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Issue: September 2015

Those Fabulous Filefish! (FULL)

Author: Scott W. Michael


Photographer: Scott W. Michael
Submerged in shallow tropical waters, coral reefs are the natural habitat for a quarter of all marine species. Though maintaining a coral reef system is a common challenge for many aquarists, some of the most colorful and popular fish in the hobby are drawn from these calcified sanctuaries. While coral reefs offer a wide range of diverse species, only five families comprise the bulk of reef communities: gobies, wrasses, groupers, damsels, and cardinalfish.

While these may be the most abundant groups found there, and some of the most prominent selections in aquarium stores, many other families are also noteworthy. One that is often overlooked by aquarists, which contains some very interesting species, is the filefish clan of the Monacanthidae family. Considered to be one of the most highly “advanced” bony fishes, filefish (also known as leatherjackets) belong to the order Tetraodontiformes, also called Plectognathi. This group also includes some very popular aquarium families: puffers, porcupinefish, and triggerfish. Let’s take a brief look at the biology and care requirements of filefish.

Meet the Family

Filefish have two dorsal fins. The first dorsal usually has two stout spines (though a few have one) and—like their cousins the triggerfish—the pelvic fins are lacking. Instead, there is an extension of the pelvic bone, known as the pelvic rudiment, that has a “spinous” knob on its end with skin attached to it (some species lack this structure). In some filefish, this bone is moveable and has an attached skin flap that is erected when the species displays toward a rival. When swimming, they propel themselves by undulating their soft dorsal and anal fins. Though they are adequate (but not strong) swimmers, they rely more on crypsis and hiding to avoid being eaten. Filefish have non-overlapping scales that have spicules (small, needle-like anatomical structures) protruding from the center of each scale. In Australia, the name “leatherjackets” has been applied to the monacanthids because of their sandpaper-like squamation.

Most members of the family fall in the 4- to 12-inch (10- to 30-cm) range, although there is one “giant” that attains more than 3 feet (90 cm) in length: the scrawled filefish (Aluterus scriptus) reaches a length of 43 inches (108 cm). It is found in the Atlantic, Indian, and Pacific Oceans. When it comes to pigmentation, some filefish are brightly colored, while many exhibit more subdued earth tone colors that facilitate their camouflage. However, some they may change their hue to better blend in with their environment. Some have filaments on the body to help break up their outline (these weed-like extensions are most well developed in tasseled filefish [Chaetodermis penicilligerus]).

Filefish Lifestyle

While there are a number of filefish found in the coral reef ecosystem, by no means is their distribution restricted to tropical seas or coral reefs. In fact, there are just as many species that make their homes on warm-to-temperate rocky reefs. Some regularly associate with floating algal species and/or debris. In many of these species, it is the juveniles that associate with flotsam. For example, I have seen groups of young scrawled filefish, numbering in the dozens, hanging out under floating plant material, trash, and Sargassum algae.

Inflator filefish (Brachaluteres spp.) often live among soft coral “trees” and grip a polyp in their jaws when they sleep at night to maintain their position among the branches. These fish are also unique in that they will expand their stomachs with water like pufferfish in order to make themselves appear more difficult to swallow to predators. Filefish spawn in pairs and are classified as egg scatterers. They vary in their choice of spawning substrate. Some lay their eggs on the sand, some on algae, and still others on stony corals. I once watched a pair of scrawled filefish spawn over a colony of Madracis, or star coral. They adopted a head-down posture and would jointly chomp on the coral they hung over. After engaging in alternating bites and displays for about five minutes, the female deposited her eggs among the intact and now-broken tips of the stony coral, followed by the male’s contribution.

These fish do not care for the fertilized gametes, at least for long. In some species, such as the threadsail filefish (Stephanolepis cirrhifer), the female may hang around the deposition site and defend the eggs for a few minutes, but otherwise will leave their eggs almost immediately to hatch on their own.

