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 stag ...
, 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 the ...
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 fins. 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 c ...
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.
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 an ...
.
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 the ...
(gulpers)
**
Monognathidae (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 (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