Idiacanthus fasciola
deepsea stalkeye fish

Idiacanthus fasciola is common in deep, temperate and equatorial oceanic waters. Specimens have been found throughout the northeast and central Atlantic Ocean as far north as 48° and in areas including the Grand Bank, Scotian Shelf, Gulf of Mexico, Straits of Florida and Caribbean Sea. It has been found in the southern Atlantic off Patagonia and as far south as 54°. In the eastern Atlantic, I. fasciola is common near the Azores, off Portugal, and off West Africa. Idiacanthus fasciola has also been found in the eastern and western Indian Ocean, off northern and eastern Australia, off Tasmania and in the western and equatorial Pacific Ocean. (Gibbs, 1964; Nakamura, 1986; Scott, 1979; Whitehead, 1984)

Females inhabit depths of 500 to 2,000 meters during the day and travel up to 250 meters or shallower at night. Most males stay between 1000 and 2000 meters at all times and do not migrate. The smallest larvae are found in great depths (probably where adults spawn), but transitional larvae live in subsurface (350 meters or shallower) areas and move to greater depths during or shortly after metamorphosis. (Coad, 1995; Whitehead, 1984)

Adult I. fasciola females range from 19 to 48.9 cm. Adult males are strikingly different from females (described below) and are between 3 and 7 cm in length (Gibbs, 1964).

Idiacanthus fasciola has a long and sleek body that is 20 to 50 times longer than it is deep. The dorsal fin is also very long, extending along the posterior two-thirds of the length of the body, and with 54 to 80 rays. The anal fin is about half as long as the dorsal fin and contains 29 to 49 rays. Both the dorsal and the anal fins extend almost until the caudal fin. Both fins also have pairs of short, bony spines that border each ray. There are no adipose or pectoral fins, but pectoral fins are present in larvae. The pelvic fins (in females only) have 6 rays and are usually found closer to the head than the caudal fin.

The skin is black or very dark brown and without scales or markings in females. Males are described as dark brown and larvae are clear with rows of pigment marks (Gibbs, 1964). The post-orbital organ is pinkish.

The head of I. fasciola has a round snout, and proportionally small eyes (larger in males). The mouth is large and at an oblique angle and with premaxilla bones that are not protractible. Nostrils are located closer to the eye than to the mouth (Gibbs, 1964). Females have teeth in the jaws that are long, barbed and fang-like, vary in size and are depressible. The gill arches do not have rakers or teeth.

There is no lateral line present, but I. fasciola has two rows of photophores on both sides that extend along the ventral part of the body. Luminous material also flanks each dorsal and fin ray and is usually yellowish in color (Gibbs, 1964). There is also a post-orbital luminous organ, but no pre- or sub-orbital luminous organs (Nakamura, 1986).

There is a chin or hyoid barbel present in females that is about two and a half times the length of the head, (absent in males). This chin barbel has two forms: one with a bulblet on the end and one without.

Males are quite different from females as are adults from larvae. Although males are generally a similar overall shape as females, they do not exceed 7 cm in length. They lack teeth, chin barbells and pelvic fins. Their post-orbital organs are proportionally larger than in females, extending from slightly longer than the diameter of the eye to up to twice the diameter of the eye. Males also have a small extension (sperm duct) of flesh that originates near the anus and attaches to the modified first anal ray. (Gibbs, 1964; Nakamura, 1986; Whitehead, 1984)

Idiacanthus fasciola goes through a remarkable metamorphosis from larva, postlarva, adolescent, transitional adolescent to adult. One common name for I. fasciola is the deep-sea stalkeye fish, which refers to the larval state. The larvae, which are long and slender and 1.6 to 2.8 cm in length, have eyes on the ends of proportionally very long, cartilaginous stalks that are up to 25% of the total body length. The eyestalks apparently allow for an increased range of vision (Nelson, 1994). The larvae do not have photophores and are clear except for a row of color spots. The head is long with no operculum and has a flat duck-like snout. The larvae have relatively large, paddle-like pectoral fins and no pelvic fins. The end of the intestine extends outside of the body and slightly beyond the caudal fin. In this early stage of development, the larvae are sexually indeterminate (Gibbs, 1964).

