Blue TangWhere and when to see them

Divers can easily observe blue tangs during daylight hours as they actively roam shallow reef flats and coral crests. When approaching schooling or feeding groups, maintain calm, neutral buoyancy and avoid sudden movements, as blue tangs easily move off if threatened. Be cautious around their caudal peduncle spines, which are razor-sharp and capable of inflicting painful wounds if a fish is cornered or handled. *Paracanthurus hepatus* also bears venomous fin spines.

Keep an eye out around coral outcrops for cleaning stations. Blue tangs act as both clients and cleaners. When visiting cleaner gobies, tangs adopt a distinctive headstand posture to signal readiness for parasite removal. At other stations, blue tangs act as cleaners for green sea turtles (*Chelonia mydas*), hovering around the turtle's flippers, head, and carapace to nip off ectoparasites and molted skin.

Night divers will find blue tangs resting motionless inside dark coral holes and crevices. During sleep, Atlantic blue tangs exhibit a stress or resting pattern marked by conspicuous white vertical stripes along their bodies. Always avoid shining dive lights directly into sleeping crevices to refrain from disturbing their rest.

Blue tangs can be encountered year-round on warm tropical reefs throughout their respective native ranges. However, specific seasonal phenomena offer unique observation opportunities for scuba divers. In Atlantic regions such as Florida, the Bahamas, and the Caribbean, spawning activity for *Acanthurus coeruleus* peaks during the winter months. Massive spawning aggregations gather three to eight days following the full moon, offering divers a chance to observe dramatic courting behaviours.

During these winter spawning events, small groups travel from adjacent reefs to coalesce into large aggregations numbering over one hundred individuals. Mating readiness is visually signaled as adults alter their body coloration, turning pale blue on their front half and dark blue on their rear half. Courting groups perform rapid upward spawning rushes toward the surface in the late afternoon to release eggs and sperm into strong offshore currents.

In Indo-Pacific locations like the Coral Sea and Australia's Great Barrier Reef, dense schools of juvenile *Paracanthurus hepatus* are commonly spotted during spring and late spring months (such as November at Osprey Reef) seeking cover among branching hard corals at shallow depths.

Blue tangs inhabit tropical marine environments ranging from 41° N to 33° S, living at depths between 2 and 40 metres in ocean temperatures typically between 24°C and 26°C. *Acanthurus coeruleus* ranges across the western Atlantic Ocean from New York and Bermuda down through Florida, the Gulf of Mexico, the Bahamas, the Caribbean Sea, and south to Brazil, as well as Ascension Island in the mid-Atlantic and the Mediterranean Sea (recorded since 2011). *Paracanthurus hepatus* occurs across the Indo-West Pacific, including nearly the entire length of the Great Barrier Reef and the Coral Sea.

Their primary habitat centers on hard-coral reefs, though they are frequently found near soft corals, coral rubble, algal beds, and seagrass beds. Juveniles heavily depend on high biogenic structure and dense coral cover to evade predators. Adults utilize open reef zones, while breeding aggregations specifically gather over flat, sandy substrate patches between coral heads where 6 to 10 metre depths and strong currents help sweep pelagic eggs out to sea. At night, individuals seek refuge inside tight coral holes and crevices.

Adult blue tangs are primarily herbivorous grazers. In the Atlantic, *Acanthurus coeruleus* feeds mainly on filamentous algae, detritus, and plankton, whereas Indo-Pacific species like *Paracanthurus hepatus* consume zooplankton alongside algae and marine plants. Because blue tangs possess digestive systems efficient at absorbing crushed cellular contents but poor at processing cellulose, they must forage heavily and continuously throughout the day on abundant, fast-growing algae. High juvenile feeding rates are required due to this low food utilization efficiency.

By constantly cropping algae from coral substrates, blue tangs perform a vital ecological service, preventing fast-growing algal turf from suffocating live coral colonies. Their population density and territory size are directly tied to food availability; lower-structure reefs require larger home ranges to provide sufficient forage. In addition to substrate grazing, blue tangs exhibit cleaning behavior, feeding on molted skin and ectoparasites picked off client animals like green sea turtles (*Chelonia mydas*).