Surgeonfish: Characteristics, Behavior, Mating

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lined surgeonfish in (Acanthurus lineatus) in Reunion

Surgeonfish are some of the most visually striking fish seen in reefs. They are vividly colored and have striking patterns and stripes. They are sought after by divers and snorkelers and are popular aquarium fish. Surgeonfish derive their name from scalpel-like spine or spines on the side of their tail. Large, colourful schools of surgeonfish are a frequent sight on the reef. Dory in Disney’s “Finding Nemo” and “Finding Dory” was a surgeonfish. [Source: Great Barrier]

Surgeonfish begin life as transparent spuds, turns yellow during adolescence and become deep royal blue when they reach adulthood. The largest bacterium known to man is visible to the naked eye and lives in the intestinal tract of surgeonfish found in the Red Sea and the Great Barrier Reef.

Surgeonfish have relatively high compressed bodies. The spiked blades and venomous fin spines which protrude from their bodies and are used both as a defense against predators and a weapon in territorial fights with rival surgeonfish. The spines are delivered with quick sideways swipes of their tail and can leave behind nasty wounds. Unicornfish are members of the surgeonfish family. They are so named because of the hornlike projection on their forehead. Many species feed on leafy algae attached to the seabed.

Acanthuridae — the Surgeonfish, Tang Unicornfish Family

Acanthuridae is the scientific name for the family of fish that includes surgeonfishes, tangs, and unicornfish. There are 72 species within the Acanthuridae family, with the the number of genera ranging from six to nine, depending on the source. Acanthurids are characterized by the existence of a “scalpel” — a distinctive spine or group of spines on either side of the tail base, the source of name surgeonfish. Color marks often emphasize the scalpels and they are important for interspecies communication. The name Acanthurus means “thorn tail”, a reference to the scalpel-like spines..[Source: R. Jamil Jonna, Animal Diversity Web (ADW) /=]

yellow tang

The majority of acanthurids have long life spans with members of many species living over 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. /=\

Acanthurids are exclusively saltwater fish and can be found in all tropical and subtropical seas — including the Atlantic, Pacific and Indian Oceans — but not the Mediterranean Sea. They inhabit offshore coral reefs as adults. Larvae are carried by the currents inshore where they quickly sink to the bottom where they begin transforming into 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. In the reef ecosystem Acanthuridae family members fill the roles of grazers and planktivores, which are important for the reef as they keep thick mats of filamentous and leafy algae from smothering the corals. They keep the mat only one to two millimeters thick and can strip vegetation from a 10 meters wide ring around the reef.

An extensive assemblage of fossils from the Eocene Period (56 million to 33.9 million years ago) suggests that Acanthuridae was more diversified in the past. Acanthurids are one of several animal families capable of exploiting reef algae and small colonial invertebrates. Reef herbivores were primarily restricted to post-Cretaceous (145 million to 66 million years ago) perciformes until Acanthuridae and other families underwent rapid evolution during the early Tertiary, 50 to 30 million years ago. /=\

Humans utilize Acanthuridae for food and in the pet trade. Many species are small and have elaborate coloration and thus are popular aquarium fish. Some acanthurids are important food but there have some reported cases of ciguatera, or fish poisoning caused by them. Currently, there is no known conservation threat to any member of this family (The World Conservation Union, 2002). They have not been evaluated for the International Union for Conservation of Nature (IUCN) Red List.

Acanthurid Characteristics

short-nosed unicornfish (Naso brevirostris)

Acanthurids range in size from 20 to 200 centimeters (8 to 80 inches) and are cold blooded (ectothermic, use heat from the environment and adapt their behavior to regulate body temperature) and have bilateral symmetry (both sides of the animal are the same). Sexual dimorphism (differences between males and females) occurs: Females usually are larger than males but sometimes males are larger. Males tend to darken during the spawning period [Source: R. Jamil Jonna, Animal Diversity Web (ADW) /=]

