The Threadfin butterflyfish (Chaetadon Auriga) is also known as the Auriga butterflyfish, Threadfin coralfish, Cross-stripe butterfly and Whip butterflyfish. Their common name comes from the filament that hangs off the back of the dorsal fin in adult fish. This can be seen in the image above. The filament is formed by the fusing of the fifth and sixth filaments of the soft dorsal fin into a thread. The thread grows longer as the fish ages.
They belong to the large subgenus Rabdophorus which some experts believe warrants distinction as a separate genus. This sub genus shares a characteristic pattern of two areas of ascending and descending oblique lines. The Threadfin butterflyfish in the Red sea lack the eye spot on the rear of the dorsal fin and are sometimes called Chaetodon auriga auriga.
The body is oval and white with dark lines running upwards away from the head and another series of lines running downwards towards the tail creating a chevron pattern. The rear of the body and caudal peduncle are yellow.
There is a dark spot on the rear of the soft dorsal fin and a black bar runs through the eye. A series of thin yellow lines runs across the face between the eyes. They have a pointed snout with sharp, narrow teeth and grow to between 20-22 cm in length. It is impossible to sex them from their looks. A fine blue line edges the rear of the dorsal fin and the caudal fin is lined with brown.
IN THE WILD
Threadfin butterflyfish are often seen in pairs the wild and as with all butterflyfish are hard to approach closely. Although they swim in the open they always have a hole nearby to pop into and are quick to do so. If they think you are following them they hide.
If one watches them feeding, they often feed in small groups often with other species of butterflyfish. When they move into the territory, of for example the jeweled damsel fish, Plectroglyphidodon dickii, which is highly territorial and chases any small fish that comes into its territory, the damsel will chase one of the shoal out the area. The others continue feeding in the damsels territory. This grouping together seems to give an advantage in this fashion. At night they darken their colours. This can be seen in the image below.
They are found from from Mossel bay in South Africa, northwards up the African coast to the Red Sea and across the Indo Pacific area up to Japan in the north to Lord Howe island south of Australia.
Threadfin butterflyfish are capable of handling much lower temperatures than most other butterflyfish. They are found across a range of environments from coral reefs, rubble covered bottoms to rocky coastlines down in the Cape province in South Africa from depths of 1 to 35 meters.
Threadfin butterflyfish are omnivores and feed on a wide variety of foods, ranging from hard and soft corals to worms, molluscs, sponges , microalgae and plankton. They browse the reef sides and tops and often form groups with members of their own species and other species of butterflyfish.
They form monogamous pairs and keep the same mates although they will find a new mate if one dies. The female swells with eggs and spawning takes place by scattering into the water table with the male fertilizing the eggs.
Spawning is thought to take place in relation to the lunar cycles but this is not certain. The eggs are spherical and buoyant and it is thought they hatch in 28 to 30 hours. Once the eggs hatch a bony plate forms over the head area and the larvae called tholichthys, are pelagic for quite some time. They slowly develop into juveniles. This phase makes it extremely difficult to breed them in aquariums.
Hybrids have been reported between Chaetodon auriga and the saddleback butterfly fish Chaetodon ephippium.
Because of their ability to survive in greater temperature ranges, as well as having a more varied diet than other butterfly fishes, this makes the threadfin butterflyfish a very easy fish to keep in an aquarium. It is colorful and probably an ideal fish for beginners. As a bonus they are also remarkably disease resistant.
They are however not reef safe and additionally it is advisable to only keep one in a tank unless one has a mated pair. Because of their territorial nature they will attack other members of their species that they deem a threat to their food source. They will readily accept most prepared foods. A tank should include hiding places and overhangs and should be fairly large as they enjoy swimming in the open.
THREADFIN BUTTERFLYFISH CLASSIFICATION
Species: C. auriga
Allen, Gerald R. 1979. Butterfly and Angelfishes of the World. Vol. 2. Aquarium Systems, OH. 203 pp.
Burgess, Warren E. 1978. Butterflyfishes of the World; A Monograph of the Family Chaetodontidae. T.F.H. Publications, NJ.832 pp.
The Reef Guide: Southern Africa’s East and South Coasts
Dennis King and Valda Fraser
The butterflyfishes: success on the coral reef
edited by Philip J. Motta