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)
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)
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)
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
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)
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)
- Key Reproductive Features
- year-round breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
There is no evidence of parental care in the Siganidae family. (Thresher, 1984)
- Parental Investment
- no parental involvement
Generally, smaller reef fishes such as siganids live between three and five years. (Moyle and Cech, 2000)
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
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)
- Known Predators
- humans (Homo sapiens)
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
- bites or stings
Currently, there is no known conservation threat to any member of this family. (The World Conservation Union, 2002)
- IUCN Red List [Link]
- Not Evaluated
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)
R. Jamil Jonna (author), Animal Diversity Web.
- 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.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
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.
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
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.
- active during the day, 2. lasting for one day.
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
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
union of egg and spermatozoan
an animal that mainly eats leaves.
A substance that provides both nutrients and energy to a living thing.
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.
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).
Having one mate at a time.
having the capacity to move from one place to another.
specialized for swimming
- native range
the area in which the animal is naturally found, the region in which it is endemic.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
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.
an animal that mainly eats plankton
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
"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.
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.
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
uses touch to communicate
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).
movements of a hard surface that are produced by animals as signals to others
uses sight to communicate
- year-round breeding
breeding takes place throughout the year
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