AcanthuridaeSurgeonfishes, tangs, unicornfishes

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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. (Allen and Robertson, 1994; Helfman, et al., 1997; Nelson, 1994; Thresher, 1984; Wheeler, 1975)

Geographic Range

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)

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. (Allen and Robertson, 1994; Johnson and Gill, 1998; Wheeler, 1975)

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. (Helfman, et al., 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)

  • 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. (Johnson and Gill, 1998; Nelson, 1994; Thresher, 1984; Wheeler, 1975)

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). (Thresher, 1984)

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. (Thresher, 1984)

There is no evidence of parental care in the Acanthuridae family. (Thresher, 1984; Thresher, 1984)

  • 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. (Allen and Robertson, 1994; Thresher, 1984)

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. (Thresher, 1984)

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. (Böhlke and Chaplin, 1994; Helfman, et al., 1997; Johnson and Gill, 1998)

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). (Böhlke and Chaplin, 1994; Johnson and Gill, 1998; 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. (Moyle and Cech, 2000)

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. (Böhlke and Chaplin, 1994; Froese, et al., 2003)

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. (The World Conservation Union, 2002)

  • IUCN Red List [Link]
    Not Evaluated

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). (Nelson, 1994)

Contributors

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

Glossary

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

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Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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Neotropical

living in the southern part of the New World. In other words, Central and South America.

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Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

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bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

diurnal
  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

herbivore

An animal that eats mainly plants or parts of plants.

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

omnivore

an animal that mainly eats all kinds of things, including plants and animals

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

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oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

pelagic

An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).

pet trade

the business of buying and selling animals for people to keep in their homes as pets.

planktivore

an animal that mainly eats plankton

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

reef

structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

visual

uses sight to communicate

year-round breeding

breeding takes place throughout the year

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 P Sale, ed. 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 W Eschmeyer, J Paxton, eds. 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.