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SiganidaeRabbitfishes

By R. Jamil Jonna

Diversity

The Siganidae family is composed of one genus, Siganus, and two subgenera, Siganus with 22 species and Lo with five species (Woodland (1990) from Nelson 1994). Siganids get their common name, rabbitfishes, from their peaceful temperament, rounded blunt snout, and rabbit-like appearance of the jaws. They are important reef herbivores that browse individually or in schools over the reef or feed on plankton within the water column. (Nelson, 1994; Wheeler, 1975)

Geographic Range

Siganids are naturally confined to the tropical Indo-Pacific, but are now found in the eastern Mediterranean as well. Siganus rivulatus is at least one species that has been able to penetrate from the Red Sea through the Suez Canal to the Mediterranean, where it is now locally common. (Johnson and Gill, 1998; Nelson, 1994; Wheeler, 1975)

Habitat

Siganids are marine and mainly inhabit reefs, shallow lagoons, sea grasses or mangrove areas. They can be found along reef edges with broken rock, reef flats with scattered coral heads or near grass flats, and often come into very shallow waters to feed in algae. They are rarely found in estuaries, and only one species, Siganus vermiculatus, is truly estuarine. Some primarily estuarine species have been successfully introduced into freshwater lake and pond habitats as well. (Helfman, et al., 1997; Nelson, 1994; Thresher, 1984; Wheeler, 1975)

Systematic and Taxonomic History

A detailed phylogenic analysis of the suborder Acanthroidei was completed by Tyler et al. (1989) and should be consulted for a list of synapomorphies (from Nelson 1994). (Nelson, 1994)

Siganids are one of several recently derived families (Acanthuridae, Monacanthidae, Pomacentridae, Scaridae, Blenniidae) 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)

Physical Description

The siganids have many strong spines with venom glands in dorsal and anal fins that contain a painful toxin. Most siganids are countershaded, but some reef species, Siganus vulpinus, have coloration similar to butterflyfishes. The teeth of siganids are compressed into a single row and asymmetrically bicuspid. The pelvic formula is unique (I, 3, I,) reflecting the hard spines at either end of the fin. The dorsal fin has 13 spines and 10 soft rays and the anal fin has 7 spines and 10 soft rays. There are 23 vertebrate and the maximum length is approximately 50 cm. (Click here to see a fish diagram). (Helfman, et al., 1997; Johnson and Gill, 1998; Nelson, 1994)

Rabbitfishes are quite colorful and can be easily identified during daylight hours. However, at night or when threatened, they change drastically as color fades and dark blotches appear. Similarly, at death colors fade rapidly, making identification after preservation difficult. Although there are no significant differences between the sexes in this group, females are larger than males in some, if not all, species. (Kuiter, 1993; Thresher, 1984)

  • Sexual Dimorphism
  • sexes alike
  • female larger

Development

Members of the Siganidae family first produce small adhesive egg sacs, which then become larvae. The larval stage is planktonic and develops into a distinctive post-larval stage called the acronurus, which is characteristic for members of the suborder Acanthuroidei. In the acronurus stage the body is transparent and individuals remain pelagic for an extended period before settling into the adult habitat and rapidly changing into the juvenile form. There is considerable morphological difference between larvae and adults and current information suggests that males reach sexual maturity before females throughout the family. (Johnson and Gill, 1998; Nelson and Wilkins, 1993; Nelson, 1994; Thresher, 1984; Wheeler, 1975)

Reproduction

Siganids characteristically school in small to large groups, with some species, such as Siganus rivulatus and Siganus luridus, breaking off into pairs or small units after spawning begins. Other members of the Siganidae family, such as the foxface, form monogamous pairs. Individual pairs or groups behave aggressively towards one another resulting in wide spacing throughout the reef during spawning. Just before gametes are released, most siganids move in a circular pattern and the males develop a marble color pattern. (Thresher, 1984)

Before spawning, siganids migrate to traditional spawning areas, with the location varying among species. Spawning peaks in spring and early summer, and, as with many other coastal species, siganids show a prominent lunar rhythm. Spawning usually takes place at night or early morning and coincides with outgoing tides. Siganid larvae also respond to the lunar cycle, as most appear inshore (after the initial pelagic stage) three to five days before the new moon. (Thresher, 1984)

