An olive-green Fan-bellied Filefish Monacanthus chinensis at low tide amongst a meadow of Fern Seagrass Halophila spinulosa.  Singapore.
 

A pale green Fan-bellied Filefish M. chinensis is well camouflaged amongst seaweed and Spoon Seagrass Halophila ovalis.  Singapore.


 

Order : Tetraodontiformes
Family : MONACANTHIDAE
Species : Over 100 species in 26 genera

References : fishbase.org

Filefishes, or leatherjackets, comprise the family Monacanthidae, of which there are over 100 species in 26 genera. They inhabit shallow marine habitats such as seagrass meadows, coral reefs and lagoons, and tidal, shallow river estuaries. At low tide they may become stranded in shallow pools.

Many species exhibit complex colours and patterning which serves as excellent camouflage in their various habitats. For example, those associated with brightly coloured coral reefs may be brightly patterned with a confusing arrangements of colours, whilst those in seagrass beds are mottled green or brown.

Their bodies are laterally compressed to an extreme degree, and rhomboid in shape. Their skin is roughly textured, hence the name 'filefish'. Their fins are generally short and soft, and the tail is fan-shaped. On the crown is a long, articulated, sharp spine which can either be held erect in a defensive posture or laid flat.

These fishes are ungainly swimmers : those in very shallow water sometimes allow themselves to  be moved gently to and fro with the lapping of waves, such that they appear to be no more than loose clumps of seaweed.

Filefishes are omnivores, with different species adapted to different food sources from different habitats. Thus, filefishes adapted to life in seagrass meadows might include seagrass in their diet, and those inhabiting coral reefs might feed on coral. Most species, however, include various marine invertebrates in their diet.

Filefishes are pan-tropical, occurring in the Pacific, Atlantic and Indian oceans.  Numerous species occur in Southeast Asia.

Illustrated here is the Fan-bellied Filefish Monacanthus chinensis which inhabits shallow coastal habitats of Southeast Asia and the western Pacific Ocean, down to a depth of around 10 metres or so. This species may reach a maximum size of 38 cm.