In the postlarval stage, 3.5 to 5.0 cm, I. fasciola begins to show adult and sexually dimorphic characteristics. The eyestalks begin to shorten and the cartilaginous rods coil up behind the eye. The head and snout begin to enlarge and the chin barbel and pelvic fins begin to grow in females.

In the adolescent stages, the skin darkens, eyestalks disappear, and the operculum grows to cover the gill openings. The intestine no longer extends beyond the caudal fin. In females, rapid growth occurs (can quadruple their length), teeth grow, and the chin barbel grows to the complete shape.

Males do not grow much in the adolescent stages and overall males retain some larval appearance in adult stage. But males do darken in these stages, the post-orbital organs grow as large or larger than the eye and the sperm duct grows out to the first anal ray. (Gibbs, 1964; Kawaguchi and Moser, 1983; Nelson, 1994; Whitehead, 1984)

Because adult males have a very diminished digestive tract, it is likely that their adult lifespan is quite short, possibly as short as a few weeks. Through collection numbers, it seems that females spend more time in the immature stages. Although they don’t seem to spend much time as mature adults, they probably live more than one year (Gibbs, 1964). (Gibbs, 1964)

Schooling behavior is inferred from collection results. Larvae are often found together, suggesting that they school. Adults have been found in groups of 2 to 7, but it is not clear is this is a result of schooling behavior (Gibbs, 1964). (Gibbs, 1964)

Idiacanthus fasciola does not have a lateral line but does have many photophores. Larvae have eyes on eyestalks, perhaps to help them see better. There is no other information availalbe on perception and communication. (Gibbs, 1964; Nelson, 1994)

The food habits of I. fasciola are described in several sources as poorly known, but adults are known to eat mid-water fishes. A myctophid or lanternfish, from the genus Diaphus was found in an adult specimen (Gibbs, 1964). Remains of diatoms and small crustaceans have been found in postlarvae. Adult males lack a functional digestive track and diet (Coad, 1995). (Coad, 1995; Gibbs, 1964; Whitehead, 1984)

There is little information available on the predators of I. fasciola. However, a specimen of I. fasciola measuring 15.5 cm was found off the coast of Tasmania in the stomach of a deep-sea trevella or Hyperglyphe antarctica or Hyperglyphe porosa in 1978. (Scott, 1979)

There is little information available on role I. fasciola plays in ecosystems. However, because it is common in deep, temperate oceans, it seems to be an important part of these ecosystems. (Fink, 1985)

There are no known negative effects of I. fasciola on humans.

There are no known positive effects of I. fasciola on humans, but it is a fish that is apparently of great interest to some ichthyologists and students of biology.

Idiacanthus fasciola is not listed as a threatened or endangered species on the IUCN, U.S. Federal List or CITES databases.

Idiacanthus fasciola was first described and named by Peters in 1876/1877.

Common names for I. fasciola include ribbon sawtailfish, black dragonfish and deepsea stalkeyefish.

The species Idiacanthus fasciola and genus Idiacanthus have gone through some classification changes over time. The remarkable stalkeye larvae of Idiacanthus fasciola were originally described as a separate species (Scott, 1979). Gibbs states that Idiacanthus is very closely related to Melanostomiatidae because they have nearly identical teeth structures. Idiacanthus is now considered the sister group to Tactostoma (Fink, 1983). The distinguishing characteristics of Idiacanthus fasciola are the extraordinary larval metamorphosis, very distinct sexual dimorphism, and pairs of spines that accompany the base of each dorsal and anal ray (Gibbs, 1964).

Many of the stomiatoid fishes have unique lower jaw barbels, although the function of this tissue is not clear. The chin or hyoid barbel found in I. fasciola females has been found to contain luminous cells (similar to those found in the photophores) near the end of the barbel. The end barbel also contains well-developed muscles, nerves and blood vessels (Hansen, 1970). (Coad, 1995; Fink, 1983; Gibbs, 1964; Hansen, 1970; Scott, 1979)