Today’s Awesome Fish Spotlight features an opulent member of the wildly popular genus Cirrhilabrus, more affectionally known as the Fairy Wrasses. Cirrhilabrus rubrimarginatus is a common species with a wide distribution across much of the Western Pacific, from Japan through to the Philippines, Palau, Indonesia, Vanuatu, Fiji and Tonga. It was first known from the Ryukyus peninsula and was officially described in 1992 along with Cirrhilabrus katherinae. The most distinctive feature of this species is the broad red margin that decorates the dorsal and caudal fins in both sexes. This trait of course is a direct translation from its specific epithet rubrimarginatus.
The Red Margin fairy wrasse is a moderately large species capable of attaining lengths of up to 12 cm (~5in). The species is dubiously related to C. lineatus which replaces it in the other parts of the Western Pacific such as New Caledonia and Australia. Like all members of the genus, C. rubrimarginatus is a protogynous hermaphrodite, where terminal males are derived from sex changed females. The species is also haremic where males are outnumbered by females.
Cirrhilabrus and the closely related Paracheilinus are remarkable for possessing the ability to “flash”, which is a term used when males of a given species display their nuptial coloration. The nuptial coloration differs from species to species, and in the wildly confusing Paracheilinus, helps the females to identify the correct males during courtship – an evolutionary advantage so to speak.
When these fishes flash their nuptial coloration, they take on a completely different set of characteristics brought upon by rapid colour change from underlying chromatophores. Very often, metallic patches may also develop in areas where colour was previously matte. This is brought upon by reorientation of reflective plates within the chromatophores. Hormonal response to excitation or aggression can trigger a species to turn on or off its nuptial coloration.
In the case of C. rubrimarginatus, the anterior half of the dorsal fin as well as the anal fin take on a lustrous white during courtship. The previously translucent pectoral fins turn orange, and the red margin on the tail intensifies. The overall coloration is also heightened, and this is very often accompanied by erratic swimming and erection of all fins.
The two photographs above of a male at the Steinhart Aquarium shows only a fish in partial courtship display. When in the fullest state of colour change, the fish as seen directly above takes on an almost silvery appearance. The red margin on the tail is no longer as it was, and is instead metallic white. This usually lasts only for the duration of courtship or sparring, at no more than a few seconds each wave. The photo directly shows a male in full nuptial display, but with an intention of sparring with an offending species instead. In this case in Japan, with the endemic and sympatric Cirrhilabrus lanceolatus.