Blue Tangs, Acanthurus coeruleus
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Description & Behavior
Blue tangs, Acanthurus coeruleus (Bloch and Schneider, 1801), aka blue tang surgeonfishes, blue barbers, blue doctors, blue doctorfishes, blur tangs, yellow barbers, and yellow doctorfishes, are one of 75 species in the surgeonfish Family Acanthuridae. This family of fishes are referred to as surgeonfish due to the very sharp, moveable spines on either side of their tails that resemble surgeons' scapels. Acanthurus is derived from the Greek "acantha" which means thorn, and the Greek "oura" which means tail.
Blue tangs are high-bodied, compressed, pancake-shaped fishes with pointed snouts and small scales. Their eyes are located high on their heads and their mouths are small and positioned low. Their dorsal fins are continuous. Of particular interest is their distinct yellow caudal (tail) spines located at the base of their tails on either side of their bodies, a characteristic shared with other surgeonfishes. This spine fits into a horizontal groove and can be extended and used to fend off rivals and predators.
Blue tangs reach 39 cm in length and have 9 dorsal spines, 26-28 dorsal soft rays, 3 anal spines, and 24-26 anal soft rays. Blue tangs have the most distinctive coloration of all western Atlantic surgeonfishes.
These fish have three color phases. In their juvenile phase, they are bright yellow, changing to a mixture of yellow and blue during adolescence. They may also have blue crescents above and below the pupils of their eyes. They may also be spotted with blue or have a yellow body and blue fins.
As these fish mature into the intermediate phase, their color darkens to a bright blue or purplish-gray with a yellow caudal (tail) fin. Gray longitudinal lines are located in their flank region with blue dorsal and anal fins banded with orange-brown diagonal lines. Their caudal spines become yellow to pale yellow or white. At night, blue tangs display white vertical stripes.
Adult blue tangs are deep blue to purplish-blue with yellow caudal spines. The change from juvenile to intermediate to adult coloration is not size-dependent as some fish in the juvenile yellow phase may be larger than the adult blue phase.
World Range & Habitat
Blue tangs, Acanthurus coeruleus, are marine fishes associated found on coral reefs at depths ranging from 2-40 m in tropical seas (in waters 24-26° C); between 41° N-33° S latitudes. They range from the western Atlantic: New York, USA and Bermuda to the Gulf of Mexico and Brazil and in the eastern Atlantic around Ascension Island. They are abundant in Florida, the Bahamas, and the Caribbean Sea.
On coral reefs, blue tangs hide in holes and crevices where they are sheltered from predators while they sleep at night. Blue tangs live singly, in pairs, or in small groups of up to 10 or 12 individuals, although occasionally they form large schools that forage about the reefs, grazing on algae. These schools sometimes include doctorfishes, Acanthurus chirurgus, and other surgeonfishes. Juvenile blue tangs are rarely seen on reefs due to their small size and need for constant cover from predators. Their intermediate phase (body is blue and tail is yellow) is often observed on reefs.
Feeding Behavior (Ecology)
Blue tangs feed entirely on algae. Other surgeonfishes have heavier-walled, gizzard-like stomachs, and are capable of handling ingested sand and other calcareous materials. Blue tangs are important in keeping algae populations under control, preventing algae from overgrowing and suffocating corals.
In the waters of the Fernando de Noronha Archipelago, juvenile blue tangs hold cleaning stations together with the doctorfishes, Acanthurus chirurgus, and sergeant majors, Abudefduf saxatilis, and graze algae as well as pick molted skin and parasites from green sea turtles, Chelonia mydas. This behavior is preceded by a characteristic inspection usually followed by feeding nips on the turtles' skin (head, limbs, and tail), as well as on their carapace. The turtle's most inspected and cleaned body parts are their flippers.
Blue tangs reach sexual maturity at 9-12 months of age and lengths of 11-13 cm. They show dioecism (refers to species in which sexes are always separate. Opposite of hermaphroditic and monoecious), practice external fertilization, are nest guarders, and open water/substratum egg scatterers.
Spawning occurs during late afternoon and evening hours. This event is indicated by a change in color from a uniform dark blue to a pale blue anterior and dark blue posterior. Males aggressively court female members of the school, leading to a quick upward spawning rush toward the surface of the water during which eggs and sperm are released. Their eggs are small, approximately 0.8 mm in diameter. The eggs are pelagic, each containing a single droplet of oil for flotation. The fertilized eggs hatch in twenty-four hours, revealing small, translucent larvae with silvery abdomens and rudimentary caudal spines.
The newly hatched larvae are referred to as acronurus because they were once thought to represent a separate genus of fish, Acronurus. The acronurus is diamond-shaped and laterally compressed, with a head shaped like a triangle. They have large eyes and prominent pectoral fins, and vertical ridges on their body. Their dorsal fins, anal fins, and scales begin to develop when the acronurus reaches 2-6 mm in length. Their caudal spine does not appear until the acronurus reaches about 13 mm in length. Late post-acronurus drift inshore, where they metamorphose into juveniles. The acronurus lose their silver color and turn brown, and their profiles become round. Their prominent dorsal and anal spines that are characteristic of the acronurus reduce, and their snout elongates. Complete metamorphosis takes about a week, after which two-inch long juveniles settle onto the bottom of a suitable inshore habitat.
Conservation Status & Comments
The spine on both sides of the caudal peduncle (tail stock) may inflict painful wounds.
References & Further Research
Research Acanthurus coeruleus » Barcode of Life ~ BioOne ~ Biodiversity Heritage Library ~ CITES ~ Cornell Macaulay Library [audio / video] ~ Encyclopedia of Life (EOL) ~ ESA Online Journals ~ FishBase ~ Florida Museum of Natural History Ichthyology Department ~ GBIF ~ Google Scholar ~ ITIS ~ IUCN RedList (Threatened Status) ~ Marine Species Identification Portal ~ NCBI (PubMed, GenBank, etc.) ~ Ocean Biogeographic Information System ~ PLOS ~ SIRIS ~ Tree of Life Web Project ~ UNEP-WCMC Species Database ~ WoRMS
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