Common surgeon, Paracanthurus hepatus, are strictly marine fish that typically inhabit tropical coral reefs in waters with a strong current. They may move seasonally, occurring at higher latitudes when water temperatures allow. Generally, common surgeon range between 30° north and south latitude and 32° east to 170° west longitude in the Indian and Pacific Oceans. Individuals found in other areas are presumed to have been released from aquaria.

Biogeographic Regions: indian ocean (Native ); pacific ocean (Native )

Common surgeon are characterized by the vibrant sky blue coloration of their oval-shaped bodies. Structures called iridophores on the exterior of the fish contribute to this coloration. Adults have dark narrow lines of dark blue on the dorsal half of their body. This color extends from the eye on the anterior end and continues to the posterior end. This coloration is darker near the posterior end and is black near the tail. A circular patch of sky blue coloration is located directly behind the pectoral fin. The pectoral and caudal fins are are bright yellow. The yellow extends in a "V" shape from the caudal fin to a point just beyond the caudal spine.

Coloration of common surgeon changes as they mature; juveniles are bright yellow with blue spots near their eyes, and their dorsal and anal fins are tipped in light blue. Their body becomes blue as they mature.

Adults range from 12 to 38 cm in length, averaging 25 to 31 cm. Common surgeon weigh on average 600 g. Males are typically larger than females.

Because many surgeonfish are similiar in size and color, species of surgeonfish are distinguished by the number of spines they possess. Common surgeon have 9 hard, sharp spines in their dorsal fin followed by 19 to 20 soft rays. Their anal fins have 3 spines and 18 to 19 rays. Their pectoral fins consist of 16 rays, and their pelvic fins have 1 spine and 3 rays.

Common surgeon have a razor-sharp caudal spine located at the base of their caudal fin. This spine contains toxins that can cause a debilitating pain to small predators and uncomfortable irritation and pain in humans. The caudal spine rests in a groove below the surface of the skin and can be extended from the body. Its base is attached to the vertebrae of the fish by a ligament directly connecting the two. The outer point of the spine is free to move with contraction of specific muscles. When threatened, a common surgeon extends its caudal spine and attempts to puncture the exterior of a predator.

Average mass: 600 g.

Range length: 12 to 38 cm.

Average length: 25 to 31 cm.

Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry ; poisonous ; venomous

Sexual Dimorphism: male larger

Common surgeon are strictly marine and can be found in tropical and sub-tropical coastal regions where temperatures are between 24 and 26 °C. They congregate near Pocillopora eydouxi, a type of coral with branching extensions, which serve as a protective hiding place when threatened. Reefs provide plant material, such as algae, necessary as food for common surgeon. Common surgeon remain at epipelagic depths between 2 and 40 m.

Range depth: 2 to 40 m.

Habitat Regions: tropical ; saltwater or marine

Aquatic Biomes: reef

Common surgeon are herbivorous. Unlike many marine fish, common surgeon rely only slightly on plankton. Instead, they graze on algae, using their small teeth to pull algae from rocks and coral. Fish of this species also feed on microalgae, other marine plants, and zooplankton.

Animal Foods: zooplankton

Plant Foods: algae; phytoplankton

Other Foods: detritus

Primary Diet: herbivore (Algivore)

Common surgeon feed largely on algae. Due to the small shape of their mouth, surgeonfish can easily pick and remove algae from uneven surfaces. Aggregations of common surgeon eat the fast growing algae from sponges in their habitat. This benefits the sponges and indirectly preserves habitat for species dependent on the steady growth of sponges. Midnight parrotfish mimic the coloration of common surgeon and often join their groups for protection.

If kept in an aquarium, common surgeon are vulnerable to many potentially lethal parasites. The most common of these is known as ich, paravortex, or marine spot disease, and is caused by the parasite, Cryptocaryon irritans. Ich causes dark spots along the sides of the fish, which may be difficult to detect against the dark blue coloration of this species. Common surgeon may be more prone to catching these types of diseases than other aquarium fish because they do not produce as much of the protective coating of slime as other species.

Ecosystem Impact: creates habitat

Mutualist Species:

• Scarus coelestinus

Commensal/Parasitic Species:

• Cryptocaryon irritans

Common predators of common surgeon include tuna, bar jacks, and tiger groupers.

Common surgeon possess multiple anti-predator adaptations. Their razor-sharp caudal spine is venomous and can cause debilitating pain to small predators. The effectiveness of this defense mechanism is enhanced by the tendency of this species to congregate. If a predator were to attack a group of common surgeon, it would become surrounded by surgeonfish that were thrashing their tails and slashing with their protrusible caudal spines. Common surgeon also display bright aposematic coloration, warning predators of their poisonous skin and venomous spine.

Other species take advantage of these defense mechanisms. Midnight parrotfish, for example, display a similar blue coloration and join groups of common surgeon for protection.

