Convict Surgeonfish

Convict Surgeonfish, Acanthurus triostegus

The Convict Surgeonfish, Acanthurus triostegus, whose common Spanish name is cirujano reo, is a member of the Surgeonfish or Acanthuridae Family, which are known collectively as cirujanos in Mexico. This fish is also known as the Convict Tang. Globally, there are forty-two species in the genus Acanthurus, seven of which are found in Mexican waters, three in the Atlantic and four in the Pacific.

The Convict Surgeonfish have deep oval highly compressed bodies with a depth that is 50 to 52% of standard length. They are light olive to almost white in color. They have six vertical black stripes on their sides: one on their head passing through their yellow eyes, four on their body, and one at the base of their caudal fin. Males and females are very similar in appearance, although males assume courtship colors when breeding. Juveniles lack the bars found in adults. They have a steep head profile with eyes located high on the head. They have a small mouth low on the head and a protrusible jaw with serrated teeth on both sides. Their anal fin has 3 spines and 19 to 22 rays; their caudal fin is slightly concave; and their dorsal fin has nine spines and 22 to 26 rays. They have short caudal peduncles and their razor sharp peduncle spines are rather small when compared to those of other tangs. They have 18 to 22 gill rakers in the anterior row and 19 to 24 gill rakers in the posterior row. Their lateral line is complete and their body is covered with tiny, rough, and tightly-spaced scales.

The Convict Surgeonfish are found in shallow and wave-affected areas in and around coral reefs at depths up to 300 feet with juveniles taking up residence in tidal pools and coral reefs. They reach a maximum length of 27 cm (11 inches). They prefer water temperatures between 24ºC (75ºF) and 26ºC (79ºF). They feed primarily on algae and are classified as grazers. They also consume benthic invertebrates on a limited basis. All developmental stages (eggs, larvae, juveniles, and adults) are preyed upon by larger fish, such as eels, groupers, snappers, and various other marine animals. They can be found as solitary individuals or in small and large schools. They can detect vibrations and possess a strong sense of smell and sound. They utilize large schools and disruptive color patterns as defense mechanisms. They can also feign death by exhibiting tonic immobility to avoid predation. They are hosts to symbiotic unicellular organisms in their gut, including one of the largest known bacterial species, Epulopiscium fishelsoni. They also host endoparasitic nematodes and trematodes, as well as at least one ectoparasitic copepod species. They are oviparous with annual breeding cycles that include large spawning aggregations numbering up to tens of thousands of fish. They are broadcast spawners with the release of pelagic eggs that are fertilized externally. During spawning, clouds of eggs and sperm are preyed upon by various predatory fish. Eggs develop rapidly and hatch into clear pelagic larvae. The larvae transition into juvenile within three months, which settle out within the reef or in tidal pools. As they mature they eventually leave the tidal pools and roam in the reefs until they join schooling groups. Adults provide no parental care to their offspring. They have a lifespan of five to seven years in captivity, however, their lifespan in the wild has not been determined.

In Mexican waters the Convict Surgeonfish has a limited distribution being found found from La Paz southward to Cabo San Lucas along the southeast coast of Baja and from Mazatlán southward to Guatemala along the coast of the mainland.

Due to its light-colored body and pattern of dark bars on the sides, the Convict Surgeonfish cannot be confused with any other species.

The Convict Surgeonfish are utilized as a food source by many cultures and are fished commercially. They are also targeted by recreational fishermen in some locations. They are considered quality food fish in some areas but in other areas are known to contain Cigua Toxin. They are a popular fish in the aquarium trade, however, require aquarium sizes of at least 75 gallons and are hostile towards other fish. They are available online at prices between $22.95 and $49.99 per fish. From a conservation perspective they are deemed of Least Concern, however, in some locations they are of concern due to overfishing and in others due to the loss of coral reef area and coral reef habitat.

A word of caution. The spines found at their tail base provide these fish with a unique defense mechanism rendering them dangerous to handle as these spines can inflict major slashing wounds.

Convict Surgeonfish, Acanthurus triostegus. Fish caught from coastal waters off He’eia, Oahu, Hawaii, February 2016, length: 15.2 cm (6.0 inches). Catch, photo and identification courtesy of Kenneth Tse, Toronto, Canada.

Convict Surgeonfish, Acanthurus triostegus. Underwater photo taken in the greater Cabo San Lucas area, Baja California Sur, November 2017. Length: ca. 20 cm (7.9 inches).

Convict Surgeonfish, Acanthurus triostegus. Underwater photo taken in the greater Cabo San Lucas area, Baja California Sur, November 2017. Length: ca. 20 cm (7.9 inches).