Saccopharyngiformes

(Swallowers and gulpers)

Class Actinopterygii

Order Saccopharyngiformes

Number of families 4

Evolution and systematics

The Saccopharyngiformes are divided into two suborders, the Cyematoidei, with the single family Cyematidae (with two monotypic genera), and the Saccopharyngoidei, which contains the other three families. Of these three families, the Monognathidae is the most diverse, with 14 identified species in the genus Monognathus. The Saccopharyngidae has 11 species in the single genus Saccopharynx; the closely related family Eurypharyngidae is monotypic. There is still controversy over the inclusion of the Cyematidae in this order, but they are placed here on the basis of reduction of skeletal features that are common among all four families. Systematists consider the Saccopharyngiformes to be quite different from anguilliform eels. The order is thought to consist of highly specialized fishes. All four families share numerous common features, most of which have to do with extreme loss of skeletal features, presumably the result of the extremely energy poor environment.

Within the Saccopharyngoidei, the eurypharyngids and saccopharyngids are superficially most similar in appearance and are considered the closest in taxonomic relationship. The Monognathidae represent a more advanced and highly specialized family, as evidenced by even greater reduction in skeletal components, that is, the loss of the upper jaw. The first fossil evidence for this order is reported to be from the Middle Cretaceous.

Physical characteristics

The loss of skeletal structures has resulted in fishes that are among the most unusual and striking in their appearance. Among other characteristics, all are scaleless, lack pelvic fins, and have very long dorsal and anal fins. All are rather "flabby" to the touch and presumably are very poor swimmers. In members of the Saccopharyngoidei, the mouths are very large to enormous, with distensible pharynges and stomachs, to allow for the capture of very large prey. Dentition varies among the families. Well-produced, posteriorly curved teeth are found in the Saccopharyngidae, with the other three families possessing small to minute teeth in the jaws.

Except for the enlarged head and mouth structure, the rest of the body of these fishes is elongated and very slender (filamentous in eurypharyngids and saccopharyngids). The body coloration varies from scattered pigment patches to a light uniform brown in monognathids, with dark brown to solid black in cyematids, eurypharyngids, and saccopharyngids. Thin white lines of unknown function extend from the head to the tail along the upper body in the saccopharyngids and eurypharyngids, and individuals in both families have luminous bulbs at the very tip of the filamentous tail. The filament may constitute 50% or more of the overall length of the fish. Overall lengths of the substantial part of the body in all saccopharyngiforms is small, not exceeding 19.6 in (50 cm).

Distribution

Saccopharyngids are most abundant and diverse in the Atlantic Ocean. Eurypharynx pelecanoides is well known from the Atlantic and central and eastern Pacific Oceans, and the monognathids are about equally diverse in the Atlantic (six species) and Pacific Oceans (seven species). Among the Cyematidae, Cyema atrum is widespread in the Atlantic, Pacific, and Indian Oceans, while Neocyema erythrostoma is only known from the eastern South Atlantic. Saccopharyngiformes have not been reported from the Mediterranean.

Habitat

These are primarily bathypelagic inhabitants, with the majority of adult specimens being collected at depths greater than 3,280 ft (1,000 m). Larvae and juveniles live in shallower waters, even into the upper mesopelagic zone below 656 ft (200 m).

Behavior

Owing to the extreme depths at which these fishes live, there are few reports of any behavior.

Feeding ecology and diet

All saccopharyngiform species are poor swimmers at best. There have been no reports on feeding in the cyematids, but it is thought that eurypharyngids and saccopharyngids draw prey close to them by means of luminescent lures on their tails and then quickly open their mouths to suck food in. Saccopharyngids are piscivorous (eat only other fish); eurypharyngids take a broader range of fish and invertebrate prey. An even more unusual form of feeding has been postulated for monognathids. It is thought that their prey (crustaceans) may be lured by scent released from glands around the mouth; when they come close enough, the fish bite them by means of a hollow fang in the mouth that injects venom, much like a rattlesnake. The fish then swallows the dead or dying shrimp whole. Little is known about the predators that feed in members of this order.