Known Predators:

• Tuna Thunnus albacares
• Bar Jacks Carangoides ruber
• Tiger Groupers Mycteroperca tigris

Anti-predator Adaptations: aposematic

Common surgeon can communicate by changing their coloration. This color change depends on the conditions and how they perceive their environment. Under stress, for example, their blue coloration deepens. The black marks along the body may become bleached slightly and the markings less visible. The iridiphores causing the bright blue coloration appear smaller and less iridescent, hence the darker shade of blue. Other fish in the community can detect this color change and infer potential problems. Color change also occurs during stimulation such as male dominance interactions or breeding.

The coloration around the caudal spine serves as a warning to other species. In common surgeon, the yellow triangular coloration extends just beyond the caudal spine. In other species of surgeonfish, the location of the caudal spine may even be emphasized by a color that is not otherwise present on the body of the fish.

Communication Channels: visual

Perception Channels: visual

Larvae of common surgeon hatch about 26 hours after the small eggs are laid. Larvae are severely underdeveloped and lack a heart beat at hatching. Larvae are nourished by yolk from the egg. Newly hatched larvae are buoyant but remain in a resting state until the heart begins to beat, up to 5 hours after hatching.

Two days after hatching, fins and pigment in the eyes begin to develop, and larvae begin to make short swimming movements. Development continues with jaws and the gut, and by the seventh day scales and intestines begin to form. Speed of development is related to light intensity. Larvae mature after about 37 days.

Juvenile common surgeon resemble adults, however, they differ in coloration. Juveniles also have a more rounded caudal fin than adults. Additionally, the ventral and poster tips the caudal fin in adults extend beyond the middle section of the fin.

Common surgeon can live more than 30 years in the wild. In aquariums, where they more readily acquire diseases, common surgeon generally do not live more than 20 years and more commonly survive only 8 to 12 years.

Range lifespan

Status: wild: 30+ (high) years.

Range lifespan

Status: captivity: 20 (high) years.

Typical lifespan

Status: captivity: 8 to 20 years.

Average lifespan

Status: captivity: 12 to 14 years.

Common surgeon congregate in breeding groups, composed of both males and females. These groups spontaneously form. Groups dissolve and reform several times prior to spawning. A group begins to swim upward and, at the crest of this upward movement, they release their gametes. Common surgeon are broadcast spawners; eggs and sperm are released directly into the water, and fertilization takes place externally. The quickened pace of their swimming during breeding is believed to allow for dispersal and mixing of the sperm and eggs. Eggs are then carried away by currents.

On occasion, common surgeon have been observed breeding with individual mates rather than in groups. In this case, a male's coloration may change. The male and female then circle around one another, showing off their coloration before breeding.

Mating System: monogamous ; polygynandrous (promiscuous)

Common surgeon breed during cooler months, though time of year varies with location and water temperature. In the Pacific, breeding activity is most intense from December to June. In locations where water temperature does not vary considerably with season, breeding can take place throughout the year. Breeding is assumed to peak during the summer in these locations, but common surgeon in these areas have spawning episodes throughout the year.

During months of prime temperature, females release their eggs about once a month. With each spawning event, females can release up to 40,000 eggs into the water column. High quantities of eggs and sperm make water cloudy in appearance.

Eggs of common surgeon hatch in 25 to 28 hours (average 26 hours). Larvae develop quickly and feed in great numbers off shore. Sexual maturity is not measured by age but rather by size. Males generally reach sexual maturity around 11 cm in length. Females, however, do not reach sexual maturity until about 13 cm in length.

Breeding interval: Common surgeon are believed to breed once monthly

Breeding season: Common surgeon generally breed during cooler months, from winter to early spring.

Average number of offspring: 40,000 eggs per spawning session.

Range time to hatching: 25 to 28 hours.

Average time to hatching: 26 hours.

Range time to independence: 4 to 7 days.

Average time to independence: 5 days.

Key Reproductive Features: seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (External ); broadcast (group) spawning; oviparous

There is no parental investment among common surgeon. As broadcast spawners, males and females disperse after releasing their gametes into the water column.

Parental Investment: no parental involvement; pre-fertilization (Provisioning)

Common surgeon have not been evaluated by the IUCN, the US Fish and Wildlife Service, or CITES. Nevertheless, human activities have negatively impacted the habitat of most coral reef inhabitants.

Common surgeon are popular in the aquarium trade, and have been harvested for this purpose for many years. Although overfishing has affected wild populations, common surgeon are not yet considered threatened.

US Federal List: no special status

CITES: no special status

The venomous caudal spine of common surgeon can inflict painful but minor wounds on humans. There are no other known adverse effects of common surgeon on humans.

Negative Impacts: injures humans (poisonous , venomous )

Common surgeon are common in the pet trade. After the release of the movie, "Finding Nemo", popularity for the species increased. Also known as regal tang, blue tang, blue hippo tang or the blue or palette surgeonfish, this species retails from $30 to well over $100 USD for breeding pairs.

Positive Impacts: pet trade