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

  • Parental Investment
  • no parental involvement

Lifespan/Longevity

Generally, smaller reef fishes such as siganids live between three and five years. (Moyle and Cech, 2000)

Behavior

Siganids are commonly known as rabbitfishes partly because of their peaceful temperament. They are diurnal herbivores, hiding in reef crevices during nighttime and browsing over reefs to feed during the day. Some species school while others browse individually among corals. (Dr. Foster and Dr. Smith, 2000; Froese, et al., 2003; Moyle and Cech, 2000)

Communication and Perception

No specific information was found concerning communication methods used by this group.

  • Communication Channels
  • visual

Food Habits

Most siganids are herbivorous and feed on phytoplankton or attached algae. (Johnson and Gill, 1998; Nelson, 1994; Wheeler, 1975)

Predation

Siganids are most threatened by predation during the planktonic, larval stage and very few larvae survive. On reefs, where most siganids live, predation is the most important cause of death (Hixon, 1991 in Moyle and Cech, 2000). The elaborate defenses in the form of poisonous spines are a testament to predation pressures. The sharp, strong spines are coated with a mucous mixed with venom and can inflict painful wounds. As discussed in physical description above, rabbitfishes lose their color at nightfall and may also change color if threatened. (Kuiter, 1993; Moyle and Cech, 2000; Wheeler, 1975)

Ecosystem Roles

All siganids are diurnal herbivores and fill the roles of grazer and planktivore. Herbivores are quite important for the reef because 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. Other siganids use the reef mainly 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 (Hixon 1991; Lewis 1986 in Moyle and Cech 2000). (Froese, et al., 2003; Moyle and Cech, 2000)

  • Ecosystem Impact
  • creates habitat

Economic Importance for Humans: Positive

Some siganids are important food fishes in many areas and colorful species are popular in the aquarium trade. The fast growth rate and shallow browsing habits of siganids make them ideal for aquaculture, as evidenced by numerous studies on their growth and reproduction. (Froese, et al., 2003; Kuiter, 1993; Thresher, 1984)

Economic Importance for Humans: Negative

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

  • Negative Impacts
  • injures 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

The fossil history of the Siganidae family contains three known fossil genera. From the Eocene epoch there is Ruffoichthys from Italy and Siganopygaeus from Turkmenistan. From the Oligocene epoch there is Archaeoteuthis from Switzerland (Tyler and Sorbini, 1990 from Nelson 1994). (Nelson, 1994)

Contributors

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

Glossary

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.

World Map

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

aposematic

having coloration that serves a protective function for the animal, usually used to refer to animals with colors that warn predators of their toxicity. For example: animals with bright red or yellow coloration are often toxic or distasteful.

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.

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

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

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

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

folivore

an animal that mainly eats leaves.

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).

monogamous

Having one mate at a time.

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.

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.

polymorphic

"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.

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

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

venomous

an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).

vibrations

movements of a hard surface that are produced by animals as signals to others

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.

Dr. Foster, , Dr. Smith. 2000. "PetEducation.Com" (On-line). Drs. Foster and Smith's source for expert pet information. Accessed May 05, 2003 at http://www.peteducation.com/.

Froese, R., D. Pauly, D. Woodland. 2003. "FishBase" (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. 268-269 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. 191 in W Eschmeyer, J Paxton, eds. Encyclopedia of Fishes – second edition. San Diego, CA: Academic Press.

Kuiter, R. 1993. Coastal fishes of South-eastern Australia. Honolulu: University of Hawaii 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.

Nelson, S., S. Wilkins. 1993. Growth and respiration of embryos and larvae of the rabbitfish Siganus randalli (Pisces, Siganidae). Journal of Fish Biology, 44: 513-525.

The World Conservation Union, 2002. "IUCN 2002" (On-line). 2002 IUCN Red List of Threatened Species. Accessed May 05, 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.

Woodland, D. 1990. Revision of the fish family Siganidae with descriptions of two new species and comments on distribution and biology. Indo-Pacific Fishes, 19: 136.

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