Filefish possess a varied diet: they are opportunistic omnivores that dine on macroalgae, filamentous algae, sea grasses, coralline algae, sponges, hydrozoans, bryozoans, and tunicates. Other animals that are typically a minor component of their diets include foraminiferans (shelled protozoa), polychaete worms, bivalves (usually smaller species), snails, ostracods, amphipods, and shrimp.

Like their triggerfish cousins, some of the large filefish also feed on sea urchins. The barred filefish (Cantherhines dumerilii) was reported to bite the tips of the club-like spines from the slate pencil sea urchin (Heterocentrotus mammillatus). Heart urchins are also sometimes a component of their diets.

There are some monacanthids that do eat gorgonians or nip the tips off of small-polyp stony corals. The Cantherhines are known stony coral eaters. Smaller Pervagor spp., which are popular with aquarists because of their more colorful “attire,” feed heavily on algae and detritus, but some also eat stony corals. Rather than biting off chunks of the coral skeleton, as well as the soft tissue, they tend to scoop the polyps from the coral’s calcareous armor, taking some skeletal material in the process.

The highly specialized orange-spotted filefish Oxymonacanthus longirostris feeds almost entirely on stony coral polyps (namely those from various Acropora spp.), and does so by neatly plucking only the polyps from the skeleton without ingesting the latter.

Aquarium Care

Most filefish are good aquarium inhabitants. They actively move about in midwater and regularly examine the substrate for food items. Because they are naturally shy and retiring, you will want to provide them with plenty of swimming room and many usable hiding places. These may include crevices, caves, and interstices behind the live rock and the back of the aquarium. These shelter sites are essential to ensure their acclimation. They will also often adapt to aquarium life more readily if their tank is not in a high-traffic area. You must not bother them during the acclimation process. This may mean targeting food into a preferred hiding place to ensure your newly acquired monacanthid gets enough to eat.

The commonly kept Pervagor spp. can be housed in tanks as small as 55 to 75 gallons (208 to 284 liters) as adults. However, adults of the larger filefish species (e.g., Cantherhines spp.) will need to be kept in tanks of 180 gallons (680 liters) or larger. They can grow quite rapidly, so be prepared for this eventuality. For example, a juvenile tasseled filefish (Chaetodermis penicilligerus) may be quite happy in a 20-gallon (76-liter) tank, but it will rapidly increase in size and need one that’s at least 100 gallons (378 liters) when it reaches maturity (around 12 inches [30 cm] long). The same is true for the juvenile scrawled filefish (Aluterus scriptus)—while it is a beautiful fish, its size can quickly become an issue.

There are also miniature species that are, unfortunately, not readily available in the aquarium trade. For example, members of the genera Brachaluteres and Rudarius excelsus only reach a maximum length of around 2 inches (5 cm). Although shy, these smaller fish would be wonderful for small tank venues.

Be careful when capturing and moving filefish. Because they are so prickly, they have a tendency to get stuck in net mesh. It is a better idea to herd them into a specimen container or shipping bag when removing them from a tank. The dorsal and pelvic spines (which are often adorned with smaller spines) are particularly prone to getting stuck. If you do this, you may have to cut the mesh away from the spine with a scalpel. This process can be very stressful for the fish. Also, while not as aggressive as their triggerfish cousins, filefish—especially larger specimens—can deliver a painful bite and should be treated with respect.

When it comes to fish neighbors, the filefishes tend to be rather benign. They (usually) do not bother any tankmates, with the possible exception of members of their own kind or close relatives. While you can put two heterospecific filefish in the same tank, the aquarium should be large. The actual size is dependent on the species in question; for example, a Pervagor sp. would require a tank of at least 125 gallons (473 liters).

Two filefish of the same species, especially of the same sex, are likely to duke it out. On the other hand, filefishes are sometimes picked on by larger angelfishes, large damsels, surgeonfishes, and triggerfishes. If a newly introduced filefish is picked on by a belligerent tankmate, it is likely to hide all the time and become anorexic.

