Rivulated Rabbitfish

Siganus rivulatus, commonly known as the marbled spinefoot, rivulated rabbitfish, or surf undefined, constitutes a prominent species within the family Siganidae. This predominantly herbivorous, ray-finned fish exhibits a gregarious nature and is the designated type species of the genus Siganus. Its indigenous range includes the undefined and the 🌊 Red Sea, from where it extended into the Mediterranean Sea via Lessepsian migration through the Suez Canal (🇪🇬 Egypt).

The taxonomic nomenclature of Siganus rivulatus is rooted in New Latin, with the generic name deriving from the Arabic "sijan," synonymous with "rabbitfish" in English. The specific epithet "rivulatus" refers to the distinctive irregular streaking characteristic of this species. The species was first identified by Swedish zoologist Peter Forsskål and subsequently published posthumously in the work "Descriptiones animalium, Flora Aundefinediaco-Arabica" in 1775 by Carsten Niebuhr. Should the 2007 proposed taxonomic revision by Kuriiwa et al., which suggests dividing the genus Siganus into three sub-genera, be adopted, the classification of S. rivulatus remains unclear, as no precise designations for the new groupings have been formalized.

Morphologically, S. rivulatus exhibits a laterally compressed body length ranging from 2.7 to 3.4 times its greatest body depth. The dorsal fin comprises 14 spines and 10 soft rays, while the anal fin contains 7 spines and 9 soft rays. The species possesses 23 vertebrae and a slightly forked caudal fin. Notably, the venomous spines are slender, barbed, and can inflict painful injuries if mishandled. The integument is covered with minute embedded scales, and the coloration varies, typically featuring dark spots and yellow undulating lines on the sides. The species generally attains a length of 10–20 centimeters but can reach a maximum recorded size of 40 centimeters.

Siganus rivulatus extends from the eastern coast of Africa, from 🇿🇦 South Africa to the 🌊 Red Sea, including regions such as the Comoros, 🇲🇬 Madagascar, and the 🇸🇨 Seychelles. The species was first documented in the Mediterranean near 🇮🇱 Israel's coast in 1924 and has since become prevalent throughout the eastern Mediterranean, reaching the Sicilian Channel and portions of the Adriatic Sea.

This species is commonly found in shallow waters over algal-covered substrates, including rocky and sandy regions, as well as among seagrass beds at depths less than 15 meters. Being euryhaline, it occasionally inhabits estuarine environments. Typically forming schools of 50 to several hundred individuals, S. rivulatus favors sheltered areas and primarily consumes algae. The larvae initially feed on plankton, including diatoms and zooplankton, while adults mainly consume algae, with occasional feeding on ctenophores and scyphozoans in certain seasons in the northern 🌊 Red Sea.

Reaching sexual maturity at approximately 13.7 centimeters, S. rivulatus spawns when water temperatures range from 24–27 °C. In the Mediterranean, spawning occurs from May to July, whereas in the 🌊 Red Sea, it occurs from March to May. The species is oviparous, with external fertilization of small adhesive eggs, and lacks parental care post-spawning. Larvae are initially planktonic, progressing to form schools before migrating to deeper waters.

S. rivulatus hosts several parasites, including Balantidium sigani, Bomolochus parvulus, and various monogeneans and digeneans. A study in 2019 indicated the presence of Gyliauchen volubilis, Procamallanus elatensis, and Sclerocollum rubrimaris, with the latter accumulating trace metals in chronically polluted areas.

In fisheries, S. rivulatus is an important species, particularly in artisanal practices, primarily captured using beach seines and gillnets, although stability in population numbers is generally observed. Nonetheless, concerns about overexploitation exist in certain regions, as indicated by data from the 🌊 Red Sea.

It is important to note that S. rivulatus has been associated with ciguatoxin-like toxins, causing poisoning incidents, particularly in polluted waters in 🇮🇱 Israel, and has been reported to induce hallucinations upon consumption.