larva settles and metamorphoses into a juvenile.
Such a long oceanic larval phase on natural plankton cannot be reproduced in a closed system, so home rearing is not feasible. Surgeonfish larviculture remains a research-aquaculture endeavor.
Common Challenges
- Adults reach up to 70 cm and need enormous swimming volume, so housing a pair is impractical.
- Pair spawning happens in open water with no eggs or nest to collect or guard.
- The pelagic acronurus larvae require ocean plankton and weeks of drift that aquariums cannot supply.