Filefish

Filefish belong to the family Monacanthidae. They are a diverse group of marine fish found in tropical and subtropical waters. These fish inhabit the Atlantic, Pacific, and Indian Oceans. You may hear them called foolfish, leatherjackets, or shingles. The family includes approximately 102 different species organized into 27 genera.

Interestingly, more than half of these species live in Australian waters. This includes 58 species across 23 different genera. They are closely related to other fish like pufferfish, trunkfish, and triggerfish.

The physical structure of a filefish is quite unique. They have rhomboid-shaped bodies that are laterally compressed. This means they are thin from side to side. Their skin is rough and covered in non-overlapping scales with small spikes. This sandpapery texture is why they are called filefish. They also have small terminal mouths with specialized incisor teeth. The upper jaw typically features four teeth in an inner series and six in an outer series. The lower jaw has four to six teeth in an outer series only.

A key mechanism for survival involves their specialized spines and bones. Most filefish have a slender, retractable spine on their head. While they often have two spines, the second is greatly reduced. This smaller spine serves to lock the first spine in an upright position. This trait is the origin of their scientific name, Monacanthidae, which comes from the Greek words for "one" and "thorn." Some species also possess recurved spines on the caudal peduncle, which is the base of the tail. Additionally, they have elongated pelvic bones that create a "dewlap." This is a flap of skin that can be moved up or down. By moving the pelvis, the fish can make its body appear much deeper than it actually is. This helps them look larger to potential predators.

Evolutionary studies provide insight into how these fish developed. Phylogenetic studies show that triggerfishes are the sister group to filefish. These two groups diverged from each other during the Middle Eocene epoch. Although they look similar, filefish are more ecologically diverse than triggerfish. They also possess more than double the species richness. This high level of diversity in shape and size may be due to their history. The crown monacanthids likely originated during the Late Eocene. In contrast, the crown balistids originated later, during the late Miocene.

Filefish occupy various habitats in the ocean. Most adults live in shallow waters, usually at depths of no more than 30 meters. They can be found in lagoons, seagrass beds, or seaward reefs. Some species even enter estuaries. Certain filefish, like the plane head filefish, live among dense mats of Sargassum seaweed. They use colors and patterns to match these weedy environments. Because they have small fins, they are not fast swimmers and move at a sluggish pace. Some species are solitary, while others live in pairs or small groups. Some filefish even drift head downward in seaweed to hide from predators.

The diet of a filefish depends on its specific species. Some only eat algae and seagrass. Others consume small benthic invertebrates like hydrozoans, gorgonians, or tunicates. Some are even corallivores, meaning they eat coral. Their life cycles involve specific mating behaviors. Males prepare and guard bottom sites for spawning. Depending on the species, either the male or both parents may guard the brood. The young filefish are pelagic, meaning they live in the open water. During this stage, they face risks from predators like tuna and dolphinfish. The smooth leatherjacket is notable for its long life, living up to 19 years in the wild.

Filefish play a significant role in global fisheries and food cultures. In recent years, the Cantherhines genus has seen annual landings of around 200,000 tonnes, mostly in China. In Australia, the ocean leatherjacket is a very important species. It is the second most valuable species in the New South Wales demersal trap fishery. Another species, the unicorn filefish, recently became the subject of a new fishery in the Arabian Sea and Bay of Bengal. In Korea, the threadsail filefish is a popular food. It is often made into a sweet and salty jerky called jwipo, which is dried and roasted.