Siganus fuscescens

Siganus fuscescens was first formally described in 1782 as Centrogaster fuscescens by the Dutch naturalist Martinus Houttuyn with the type locality given as Nagasaki.[3] This taxon is largely sympatric with the white-spotted spinefoot (Siganus canaliculatus) and these taxa are also very similar in appearance. Molecular analyses in 2011 found that there were three separate lineages within S. canaliculatus and S. fuscescens, that these lineages interbred and that each lineage had specimens which referred to either taxon. The authors thus concluded that S. canaliculatus is in fact a synonym of S. fuscescens.[4] This, however, is not the position taken by FishBase,[5] although it does note that the two species are sometimes confused.[2] The Catalog of Fishes, on the other hand, recognises the third lineage as the species Siganus margaritiferus, found from the Andaman Islands to the Solomon Islands, north to Japan and south to Australia.[3] Further molecular analyses have found that this may be a species complex and, while not resolving the potential synonymy of S. canaliculatus, these have suggested that there are cryptic species within S. fuscescens.[6] The specific name fuscescens means “darkened”, thought to be a reference to the dark upperbody.[7]

Siganus fuscescens has a moderately slender, laterally compressed body, the standard length being 2.3 to 2.9 times its depth. The dorsal profile of the head is weakly to notably concave over the eyes with either a blunt or a pointed snout. The front nostril has a flap which becomes shorter as the fish grows and is reduced to a small peak in the oldest fishes.[8] The dorsal fin has 13 spines and 10 soft rays while the anal fin has 7 spines and 9 soft rays.[2] The front spine on the dorsal fin points forwards. The caudal fin is nearly emarginate in smaller individuals of less than 10 cm (3.9 in) standard length becoming forked in larger fish.[8] This species attains a maximum total length of 40 cm (16 in), although 25 cm (9.8 in) is more typical.[2] The overall colour of this rabbitfish is greenish-grey to brown, fading to silvery on the lower body, and it has a large number of small light-bluish spots on the flanks, and a slender brown bar along the upper margin of the operculum. Additionally, they frequently possess a dark patch under the start of the lateral line. When they are asleep or threatened, the adults adopt a mottled pattern.[9]

Siganus fuscescens is found in the eastern Indian Ocean and the Western Pacific Ocean from the Andaman Sea east to Fiji and Samoa, north to Japan and south to Australia.[1] In Australia, its range extends from Busselton, Western Australia around the tropical northern coast to the Nadgee River in southern New South Wales. It is found around Lord Howe Island in the Tasman Sea.[9] There is a single record from the Italian Mediterranean Sea, but it is thought that this record probably refers to an individual transported by shipping.[10] It is found as deep as 50 m (160 ft) in shallow coastal waters in algae, sea grass and coral or rocky reefs.[1][8] It is frequently encountered in large estuaries.[2]

Siganus fuscescens is a schooling species and is a mostly diurnal fish. The juveniles have a diet dominated by filamentous algae while the adults prefer leafy algae and sea grass.[2] When they arrive on coral reef flats, the larvae aggregate in schools with a normal size of 200 individuals, but may hold as many as 5,000. When they have reached 3 months old, the number of fishes in a school declines to about 12.[8] Prior to spawning, this species forms aggregations of 30-60 individuals in raised areas of the inner reef flats and they spawn on the 4th or 5th day of a new moon. Spawning occurs near the edge of the reef, and each female lays around 300,000 eggs in a single spawning. Individuals will spawn in consecutive years and fish more than two year old may spawn more than once in a season.[2]

Siganus fuscescens is caught using small seine nets, set nets, fish traps, and by spearfishing. Landed adults are sold as fresh fish, but juveniles are frequently preserved as dried fish and sold in large quantities.[1]