Siganus insomnis

Lore & Background

Siganus insomnis was first formally described in 2014 by David J. Woodland and R. Charles Anderson, with the type locality given as Addu Atoll, Feydhoo Island in the Maldives. The populations of spinefoots similar to Siganus lineatus in the waters off the Maldives, southern India and Sri Lanka had been considered to be conspecific with S. lineatus, but differences in colour and pattern as well as genetic differences showed that this was a valid, allopatric species. The specific name insomnis means 'sleepless', a reference to its preference for nocturnal feeding.

This species is a deep-bodied rabbitfish told apart from other rabbitfishes by having the flanks (apart from possibly the belly and a narrow strip along the base of the spiny part of the dorsal fin) marked with horizontal, parallel bronze coloured lines which extend the whole length of the flank from the nape and gill slit to underneath the large yellow spot below the base of the soft rayed part of the dorsal fin. Most of these lines on the flanks are uninterrupted for the whole of their length. There may be a row of bronze spots along the base of the spiny part of the dorsal fin. In small juveniles the caudal fin is emarginate but as the fish grows it becomes more forked. This species attains a maximum total length of 35 cm.

Siganus insomnis is found in the northern central Indian Ocean in the Maldives, southern India and Sri Lanka. Adults are normally found on coral or sandstone reefs, as well as beds of seagrass. The juveniles live in large estuaries. Estuaries are important habitats for the younger fishes who move onto reefs when they mature. Adults live in schools and are largely herbivores that scrape algae from rock and coral substrates as well as browsing on seaweeds and seagrasses. It likely also eats small amounts of animal matter. Analyses of intestinal contents of specimens with standard lengths between 2 and 27 cm collected from lagoons and coastal areas of Sri Lanka showed they had consumed diatoms, green, blue-green, red and brown algae and a seagrass. This species appears to prefer feeding at night, and was apparently mostly inactive in the day.

Reader's Guide

Siganus insomnis is notable primarily for its relatively recent recognition as a distinct species in 2014, resolving a long-standing confusion with Siganus lineatus. Its discovery was based on differences in colour and pattern as well as genetic differences, highlighting the importance of detailed taxonomic work in understanding biodiversity in the northern central Indian Ocean. The species is fished for using gillnets and hand line in the Maldives and is also spearfished at night; it is also part of a major fishery in Sri Lanka. Its nocturnal feeding habit, reflected in its specific name insomnis meaning 'sleepless', distinguishes it ecologically from many other rabbitfish. The species' reliance on both estuarine habitats for juveniles and coral or sandstone reefs for adults underscores the connectivity of coastal ecosystems. Its diet, primarily herbivorous but with possible animal matter, and its schooling behavior as adults, are typical of rabbitfishes but documented specifically for this species through intestinal content analysis. The bronze-lined pattern and yellow spot are key field identification features.

Did You Know?

• Its specific name insomnis means 'sleepless', referring to its preference for nocturnal feeding.
• Juveniles live in large estuaries and move onto reefs when they mature.