Family Acanthuridae
surgeonfishes and tangs

Most sources agree on the existence of 72 species within the Acanthuridae family but the number of genera ranges from 6 to 9, depending on the source. Acanthurids, commonly known as surgeonfishes, are characterized by the existence of the “ scalpel ,” a distinctive spine or group of spines on either side of the tail base, hence the common name surgeonfish. Color marks often emphasize the scalpels and they are important for interspecies communication, as discussed in Communication below. (Allen and Robertson, 1994; Helfman, Collete, and Facey, 1997; Nelson, 1994; Thresher, 1984; Wheeler, 1975)

Acanthurids are exclusively marine-dwelling and can be found in all tropical and subtropical seas but are absent from the Mediterranean. (Johnson and Gill, 1998; Nelson, 1994)

Acanthurids inhabit offshore coral reefs as adults but larvae are carried by the currents inshore where they quickly sink to the bottom and begin transformation to the juvenile form. Bottom-dwelling species are often found along shallow rocky shores, or exposed coral reefs in surge areas while plankton feeders are generally found well above the bottom over sandy areas. (Allen and Robertson, 1994; Johnson and Gill, 1998; Wheeler, 1975)

Guiasu and Winterbottom (1993) (from Nelson 1994) have investigated the possibility that two genera within the Acanthuridae, Prionurus (surgeonfishes, doctorfishes, lancetfishes) and Acanthurus (surgeonfishes and tangs) might not be monophyletic (from Nelson 1994). A detailed listing of synapomorphies within the suborder Acanthuroidei can be found in Tyler et al. (1989) (from Nelson 1994). (Nelson, 1994)

Acanthurids are one of several recently derived families (Blenniidae, Monacanthidae, Pomacentridae, Scaridae, Siganidae) capable of exploiting reef algae and small colonial invertebrates. Reef herbivory was primarily restricted to post-Cretaceous perciformes until these families underwent rapid evolution during the early Tertiary, 50 to 30 million years ago. (Harmelin-Vivien, 2002)

Acanthurids range in size from 20 to 200 cm and have deep, compressed bodies with small mouths adapted for nibbling and scraping small organisms from the rocks and coral. They are distinguished by a modified scale on the caudal peduncle, which forms a knife blade that is often covered with a toxic slime. In some genera, this blade exists as fixed, laterally projecting plates (Prionurus and Naso) or spines that project forward as the fish flexes its body. The pelvic fin has one spine and three (Naso and Paracanthus) or five soft rays. The dorsal fin usually has four to nine spines and the anal fin commonly has 19 to 36 soft rays and two or three spines. (Helfman, Collete, and Facey, 1997; Johnson and Gill, 1998; Nelson, 1994)

There is variation in the degree and type of sexual dimorphism exhibited by acanthurids, although permanent dimorphism is relatively uncommon. There are either size (male or female may be larger depending on the species) or morphological differences between the sexes. Only members of the genus Naso exhibit morphological differences while size difference has been reported for various species. Additionally, males tend to darken during the spawning period (see Reproduction and Communication below). (Click here to see a fish diagram). (Thresher, 1984)

The larval stage of acanthurids, termed the acronurus, differs considerably from both adult and juvenile stages. The acronurus is transparent and the scales along the ridges of the body are absent. The acronurus is planktonic and remains pelagic (in deep water) for an extended period before settling to the bottom near shore where it rapidly develops into the juvenile form. Depending on the species sexual maturity is reached after one to two years and length at maturity ranges from 10 cm, Acanthurus triostegus, to 15-19.5 cm (depending on sex and species) for some western Atlantic species, such as Acanthurus coeruleus and Acanthurus bahianus. (Johnson and Gill, 1998; Nelson, 1994; Thresher, 1984; Wheeler, 1975)

The majority of acanthurids are long-lived with many species exceeding 30 years. Acanthurids from the tropical Atlantic, especially from the Carribbean, are short-lived while those living in the West Pacific reach greater maximum ages. There is no relationship between age and size in species of acanthurids.

A key behavior in acanthurids is intraspecific competition between males when defending harems or territory. Males circle each other warily with scalpels pointed towards the opposing male during combat. As discussed above, acanthurids are organized in a wide variety of social systems at different times. They may be found in monogamous pairs, small foraging units, harems, or enormous spawning or feeding groups. During feeding, hordes of acanthurids descend on the reef so that attempts by bottom-dwellers to defend their territory are thwarted. (Allen and Robertson, 1994; Thresher, 1984)

One important form of communication in acanthurids is color changes that occur when males are aroused through intraspecific competition or spawning. The scalpel is also emphasized with bright colors and is angled at the opposing male during combat. This emphasis of the scalpel facilitates its role during competition. Additionally, individuals change colors as they become aroused, whether in combat or spawning. (Thresher, 1984)

Acanthurids have small mouths and incisor-like, lobate teeth used to probe the reef for small animals (zooplankton) and plants. Acanthurids are diurnal fishes, seeking cover at night, and most are herbivorous except one genus, Naso (unicornfish), which is planktivorous. Some herbivorous species may have heavy-walled gizzard-like stomachs as they pick up large quantities of coral and sand when feeding on short algal growths on the sea bottom. Others have thin-walled stomachs and graze mainly on algae, fronds, or filaments connected to rocky substrates and pick up very little calcareous material while feeding. (Böhlke and Chaplin, 1994; Helfman, Collete, and Facey, 1997; Johnson and Gill, 1998)

A scalpel-like spine on each side of the caudal peduncle (just in front of the tail) serves as the main defense mechanism of acanthurids. The spines make a slashing motion by powerful bursts of the tail and can inflict serious wounds on the victim. Acanthurids feed only during daylight hours and seek out reef crevices for protection at night. For most reef fishes predation pressures are highest during the planktonic life stages (Hixon 1991 from Moyle and Cech 2000). (Böhlke and Chaplin, 1994; Johnson and Gill, 1998; Moyle and Cech, 2000)

Members of the Acanthuridae family fill the roles of grazer and planktivore. Herbivores are quite important for the reef as they keep thick mats of filamentous and leafy algae from smothering the corals. They keep the mat only 1 to 2 mm thick and can strip vegetation from a 10 m wide ring around the reef (Hixon 1991; Lewis 1986 from Moyle and Cech 2000). One genus within the Acanthuridae, Naso (unicornfish), is classified as diurnal planktivores (feeding twice during the day on organisms floating in the water column). Unicornfishes mainly use the reef for shelter but “hover above it in brilliant, shifting shoals, while feeding on plankton.” These fish deposit feces in the small crevices where they hide, which is important in promoting the growth and diversity of corals. (Moyle and Cech, 2000)

No specific information was found concerning any negative impacts to humans.

Many species within this family are small and exhibit elaborate coloration. Consequently, they are popular aquarium fish. Some acanthurids are important food fish as well, but there have some reported cases of ciguatera, or fish poisoning. (Böhlke and Chaplin, 1994; Froese, Pauly, and Woodland, 2003)

Currently, there is no known conservation threat to any member of this family. (The World Conservation Union, 2002)