Reproductive biology

Nothing is known about the cyematoids, other than that they have separate sexes that do not appear to exhibit dimorphism. They also have leptocephalus larvae—a thin, largely transparent, ribbonlike stage that is common to several primitive orders of bony fishes (Elopiformes and Anguilliformes), including all saccopharyngiforms. In all three families of saccopharyngoids, sexually mature individuals are dimorphic. Males have greatly enlarged nasal structures and slightly enlarged eyes, and the jaws and stomachs in both males and females atrophy. It is widely believed that males locate their mates by following scent trails of pheromones released by the females and that spawning is a terminal event, with both individuals dying after mating. This reproductive pattern has been found in a number of shallow water eels and other fish species.

Conservation status

There are no known conservation measures specific to these families. No species from either family is listed on the IUCN Red List.

Significance to humans

Owing to their rarity and poorly studied biological characteristics, no significance can be attributed to saccopharyngiforms. They are objects of curiosity because of their extreme body specializations.

Species accounts

List of Species

Bobtail snipe eel

Cyema atrum

family

Cyematidae

taxonomy

Cyema atrum Günther, 1878, South Pacific, Challenger station 1,770 ft (539 m); Antarctic, Challenger station 948; 9,000; and 10,800 ft (289; 2,743; and 3,292 m).

other common names

English: Bobtail eel, deepwater eel; Danish: Korthalet ål; Finnish: Nuoliankerias.

physical characteristics

This species has a rather striking appearance that is quite different from that of other saccopharyngiforms. Adults are black in coloration. This species is scaleless, like all members of the order. The eyes are very small. The jaws are thin and long, with numerous very fine teeth, and the jaws curve slightly away from each other at their tips. The dorsal and anal fin rays become progressively more elongated toward the rear of the body and extend well past very short caudal rays; the effect is that in side view the fish looks like an arrow! It is a small species, with a maximum reported size of about 6.3 in (160 mm).

distribution

It has been reported from all oceans between about 70° north and 55° south. Most collections have been from the Atlantic and Pacific Oceans.

habitat

The species is oceanic, lower mesopelagic to bathypelagic. Although it has been reported from collections made as shallow as 1,148 ft (350 m), most records are from depths exceeding 4,921 ft (1,500 m).

behavior

Nothing is known.

feeding ecology and diet

There has been little research on the feeding habits of this eel. Because of its jaw structure, it is suggested that the species feeds on comparable prey types and in a fashion similar to that of the anguilliform eels of the family Nemichthyidae, commonly known as snipe eels. Nemichthyids use their thin, recurved jaws to feed on crustacean shrimps, especially those in the family Sergestidae. Predators of this species are unknown.

reproductive biology

Unlike the other saccopharyngiforms, there is no apparent sexual dimorphism in adults. No other reproductive data have been reported for this species. The leptocephalus stage is rather distinctive; the deep oval body has a very small pointed head and a pointed caudal extension. These features grow a bit larger than in other saccopharyngiform leptocephali, with a maximum recorded total length of 2.8 in (70 mm).

conservation status

Not threatened.

significance to humans

None known.

Pelican eel

Eurypharynx pelecanoides

family

Eurypharyngidae

taxonomy

Eurypharynx pelecanoides Vaillant, 1882, off New England, United States, about 40°N, 68°W, 3 Albatross stations, 2,334–8,802 ft (711–2,683 m).

other common names

English: Big mouth gulper eel, pelican gulper, pelican gulper fish, pelican fish, deep-sea gulper, umbrella mouth gulper; French: Grand-gousier pelican; German: Pelikanaal; Spanish: Pez pelicano; Danish: Pelikanål; Finnish: Pelikaaniankerias; Icelandic: Gapaldur; Japanese: Fukuro-unagi; Polish: Polykacz.

physical characteristics

Superficially similar to species in the genus Saccopharynx, with which it shares the closest taxonomic relationship within the order, this species is coal black overall, except for a tiny white region on the caudal organ. It is scaleless. Probably the most striking differences between the pelican eel and Saccopharynx species are that the jaw length is extreme, almost 50% of the distance to the anus; the jaw teeth are very small; and there is a gradual narrowing of the body posterior to the abdomen. Other similarities to Saccopharynx species include small eyes that detect light rather than form visual images, the presence of a presumably luminous caudal organ at the end of a very long filamentous tail, an expansible stomach, and a weakly ossified and poorly muscled body. Because the delicate tail is usually broken, the maximum size is uncertain, but the largest intact specimen ever collected measured 25.9 in (750 mm) in total length.

