Forktail Rabbitfish

The streamlined spinefoot (Siganus argenteus), also known as the forktail rabbitfish, schooling rabbitfish, or silver spinefoot, is a marine ray-finned fish species within the family Siganidae, commonly located in the Indo-Pacific region.

Originally described in 1825 under the name Amphacanthus argenteus by French naturalists Jean René Constant Quoy and Joseph Paul Gaimard, the type locality for this species is recognized as 🇬🇺 Guam in the Marianas. The specific epithet "argenteus" refers to the fish's silvery coloration, especially noted on its cheeks and lower body.

Characterized by an oval, compressed, slender, and fusiform body, the streamlined spinefoot has a standard length ranging from 2.4 to 3 times its body depth. Its small head is marked by a pointed snout rather than a steep dorsal profile. Additionally, a notable feature is the flap at the front nostril that extends beyond the rear nostril. The species presents a dorsal fin with 13 spines and 10 soft rays, and an anal fin with 7 spines and 9 soft rays, alongside a deeply forked caudal fin. Typically reaching up to 40 cm (16 in) in length, although more commonly found at 25 cm (9.8 in), the species exhibits blue coloration on the upper body with a silvery lower body, featuring variable patterns, spots, and curved lines, particularly along the lower flanks. The eye displays a silvery-yellow iris, and the pectoral fin's axil is yellow. Additionally, a yellow stripe usually runs along the dorsal fin base, with a dark brown bar present on the upper edge of the operculum. When alarmed or resting, this fish transitions to a pale and dark brown mottling, creating seven diagonal zones across the flanks, with mottled fins.

The streamlined spinefoot enjoys a broad Indo-Pacific distribution, spanning from the 🌊 Red Sea and extending southward to 🇲🇿 Mozambique and 🇲🇬 Madagascar, east to Pitcairn Island in the 🌊 Pacific Ocean, further north to 🇯🇵 Japan, and south to 🇦🇺 Australia. Within 🇦🇺 Australia, the species is found from Shark Bay in Western Australia to the Ashmore Reef in the Timor Sea and on the east coast from the northern Great Barrier Reef near Cape York down to Moreton Bay in Queensland. Additionally, it is present on reefs in the Coral Sea and at the Cocos (Keeling) Islands. There has even been a singular recording off Libya in the Mediterranean Sea, though the means of its arrival remain uncertain. On isolated islands such as Rapa, Pitcairn, and the Line Islands, it is often the sole rabbitfish species. It typically inhabits depths up to 40 m (130 ft) in lagoons, reefs, seaweed, and seagrass beds.

In terms of behavior, the streamlined spinefoot forms large, fast-swimming schools that reside in the water column above the seabed, periodically diving to the substrate to feed. Juveniles and adults can be observed in smaller schools comprising 2–100 individuals near coral reefs, particularly within the surge zones. Juveniles tend to occupy surface waters in dense schools several kilometers offshore, transitioning to reef flats prior to metamorphosis. Their diet is primarily composed of red and green macroalgae, with the ability to consume the chemically rich macroalgae Chlorodesmis fastigata, albeit opportunistically and likely as a minor dietary component. Notably, their spines are capable of delivering venom to potential attackers.

Distinct from other rabbitfish, the streamlined spinefoot engages in pelagic spawning and features a relatively prolonged larval stage along with a unique prejuvenile stage. These extended developmental phases facilitate widespread dispersion and reduce the species’ susceptibility to overfishing compared to its congeners.