Feeding Filefish

One major concern with filefish is providing them with enough to eat. While they are naturally laterally compressed (when viewed head-on, they look thin), captive specimens are often emaciated. This is evident when you look at the stomach and along the back of the fish. In an underfed individual, the stomach will be pinched in and the dorsum will be indented due to atrophied muscle tissue. The latter is evidence of more long-term starvation.

These fish feed almost continuously in the wild and should be fed numerous small meals in the aquarium—three times a day at a minimum. Younger fish have greater metabolic needs and will require even more food than adults. Of course, in many cases it is the juveniles that are sold in the aquarium trade. For aquarists who like to tinker with controllers and feeders, several companies offer systems that provide small amounts of food throughout the day.

It is important that any filefish’s diet contains plant material. This may include algal flake food, frozen preparations that contain algae, and dried algae sheets or nori. A tank with some filamentous algae and macroalgae can also be helpful in supplying their nutritional needs, especially with a specimen that is reluctant to ingest introduced fish foods. Meaty foods, like fresh seafood shavings, frozen mysid shrimp, frozen preparations, and flake foods should round out the diet.

Filing Down Corals

Filefish certainly have the dentition to damage both stony and soft corals and, as mentioned above, some regularly include them in their diets. However, they have been housed in reef tanks without damaging their scleractinian neighbors.

In fact, some filefish may be used to help control pestilent invertebrates in tanks. One of these is the majano anemone. The seagrass filefish (Acreichthys tomentosus) will feed on—and wipe out—majano populations, as well as Aiptasia (they usually feed on the latter first, and then move on to the former). And be warned, when they run out of these small sea anemones, they may begin feeding on your more desirable cnidarians, including xenia corals, leather corals (they pick at Sarcophyton coral polyps), and large-polyp stony corals.

The exquisite orange-spotted filefish will eat the polyps of some stony corals. Due to its highly specialized diet and poor track record when it comes to captive survival, it is best left on the reef.

If you decide to try a filefish in your reef tank, know that there is an inherent risk involved. These fish have the dentition and the natural propensity to feed on sessile invertebrates, so you are tempting fate if you attempt to keep them in your reef tank. Remember that hungry filefish are more apt to stray, so feed them well. Ornamental invertebrates that may be accosted by these fish include polychaete worms (including tube worms), shrimp, small crabs, serpent stars, sea urchins, and small sea cucumbers.

Common Filefish

Provided below is a short guide to some of the more popular filefish species entering the aquarium trade, with a brief husbandry summary for each one.

SEAGRASS OR BRISTLE-TAILED FILEFISH
The seagrass filefish (A. tomentosus) is often employed to rid tanks of noxious sea anemones. While it usually behaves well in the presence of other cnidarians, it may occasionally stray and nip at coral polyps. It usually ignores other fish species. Keep one per tank, unless the aquarium is extra large (180 gallons [680 liters] or more), or if you can acquire a male-female pair. Reaching a length of 4 inches (10 cm), individuals can be housed in tanks of 55 gallons (208 liters) or more.

SCRAWLED FILEFISH
As a juvenile, Aluterus scriptus will acclimate to a larger aquarium, but because it gets so large, adults are usually not suitable for the vast majority of home aquariums. Its normal diet includes algae, gorgonians, and zoanthids. Adults grow up to 36 inches (90 cm) and need to be kept in tanks of 500 gallons (1900 liters) or more.

TASSELED OR LEAFY FILEFISH
Young C. penicilligerus are adorned with dermal appendages, which tend to decrease in size as the fish grows. This is a durable aquarium species, but it has a dubious reputation when it comes to living with invertebrates. Adults, which reach a length of 12 inches (30 cm), have been known to eat polychaete worms, crustaceans, and may nip at corals on occasion. Keep one per tank. A 125-gallon (473-liter) tank will be needed to house an adult.

