Icosteoidei (Ragfish)

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Class Actinopterygii

Order Perciformes

Suborder Icosteoidei

Number of families 1

Evolution and systematics

When the juvenile ragfish was first described by Lockington in 1880, there was apparently a premonition of the difficulty in placing this species into any existing taxonomic groupings. Indeed, the name of the single icosteoid representative, Icosteus aenigmaticus, means "puzzling fish with yielding bones." Lockington hypothesized the ragfish's probable relationship to the blennioid fishes. He questionably assigned it to the family Blenniidae (the blennies) but stated that it probably should be in its own family.

In the next twenty years, Lockington's contemporaries offered other taxonomic schemes. In 1881 Steindachner also examined a juvenile ragfish and concluded that it was not a blennioid and should be positioned with Icichthys (the medusafish). In 1887 Günther examined two juveniles and proposed a coryphaenid (dolphin) affinity. In 1887 Tarleton H. Bean described the first adult ragfish; being of such dissimilar appearance to the juvenile form, it was recognized as its own species, Acrotus willoughbyi, and placed in the family Icosteidae, along with Icosteus and Icichthys. Several years later Acrotus was elevated to its own family, Acrotidae.

The next 70 years yielded few studies of ragfish (actually, ragfishes at this time) systematics. During this period, the ragfish received little attention, doubtless because of its rare occurrence. Most ichthyologists still agreed that it was a percomorph (typical spiny-rayed fishes), though degenerate and specialized. The majority of researchers recognized a stromateoid (medusafish, butterfishes, and squaretails) affinity. A few, such as Berg in 1940, believed that the ragfish should be elevated to its own order, Icosteiformes.

In 1961 Clemens and Wilby observed that Icosteus (the fantailed ragfish) was the juvenile form of the adult Acrotus (the brown ragfish). At that time, interest in the ragfish was renewed, and since then the science of systematics has advanced in the methods of interpreting characteristics that elucidate affinities between groups. Studies on internal anatomy, external morphological features, and early life history have been adding to our knowledge of ragfish. With this information, investigators may be able to piece together the "puzzling" systematic relationships of the ragfish.

The taxonomy for this species is Icosteus aenigmaticus Lockington, 1880.

Other common names include ragfish, spotted ragfish, speckled ragfish, fan-tailed ragfish, giant ragfish, and brown ragfish.

Physical characteristics

Juvenile ragfish have limp bodies that are easily doubled over like a wet rag (hence the name ragfish). They are deep-bodied and laterally compressed (especially the region along the dorsal and anal fin base), with a slender caudal peduncle. The body is smooth-skinned and scaleless, except along the lateral line, where the scales produce groups of numerous small spines. The lateral line is minimally arched anteriorlyand then straight. The head is short, with an abrupt, high predorsal profile. The eyes are small. The mouth is terminal, with limited protrusibility. The teeth are minute and comb-like. The snout is blunt and has been described as calflike. The dorsal fin has one spine and 52–56 soft rays, and the anal fin has 37–40 soft rays. The caudal fin is rounded and

fanlike. The pelvic fins have one spine and four soft rays. All fins have small spinules on the surface. Smaller juveniles (less than 4.7 in, or 120 mm, in length) have mottled dark blotches over a light grayish purple background. Juveniles longer than 4.7 in (120 mm) have numerous irregular spots over a light grayish purple background, which are less prominent along the ventral region of the body.

At 14.2–16.5 in (360–420 mm), ragfish undergo a remarkable transformation into an adult form that is very different from the juvenile form. The body becomes more elongated, and a fleshy keel becomes pronounced along the ventral midline. The lateral line scale spines disappear, and the number of dorsal and anal fin rays decrease as the skin along the anterior origins encroaches on the leading spine and rays. The pelvic fins disappear. The caudal fin margin broadens and becomes emarginate to slightly forked. The spinules on the fins are reabsorbed. Finally, the coloration turns to a uniform dark brown over the entire body and fins.

