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 eels (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
myomeres (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 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 (gulpers)
**
Monognathidae (one-jaws)
**
Saccopharyngidae (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