Acanthuridae

Diversity

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.

Geographic Range

Acanthurids are exclusively marine-dwelling and can be found in all tropical and subtropical seas but are absent from the Mediterranean.

• Biogeographic Regions
• oriental oriental
  • native native
• ethiopian ethiopian
  • native native
• neotropical neotropical
  • native native
• australian australian
  • native native
• indian ocean
  • native native
• atlantic ocean atlantic ocean
  • native native
• pacific ocean pacific ocean
  • native native

Habitat

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.

• Habitat Regions
• saltwater or marine saltwater or marine

• Aquatic Biomes
• pelagic pelagic
• reef reef

• Other Habitat Features
• intertidal or littoral intertidal or littoral

Systematic and Taxonomic History

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).

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.

Physical Description

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.

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 ).

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger
• male larger

Development

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 .

• Development - Life Cycle
• metamorphosis metamorphosis

Reproduction

Acanthurids spawn by forming individual pairs or groups of pairs, but in some species, both paired and group spawning have been observed. Paired spawning can occur in three different situations: between members of a stable pair or harem defining a common territory, between individual males with temporary spawning areas at the reef edge and passing females, and between members of a foraging group, also at the reef edge. However, group spawning is most common in acanthurids. Acanthurids exhibit color changes during spawning, but also through other forms of arousal, such as intraspecific competition (see Communication below).

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Acanthurids aggregate in huge numbers prior to spawning. Spawning can occur throughout the year in some species but peaks in late winter and early spring. The existence of harem-based social systems and consistent size differences between the sexes suggests that sequential hermaphrodism is exhibited by some acanthurids.

• Key Reproductive Features
• iteroparous iteroparous
• year-round breeding year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sequential hermaphrodite
• sexual sexual
• fertilization fertilization
  • external external
• oviparous oviparous

There is no evidence of parental care in the Acanthuridae family.

• Parental Investment
• no parental involvement

Lifespan/Longevity

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.

Behavior

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.

• Key Behaviors
• natatorial natatorial
• diurnal diurnal
• motile motile
• social social
• colonial colonial
• dominance hierarchies dominance hierarchies

Communication and Perception

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.

• Communication Channels
• visual visual
• tactile tactile

• Perception Channels
• visual visual
• tactile tactile
• chemical chemical

Food Habits

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.

• Primary Diet
• carnivore carnivore
  • eats non-insect arthropods
• herbivore herbivore
• omnivore omnivore
• planktivore planktivore

Predation

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).

Ecosystem Roles

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.

Economic Importance for Humans: Positive

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.

• Positive Impacts
• pet trade pet trade
• food food

Economic Importance for Humans: Negative

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

Conservation Status

Currently, there is no known conservation threat to any member of this family.

Other Comments

A rich fossil history from the Eocene epoch suggests that this group was more diversified in the past (Blot and Tyler 1990 from Nelson 1994 pg. 422).

Additional Links

Encyclopedia of Life

Contributors

R. Jamil Jonna (author), Animal Diversity Web.

References

Allen, G., D. Robertson. 1994. Fishes of the Tropical Eastern Pacific . Honolulu, HI: University of Hawaii Press.

Böhlke, J., C. Chaplin. 1994. Fishes of the Bahamas and Adjacent Tropical Waters . Wynnewood, PA: Published for the Academy of Natural Sciences of Philadelphia by Livingston.

Froese, R., D. Pauly, D. Woodland. 2003. "Fish Base" (On-line). FishBase World Wide Web electronic publication. Accessed May 05, 2003 at http://www.fishbase.org/ .

Harmelin-Vivien, M. 2002. Energetics and Fish Diversity on Coral Reefs. Pp. 269 in Coral Reef Fishes: Dynamics and Diversity in a Complex Ecosystem . San Diego, CA: Academic Press.

Helfman, G., B. Collete, D. Facey. 1997. The Diversity of Fishes . Malden, MA: Blackwell.

Johnson, G., A. Gill. 1998. Perches and Their Allies. Pp. 192 in Encyclopedia of fishes – second edition . San Diego, CA: Academic Press.

Moyle, P., J. Cech. 2000. Fishes: An introduction to ichthyology – fourth edition . Upper Saddle River, NJ: Prentice-Hall.

Nelson, J. 1994. Fishes of the World – third edition . New York, NY: John Wiley and Sons.

The World Conservation Union, 2002. "IUCN 2002" (On-line). 2002 IUCN Red List of Threatened Species. Accessed July 08, 2003 at http://www.iucnredlist.org/ .

Thresher, R. 1984. Reproduction in reef fishes . Neptune City, NJ: T.F.H. Publications.

Wheeler, A. 1975. Fishes of the World, an Illustrated Dictionary . London: Ferndale Editions.