distribution

This a circumglobal species, found in temperate and tropical waters of all oceans. It is best known from the Atlantic and eastern and central Pacific Oceans.

habitat

The species is oceanic and bathypelagic. Although there are some shallow-water capture records at less than 1,640 ft (500m), most individuals are collected between 3,281 and 9,842 ft (1,000–3,000 m).

behavior

Nothing is known.

feeding ecology and diet

This species takes in a wider range of prey than do species in the genus Saccopharynx. Prey items include fishes, various crustaceans (especially caridean decapod shrimps), and cephalopod mollusks. In addition, there have been several reports of benthic prey items in the stomachs of pelican eels. Predators are unknown.

reproductive biology

Reproduction is similar to that of species in the genus Saccopharynx, in that sexually mature males have greatly expanded nasal structures, accompanied by stomach atrophy, loss of dentition, and reduction in jaw structure. Reproduction is apparently a terminal event. Leptocephalus larvae are oval and deep-bodied, like Saccopharynx species, but they are smaller, with a maximum length of about 1.6 in (40 mm). They have several greatly elongated larval teeth in the upper jaw.

conservation status

Not threatened.

significance to humans

None known.

No common name

Monognathus rosenblatti

family

Monognathidae

taxonomy

Monognathus rosenblatti Bertelsen and Nielsen, 1987, Central North Pacific, 31°N, 159°W, 14,300–17,300 ft (4,853–5,266m)—bottom is 19,000 ft (5,800 m).

other common names

None known.

physical characteristics

The monognathids are truly bizarre in appearance. Probably the two most striking features are the complete lack of an upper jaw (which gives the family its name) and the presence of a hollow rostral fang that extends downward from the roof of the mouth. The fang is associated with a glandular mass thought to secrete venom. The body is scaleless, very slender, and pale tan to light brown in color. As with other saccopharyngoids, the eyes are very small, the stomach is distensible, the body skeleton is poorly ossified, and the musculature is weak. The caudal region is flattened laterally but does not extend into the long filament seen in saccopharyngids and eurypharyngids. Monognathid species have been divided into two groups based on the relative shape and length of their skulls; there is a "short-skulled" group and a "long-skulled" group, with M. rosenblatti belonging to the latter. The largest specimen collected measured 2.8 in (70 mm) in length, although the largest monognathid reported thus far was 11.4 in (290 mm).

distribution

This species is known only from the northeastern Pacific Ocean.

habitat

As with others in this genus, this species is found in oceanic, deep bathypelagic habitats. The shallowest record for M. rosenblatti is 6,889 ft (2,100 m). Due to their habitat, this family is exceptionally rare. All 14 species are known form a combined total of fewer than 80 individuals, about 50% of which belong to M. rosenblatti.

behavior

Nothing is known.

feeding ecology and diet

No stomach contents have been reported from any M. rosenblatti specimens, but prey from other monognathid species have all been crustacean shrimps. All of the shrimps were quite large relative to the body size of the fish. It has been hypothesized that these weak fish inject their prey with venom using the rostral fang, in much the same fashion as certain venomous snakes overcome their prey. Predators are unknown.

reproductive biology

As with the other saccopharyngoids, the sexually mature collected specimens of monognathids (none of which were M. rosenblatti) exhibit dimorphism and evidence that spawning is a one-time terminal event. Males possess greatly enlarged nasal structures, suggesting that locating of mates takes place by scent. Although it is believed that the larval form is a leptocephalus, as yet none has been positively identified as belonging to this family.

conservation status

Not threatened.

significance to humans

None known.

