Male unicorn tangs change the colour of their horn to signal to rival males and prospective partners a new study has shown.
Scientists believe that the horn plays a role in courtship as well as maintaining dominance over subordinate members of the same species.
Hiroshi Arai of the Tokyo Sea Life Park and Tetsu Sato of Nagano University studied two unicorn tang species, Naso unicornis and Naso vlamingii, at a public aquarium in Japan.
They noticed that males of both species change the colour of the protuberance, and the colour of the body to highlight the protuberance, during displays between prospective partners and rival males.
Their findings, which were published in the latest issue of Ichthyological Research, the journal of the Ichthyological Society of Japan, describe how the horn is used during displays at night and during the day:
"The displays with colour changes of the protuberance took place when a male courted a female in the evening or at night. The same displays were occasionally performed by males toward females throughout the daylight hours", wrote Arai and Sato.
"In N. vlamingii, dominant males displayed the protuberance toward subordinate smaller males. Although the sizes and shapes of the protuberance were sexually monomorphic, females of both species rarely made displays of the protuberance."
Both Naso vlamingii and Naso unicornis were observed spawning in the aquarium.
Sexual selectionArai and Sato believe that female unicorn tangs have sexually selected the trait in males which allows they to quickly change the colour of their protuberance, as well as the protuberance itself, because the characters appear to play important roles in sexual interactions.
Arai and Sato wrote: "Males of both species used the protuberance in displays, combined with quick and drastic changes in the colours of the protuberance and other parts of their body. The protuberance was well contrasted by the colour arrangement (flashing).
"The displays with quick colour change of the protuberance were performed when a male courted a female. The same displays by males were occasionally observed throughout the daylight hours. Females of both species scarcely displayed the protuberance. The protuberance was not used as a device for attach or defense nor as a feeding defense."
Naso unicornis develops a horn-like protuberance on its forehead, while Naso vlamingii develops a round protuberance. Both species are sold in the aquarium trade, but their large size (over 30cm) makes them unsuitable for all but the largest marine aquariums.
For more information see the paper: Arai H and T Sato (2007) - Prominent ornaments and rapid color change: use of horns as a social and reproductive signal in unicornfish (Acanthuridae: Naso). Ichthyological Research (2007) 54: 49-54.