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

Anguilliformes Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera, and about 800 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...

, 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 their ...

s, bobtail eels, and gulper 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 aphot ...

, approximately 500–1800 meters 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 oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

) are found in all major oceans.

Description

They have multiple internal differences from the rest of Anguilliformes. Notably, they have no symplectic bone, 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 surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods. Structure and function Structure In actinopterygians, the pelvic fin consists of two en ...

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 connective fascia (myosepta) and most easily seen in larval fishes or in the olm. Myomere ...

s (muscle segments) are V shaped instead of W shaped as in all other fishes, and their lateral lines 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 production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some Fungus, fungi, microorganisms including ...

. 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 the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...

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 The bobtail snipe eels are two species of deep-sea eels in the family Cyematidae, one only in each of two genera. They are small elongate fishes, growing up to 16cm (6 in) in length. The family Cyematidae is characterized by thin, short bodies w ...

(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 ...

(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 Saccopharyngiforms. 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 genomi ...

(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