Ragfish have been reported to reach a length of more than 6.9 ft (2.1 m). Adult males seem to attain the greatest length, while most females grow to about 5 ft (1.6 m). Adult females appear to be deeper bodied and weigh nearly twice as much as a male of similar size.


Ragfish are distributed from San Onofre, California, to immediately north of the Aleutian Islands and across the northern Pacific to Kochi City, southeastern Japan. They are considered rare but are less so north of Cape Mendocino, California.


Ragfish are captured from the surface to depths of more than 1,640 ft (500 m). They usually are offshore over deeper water, which suggests an epipelagic or mesopelagic habitat. The juvenile's spotted coloration, however, might imply a benthic habitat.


There is little information on ragfish behavior. They most likely are not schooling fish, because they always are captured individually. Rarely, large ragfish are found stranded on the shore. Ragfish have never been observed in their natural habitat.

Feeding ecology and diet

Little is known of the feeding ecology of the ragfish. Those that are collected rarely have identifiable food items in the stomach. In 1968 Fitch and Lavenberg reported small fishes, squid, and octopods as possible prey items. There also is evidence of gelativory in ragfish. Gelativory is the consumption of jellyfishes and their relatives. Preliminary investigations of the internal anatomy suggest morphological specializations to accommodate a gelativorous diet.

Ragfish probably are preyed upon by piscivorous (fisheating) fishes. There is one record from Washington of a small juvenile ragfish found in the stomach of a tuna. In 1938 Cowan reported that a colleague identified the adult ragfish as the whaler's "bastard halibut," which forms a large part of the diet of sperm whales taken in deep water off Queen Charlotte Island, British Columbia.

Reproductive biology

Ragfish are oviparous and ready to spawn at seven to nine years of age, according to Fitch and Lavenberg, and they spawn in summer. In 1968 Allen studied four adult females and postulated that spawning occurs in late winter and early spring. Allen observed that eggs were largest (to 0.12 in, or 3.0 mm, in diameter) at that time and estimated fecundity to be 430,000 eggs for a 59-in (1.5-m) female. It is not known whether ragfish undergo spawning migrations or become territorial. Eggs are nonadhesive and planktonic, with a single oil globule. The larvae are distinctly pigmented and planktonic.

Conservation status

This species is not listed by the IUCN.

Significance to humans

Ragfish have no sport or commercial fishing value. They occur rarely in the bycatch of commercial shrimp and bottom fish trawlers.



Fitch, J. E., and R. J. Lavenberg. Deep-Water Teleostean Fishes of California. Berkeley: University of California Press, 1968.


Allen, G. H. "Fecundity of the Brown Ragfish, Icosteus aenigmaticus Lockington, from Northern California." California Fish and Game 54, no. 3 (1968): 207–214.

Bean, T. H. "Description of a New Genus and Species of Fish, Acrotus willoughbyi, from Washington Territory." Proceedings of the U.S. National Museum 10 (1887): 631–632.

Clemens, W. A., and G. V. Wilby. "Fishes of the Pacific Coast of Canada." Bulletin (Fisheries Research Board of Canada) 68(1961): 1–443.

Cowan, I. M. "Some Fish Records from the Coast of British Columbia." Copeia 1938, no. 2 (1938): 97.

Günther, A. "Report on the Deep-Sea Fishes Collected by the H.M.S. Challenger during the years 1873–1876." Zoology 22(1887): 46–47.

Lockington, W. N. "Description of a New Genus and Some New Species of California Fishes (Icosteus aenigmaticus and Osmerus attenuatus)." Proceedings of the U.S. National Museum 3 (1880): 63–68.

Steindachner, F. "Ichthyologische Beitrage (11)." Sitzungsber. Kais. Akad. d. Wiss. Wien. Math. Naturw. Kl. Bd. 83 (1881): 396–397.

K. Gus Thiesfeld, BS