The Saccopharyngiformes are a derived lineage of unusual eels within the order

Anguilliformes Eels are ray-finned fish belonging to the order (biology), order Anguilliformes (), which consists of eight suborders, 20 Family (biology), families, 164 genus, genera, and about 1000 species. Eels undergo considerable development from the earl ...

, and includes families Cyematidae, Monognathidae, Eurypharyngidae, Saccopharyngidae, and the proposed family Neocyematidae. Most of the fish in this group are deep-dwelling and rarely seen, typically known from only a handful of specimens. Species include recognizable fish such as

pelican eel The pelican eel (''Eurypharynx pelecanoides'') is a deep-sea eel. It is the only known member of the genus ''Eurypharynx'' and the family Eurypharyngidae. It belongs to the " saccopharyngiforms", members of which were historically placed in thei ...

s (also commonly known as gulper eels) and bobtail eels. Some can live deep in the ocean, well into the

aphotic zone The aphotic zone (aphotic from Greek prefix + "without light") is the portion of a lake or ocean where there is little or no sunlight. It is formally defined as the depths beyond which less than 1 percent of sunlight penetrates. Above the apho ...

, approximately deep. Extensive research has not been conducted on them due to being indirectly observed, with some species known only from their larvae. All families except for the exceptionally rare individuals of proposed family Neoceymatidae (known only from the

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

) are found in all major oceans.

Description

They have multiple internal differences from the rest of Anguilliformes. Notably, they have no

symplectic bone The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

, opercular bones, ribs, or swim bladders. Like many other eels, they lack scales and

pelvic fin Pelvic fins or ventral fins are paired fins located on the ventral (belly) surface of fish, and are the lower of the only two sets of paired fins (the other being the laterally positioned pectoral fins). The pelvic fins are homologous to the hi ...

s. Their

myomere Myomeres are blocks of skeletal muscle tissue arranged in sequence, commonly found in aquatic chordates. Myomeres are separated from adjacent myomeres by fascia consisting of connective tissue, known as myosepta. Myomere counts are sometimes us ...

s (muscle segments) are V-shaped instead of W-shaped as in all other fishes, and their

lateral lines The lateral line, also called the lateral line organ (LLO), is a system of Sensory perception, sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via ...

have no pores, instead being modified to groups of elevated tubules. The jaws are quite large, lined with small teeth, and several types are notable for being able to consume fish larger than themselves. Some species in families Eurypharyngidae and Saccopharyngidae are

bioluminescent Bioluminescence is the emission of light during a chemiluminescence reaction by living organisms. Bioluminescence occurs in multifarious organisms ranging from marine vertebrates and invertebrates, as well as in some fungi, microorganisms inc ...

. Like other eels, saccopharyngids have leptocephalus larvae. However, these larvae also have a number of unusual characteristics, such as remarkably deep bodies in the Cyematidae, long lower jaws in the Eurypharyngidae, and unique pigmented swellings at the ends of the gut in Saccopharyngidae and Eurypharyngidae. The three established families of the suborder Saccopharyngoidei (Eupharyngidae, Monognathidae, Saccopharyngidae) all exhibit

sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

Taxonomy

Until recently, the order "Saccopharyngiformes" was accepted as a separate order from Anguilliformes, based on their distinct morphological differences from the rest of the "true eels". Genetic work over the past few years has shown that it is instead a derived lineage within the Anguilliformes. The four (proposed five) families in two suborders are: * Suborder Cyematoidei ** Cyematidae (bobtail snipe eels) * Suborder Saccopharyngoidei **

Eurypharyngidae The pelican eel (''Eurypharynx pelecanoides'') is a deep-sea eel. It is the only known member of the genus ''Eurypharynx'' and the family Eurypharyngidae. It belongs to the "saccopharyngiforms", members of which were historically placed in their ...

(gulpers) **

Monognathidae ''Monognathus'', or onejaw, is the only genus of the family Monognathidae of deep-sea eels. The name comes from the Greek ''monos'' meaning "one" and ''gnathos'' meaning "jaw", a reference to the large mouth in comparison with the rest of the fi ...

(one-jaws) **

Saccopharyngidae ''Saccopharynx'' is a genus of deep-sea eels with large mouths, distensible stomachs and long, scaleless bodies. Commonly, these fish are called gulpers or gulper eels. It is the only genus in the family Saccopharyngidae, and is part of the deriv ...

(swallowers) **

Neocyematidae Neocyematidae is a family of fishes in the order Anguilliformes. This family, established from five described adult specimens of the species '' Neocyema erythrosoma'', was originally thought to be a part of the family Cyematidae, until genomic s ...

(proposed)

References

External links

Rarely-seen deep sea gulper eel (''Eurypharynx pelecanoides'') in the first time ever video of its massive jaws
November 2020. {{Taxonbar, from=Q621582 Bioluminescent fish Deep sea fish Eels