Family Siganidae
rabbitfishes and spinefishes

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. ()

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. ()

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. ()

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). ()

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. ()

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). ()

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. ()

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. ()

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

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. ()

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

Most siganids are herbivorous and feed on phytoplankton or attached algae. ()

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. ()

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). ()

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

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. ()

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