Gulper eel

Saccopharynx ampullaceus

family

Saccopharyngidae

taxonomy

Ophiognathus ampullaceus Harwood, 1827, NW Atlantic Ocean, 32°20'N, 30°16'W, 0–6,234 ft (0–1,900 m). Neotype: ISH 3288/79. Original locality 62°N, 57°W. Neotype selected by Nielsen and Bertelsen (1985).

other common names

English: Pelican fish; Danish: Slugål; Finnish: Ahmattiankerias; Icelandic: Pokakjaftur; Polish: Gardzielec.

physical characteristics

The body is attenuated and very flabby, with poorly ossified bones and weakly developed muscles. The most striking attributes are tiny eyes that function as light detectors; a greatly enlarged mouth with numerous slightly recurved teeth; an elongated stomach region, with the posterior end of the abdomen clearly demarcated from the tail, and an extremely long tail (about 75% of body length), with an elongated caudal filament that terminates in a "caudal organ" believed to be luminescent. Because of the delicacy of the body, the filaments often are broken off in captured specimens. The body is scaleless. The largest intact specimen measured 5.2 ft (1.6 m), although much of the body length consists of the elongated whiplike tail and caudal filament.

distribution

This species is the best known of the genus. It has been collected only from the North Atlantic Ocean between 10° and 65° north latitude.

habitat

The gulper eel is oceanic and bathypelagic. Only juveniles have been captured at depths of less than 2,624 ft (800 m). It is believed that adults typically reside deeper than 6,561 ft (2,000 m).

behavior

Because of the great depths of its habitat, aspects of the behavior of this species are largely the subject of conjecture.

feeding ecology and diet

The species is piscivorous. Relatively few saccopharyngids have been recovered with intact stomach contents, but in all cases various fish species were the prey. The gulper eel has an extremely distensible stomach, allowing it to ingest very large prey. Because of its weak skeleton and body muscles, it is believed to be a very poor swimmer. It is thought to lure prey within range by means of the luminescent caudal organ, which it may suspend in the water near its mouth. The jaw muscles are the only well-developed muscles and probably allow the gulper eel to suck its prey into the large mouth by quickly opening the jaws. Predators are unknown.

reproductive biology

Males and females are sexually dimorphic. Sexually mature males show extreme degeneration of the jaws, along with a loss of teeth and reduction in abdominal size. In addition, the eyes become somewhat enlarged, and the nasal apparatus is significantly enlarged. It has been suggested that males locate females by tracking pheromone (scent) trails released by the females. As with numerous eel species as well as some other deep-sea fish species, reproduction is thought to be a terminal event. As with eels in general, larval gulper eels have a leptocephalus, a ribbon-like transparent stage. Relatively few leptocephali have been collected, but all are deep-bodied and small, with a total length of 1.47–1.9 in (40–50 mm).

conservation status

Not threatened.

significance to humans

None known.

Resources

Books

Bertelsen, E., Jørgen Nielsen, and David G. Smith. "Families Saccopharyngidae, Eurypharyngidae, and Monognathidae." In Fishes of the Western North Atlantic, edited by Eugenia B. Böhlke. Part 9. New Haven: Sears Foundation for Marine Research, 1989.

Nelson, Joseph S. Fishes of the World. 3rd edition. New York: John Wiley and Sons, 1994.

Smith, David G. "Order Saccopharyngiformes, Family Cyematidae." In Fishes of the Western North Atlantic, edited by Eugenia B. Böhlke. Vol. 9, Part 1. New Haven: Sears Foundation for Marine Research, 1989.

——. "Families Cyematidae, Saccopharyngidae, Eurypharyngidae, and Monognathidae: Leptocephali." In Fishes of the Western North Atlantic, edited by Eugenia B. Böhlke. Vol. 9, Part 2. New Haven: Sears Foundation for Marine Research, 1989.

Periodicals

Bertelsen, E., and Jørgen G. Nielsen. "The Deep-Sea Eel Family Monognathidae (Pisces, Anguilliformes)." Steenstrupia 13, no. 4 (1987): 141–198.

Gartner, John V. Jr. "Sexual Dimorphism in the Bathypelagic Gulper Eel Eurypharynx pelecanoides (Lyomeri: Eurypharyngidae), with Comments on Reproductive Strategy." Copeia 2 (1983): 446–449.

Nielsen, Jørgen G., and E. Bertelsen. "The Gulper-Eel Family Saccopharyngidae (Pisces, Anguilliformes)." Steenstrupia 11(1985): 157–206.

Other

"FishBase: A Global Information System on Fishes." 7 Nov. 2002 (12 Nov. 2002). <http://www.fishbase.org/home.htm>

John V. Gartner Jr., PhD