FRINGED FILEFISH
A voracious feeder in captivity, the fringed filefish (Monacanthus ciliatus) reaches 8 inches (20 cm) and does well in tanks as small as 75 gallons (284 liters). Main components of its diet include algae, sea grass, and tiny crustaceans. It will not behave aggressively toward unlike species, but may spar with close relatives or other fringed filefish of the same sex. It can be housed in a reef tank.

SLENDER FILEFISH
One of the best aquarium filefish, the slender filefish (M. tuckeri) remains small (reaching 4 inches [10 cm]) and readily acclimates to a new environment. It is one of the better suited species for the reef tank because it feeds mostly on small planktonic crustaceans. You can keep more than one M. tuckeri in a larger tank (180 gallons [681 liters] or more). A 55-gallon (208-liter) tank will adequately house an adult of this species. In the wild, it often associates with gorgonians.

ORANGE-SPOTTED FILEFISH
The attractive orange-spotted filefish (Oxymonacanthus longirostris), which grows to 3½ inches (9 cm), should be avoided by aquarists as it rarely, if ever, survives long in captivity. It has a specialized diet that consists almost entirely of Acropora coral polyps. Some people have kept them in small-polyp stony coral aquariums, where they have a constant supply to dine on. However, only experienced marine hobbyists should attempt to keep them.

MIMIC FILEFISH
A fascinating species that looks just like the poison-laden saddled toby (Canthigaster valentini), the mimic filefish (Paraluteres prionurus) is merely displaying a case of Batesian mimicry—that is, a harmless species mimicking a harmful one. Unless you know what to look for, it can be easy to confuse these species (the filefish has two dorsal fins, while the toby has one). It is a great aquarium fish that can be kept in a reef tank, as long as it remains well fed—if not, it will damage corals. That being said, there is always a degree of risk when adding a filefish to a venue with cnidarians. A 30- to 55-gallon (113- to 208-liter) tank will adequately house an adult, which can reach a length of 4 inches (10 cm).

BLACKBAR FILEFISH
A very shy fish that will spend most of its time peering from a reef crevice when the aquarist is near the tank, the blackbar filefish (Pervagor janthinosoma) will acclimate to its keeper in time and move into the open when you are present. However, it will settle in more readily if housed in a tank placed in a low-traffic area of the home or office. The members of this genus are known stony coral predators, so adding it to a reef tank is not without risk. They grow to 5½ inches (14 cm), so a tank of 55 gallons (208 liters) can house a single adult.

BLACKHEADED FILEFISH
Like others in the genus, the blackheaded filefish (P. melanocephalus) can be reclusive when first added to its aquarium home. This will be exacerbated if it is harassed by aquarium neighbors. It does not tend to be as shy as P. janthinosoma. It is a potential coral polyp nipper. Unless you can acquire a male-female pair, it is best to keep one per tank. This fish grows to 4 inches (10 cm) in length, so a single adult should be kept in a tank of 55 gallons (208 liters) or larger.

FANTAIL FILEFISH
A Hawaiian beauty, the fantail filefish (P. spilosoma) is one of the most common filefish species in the aquarium trade. It tends to be hardy and readily acclimates to captivity. While it tends to be less shy than others in the genus, provide it with plenty of suitable hiding places. Stony coral polyps are an important component of its natural diet, but it readily accepts and thrives on aquarium fish foods. Because of its bill of fare, it is not usually welcome in the reef tank. Keep one per aquarium, as they grow to 7 inches (18 cm), unless you can acquire a male-female pair. Males will quarrel and may damage each other. Adults are best housed in tanks of 75 gallons (284 liters) or larger.

Rank and File

While often overlooked, filefish are truly worthy candidates for the live rock–only fish aquarium and may be kept with some of the more noxious soft corals. If you feed them often, provide suitable hiding places, and nurture a peaceful setting, filefish should live happily for many years in your home aquarium.

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