Acanthurids have deep, compressed bodies with small mouths adapted for nibbling and scraping small organisms from the rocks and coral. According to Animal Diversity Web: They are distinguished by a modified scale on the caudal peduncle (just in front of the tail), which forms a knife blade that is often covered with a toxic slime. In some genera, this blade exists as fixed, laterally projecting plates or spines that project forward as the fish flexes its body. Some species of Acanthurus may have venom associated with the spine as well. The pelvic fin has one spine and three 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. The scalpel-like spine on each side of the caudal peduncle 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 victim. Because many surgeonfish are similiar in size and color, species of surgeonfish are distinguished by the number of spines they possess. /=\

Acanthurids communicate with vision and touch and sense using vision, touch and chemicals usually detected with smell. One important form of communication 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. Individuals may change colors as they become aroused, whether in combat or spawning. /=\

Acanthurid Behavior and Feeding

Acanthurids are diurnal (active mainly during the daytime), motile (move around as opposed to being stationary), social (associates with others of its species; forms social groups), colonial (living together in groups or in close proximity to each other), and have dominance hierarchies (ranking systems or pecking orders among members of a long-term social group, where dominance status affects access to resources or mates). [Source: R. Jamil Jonna, Animal Diversity Web (ADW) /=]

Acanthurids have small mouths and incisor-like, lobate teeth used to probe the reef for small animals and plants. They are mostly herbivorous. 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.

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 "Coral eating" fish such as parrotfish and surgeon fish feed on the algae rather than the coral polyps themselves. Many do this by eating the rocky coral itself, extracting the food the need and grinding up rocky material and expelling it as sand. The source of much of the recently made sand sediments around the reef is from parrotfish and other coral eaters.

Around dusk at certain times of the year, surgeonfish suddenly stop eating and organize themselves into long chains, with individuals about half a meter from one another, and head to the open ocean side of the reef. Many chains of surgeonfish join together to form dome-shaped school with around 2,000 fish that begins an unusual reproduction ritual. [Source: Joseph Levine, Smithsonian magazine and the book “The Coral Reef at Night” ]

Acanthurid Mating, Reproduction and Offspring

Acanthurids are oviparous (young are hatched from eggs) and iteroparous (offspring are produced in groups). They may engage in year-round breeding, with peaks in late winter and early spring, and are sequential hermaphrodites in which individuals change their sex at some point in their lives and typically produces eggs and sperm at different stages their lives. Reproduction is external, meaning the male’s sperm fertilizes the female’s egg outside her body. Acanthuridae are polygynandrous (promiscuous), with both males and females having multiple partners. There is no evidence of parental care in the Acanthuridae family.

According to Animal Diversity Web: 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. /=\

Acanthurids aggregate in huge numbers prior to spawning. The existence of harem-based social systems and consistent size differences between the sexes suggests sequential hermaphrodism. 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.

Describing the mating ritual of surgeonfish in the Red Sea, Joseph Levine wrote in “The Coral Reef at Night”, "As the light dims noticeably, a dozen individuals gather more closely together within the pulsing mass. They separate slightly from the group as the dome reaches it peak. Finally, as though released from a tautly strung bow, first one group, then two groups, then five, sharply fling themselves toward the surface, pivot sharply and dash back down into the milling swarm. At the apex of each group's ascent, its members leave something behind small clouds of transparent eggs and white sperm that float slowly away...After the mating dance, the surgeonfish return single file to their reef haunts and dart into their favorite hiding place.

Mating and fertilization often occur as the sun is getting ready to set. Why do surgeon fish and other reef species spawn at dusk and loose themselves in what they are doing when they are most vulnerable to predators? They do it for their offspring eggs and larvae which are fed on by sharp-eyed daytime reef fish that see less well and seek cover in the evening.

The life cycle of Acanthurids is characterized by metamorphosis — a process of development in which individuals change in shape or structure as they grow. 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 (living in the open ocean, far from land) (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 centimeters, Acanthurus triostegus, to 15-19.5 centimeters (depending on sex and species) for some western Atlantic species, such as Acanthurus coeruleus and Acanthurus bahianus. /=\

Image Sources: Wikimedia Commons; YouTube, Animal Diversity Web, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated March 2023

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