Unicorn Filefish, Aluterus monoceros
The Unicorn Filefish, Aluterus monoceros, whose common Spanish name is lija barbuda, is a species in the family Monacanthidae, the Filefishes, known as lijas in Mexico. Globally, there are only four species in the genus Aluterus, and all four are found in Mexican waters, four in the Atlantic and two in the Pacific.
The Unicorn Filefish have very compressed elongated oval bodies. They are gray to gray-brown in color with brown spots and blotches on their upper sides. Their anal and dorsal fins are pale yellowish brown and their tail membranes are dark brown. They have the ability to change color to match their surroundings. Upon death, their coloring transitions to a bland and uniform tan. Their head has a convex upper and concave lower profile and features a long pointed snout, a small centrally located upturned mouth, and small beady eyes located high on the back of the head. Their caudal fin base is longer than it is deep; the fins are relatively short with a straight or concave edge. They have two dorsal spines, the first of which is long and slender and is located directly over the eye; it can be locked in position by the second smaller spine. When threatened, the Unicorn Filefish will dive quickly into a crevice in the reef, wedge themselves into the shelter by erecting and locking their first dorsal spine and another spine located on their belly. This behavior is also used when they rest on the reef at night. They lack pelvic fins. Their body is covered with small scales and small hairs creating a coarse sandpaper-like texture.
The adult Unicorn Filefish are benthic, being found at depths up to 165 feet, and inhabit coral and rocky reefs; the juveniles are pelagic and found in and around floating debris at large distances from land. They have a wide circumglobal distribution in tropical and subtropical waters which includes the Eastern Atlantic (west coast of Africa including the Mediterranean Sea), the Western Atlantic (Massachusetts to Argentina), and the Western Indian and Eastern Pacific Oceans (Baja California to Chile). In Mexican oceanic waters they are found throughout the Gulf of Mexico and in the Pacific from Magdalena Bay south along the west coast of Baja, in the southern two thirds of the Sea of Cortez, and along the coast of the mainland south to Guatemala. I have photographic documentation of a fish caught in the Santa Rosalia area of the Sea of Cortez which establishes the northern range for this species. This is one of the very few species that are found in both the Atlantic and Pacific waters of Mexico.
The Unicorn Filefish are secretive fish that hide in caves. They are omnivorous and feed on algae, anemones, gorgonians, sponges, and tunicates. In turn, they are preyed upon by larger fish including Dorado, Coryphaena hippurus, and Bluefin Tuna, Thunnus thynnus. They reach a maximum length of 76 cm (30 inches). They breed in groups consisting of one male and two to five females. The females lay demersal eggs in safe areas, such as depressions in the sand, and the eggs are then fertilized by the males. Both the males and the females will guard these fertilized eggs from predators. Upon hatching, the females will care for the young.
The Unicorn Filefish is easily confused with the Scrawled Filefish, Aluterus scriptus (upper snout concave, rounded caudal fin).
The Unicorn Filefish are considered to be game fish in some parts of the world but in Mexico they are rare and of limited value, thus normally a “catch and release.” They are reported to contain Ciguatera Toxin making them a poor food choice. They are collected and sold for the aquarium trade at a modest level. As they are a relatively shallow water slow-moving species, they are often the subject of underwater photographers.