Surgeonfish (family Acanthuridae) are an important group of reef fishes with a small mouth; a deep, compressed body; and sharp caudal spines, from which they derive their name. The family comprises 80 species, of which 73 occur in the Indo-Pacific. Many species are widely distributed as a result of their long period of development as pelagic larvae. Genera include Acanthurus, Ctenochaetus, Naso, Paracanthurus and Zebrasoma.

Convict tang (Acanthurus triostegus) grazing on a reef off Oahu.
Convict tang (Acanthurus triostegus) grazing on a reef off Oahu.

Most surgeonfish are herbivorous and travel in schools, using their small, highly adapted mouth to scrape algae and invertebrates off rocks and corals. Such grazing is critical to preventing algae from smothering corals thereby keeping coral reefs healthy. Nasos (unicornfish) are primarily planktivorous. Colorful surgeonfishes (gem tang, powered blue tang, Pacific purple tang, chevron tang and yellow tang) are popular in the aquarium trade.

The convict tang (Acanthuris triostegus) or manini, as it is referred to in Polynesia, occurs throughout the Indo-Pacific and is one of the most widespread surgeonfish in the world. In Hawaii’s coastal waters, they are the most abundant member of the surgeonfish family. The Hawaiian A. triostegus population was once referred to as Acanthurus sandvicensis due to its additional fin rays and the black streak (rather than black dots) across the fin base. A. triostegus group spawning occurs at dusk with peak spawning from 12 days before to 2 days after the full moon. The natural pelagic larval duration (PLD) is about 75 days. Juveniles settle in tidepools. Manini, which means small in Hawaiian, rarely exceed 7” in length (Randall 2007). They are popular local food fish and make attractive additions to larger saltwater aquariums.

A. triostegus juveniles were raised from eggs collected in coastal waters off Oahu’s east side in January, 2018. The eggs were collected outside of the A. triostegus peak spawning season, which is February through June in Hawaii. The larvae were raised in a 20-gallon tank (76-77F, 15:9 photoperiod) throughout most of the rearing period on cultured copepods, microparticulate diets and artemia. Juveniles were grown out in a 40-gallon aquarium and a 55-gallon barrel system.

110-day-old convict tang (Acanthurus triostegus) juveniles raised in the laboratory at Reef Culture Technologies for the Hawaii Larval Fish Project.
110-day-old convict tang (Acanthurus triostegus) juveniles raised in the laboratory at Reef Culture Technologies for the Hawaii Larval Fish Project.

A. triostegus eggs are spherical, clear, average 0.7 mm in diameter and contain a single oil globule. The newly hatched larvae measure 1.6 mm TL (total length) and lack a mouth, eyes and a digestive track. They began to feed 3 dph (days post hatch) at 2.3 mm TL. By 5 dph, the yolk-sac and most of the the oil globule is absorbed. Flexion occurs from 20-30 dph between 4.5-5.5 mm TL. Flexion larvae develop a long 1st dorsal and anal spine and a deep, triangular-shaped body. The dorsal and anal fins reach maximum relative length during postflexion. The distinctive acronurus stage (late postflexion stage), for which the Acanthurid family was once named, begins with the formation of the caudal peduncle spine (Leis and Carson-Ewart 2000). At this stage, the larval body becomes extremely laterally compressed and oblong.

Convict tang (Acanthurus triostegus) larvae raised in the laboratory at Reef Culture Technologies for the Hawaii Larval Fish Project.
Convict tang (Acanthurus triostegus) larvae raised in the laboratory at Reef Culture Technologies for the Hawaii Larval Fish Project.

A. triostegus larvae begin to settle at 93 dph when they are 23-25 mm TL. As they become more bottom-oriented, they develop black bars from the tail forward. They are fully colored after 7-20 days. Specializations to pelagic life include the elongate dorsal and pelvic spines, relatively strong head spination, early-forming larval scales, and bizarre morphology of the acronurus stage (Leis and Carson-Ewart 2000).

A. triostegus larvae are difficult to culture due to their relative small mouth size, voracious appetite, sensitivity to microbial infections and long larval period. High water flow rates, exceptional water quality and frequent additions of relatively small nutritious foods were critical to raising this species. Postflexion larvae were fed every 2 hours, on average. Feeding automation would help to reduce labor and make the rearing method more cost-effective.

Late stage A. triostegus larvae are more robust than expected. They survive surprisingly well in a small rearing tank, despite their relative large size, high activity level and long larval phase. That said, A. triostegas would likely grow and settle considerably faster in a larger tank, similar to yellow tang (Zebrasoma flavescens) and Pacific blue tang (Paracanthurus hepatus)(Callan et al. 2018, DiMaggio et al. 2017). A. triostegus, Z. flavescens and P. hepatus appear to have similar culture requirements. This is the first documented larval rearing of the genus Acanthurus.

Interesting note
The Oceanic Institute cracked the code for surgeonfish culture in 2015, raising the first ever yellow tang after many years of research. Thousands of yellow tang have since been produced and efforts are underway to commercialize the still somewhat costly culture technology.

Using a similar technique, University of Florida’s Tropical Aquaculture Laboratory raised the first Pacific blue tang in 2016, the second Acanthurid to be successfully cultured. The following year, the University of Florida’s Indian River Research and Education Center also raised blue tang at their facility.

Callan, C.K., A.I. Burgess, C.R. Rother, R. Touse. 2018. Development of Improved Feeding Methods in the Culture of Yellow Tang, Zebrasoma flavescens. Journal of the World Aquaculture Society. 10.1111/jwas.12496.

DiMaggio, M. A., E. J. Cassiano, K. P. Barden, S. W. Ramee, C. L. Ohs, and C. A. Watson. 2017. First record of captive larval culture and metamorphosis of the Pacific blue tang, Paracanthurus hepatus. Journal of the World Aquaculture Society 48: 393 – 401.

Leis, J.M. and B.M. Carson-Ewart. (editors). 2000. The larvae of Indo-Pacific coastal fishes. An identification guide to marine fish larvae. (Fauna Malesiana Handbooks 2). E.J. Brill, Leiden. (of 130 chapters, 110 are co-authored by Leis) 870 pp

Randall, J.E., 2007. Reef and shore fishes of the Hawaiian Islands. Sea Grant College Program, University of Hawai’i, Honolulu. i-xivb + 1-546.