|Pelican eel (Eurypharynx pelecanoides)|
The saccopharyngiforms are a derived lineage of unusual eels within the order Anguilliformes, 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, bobtail eels, and gulper eels. Some can live deep in the ocean, well into the aphotic zone. 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) are found in all major oceans.
They have multiple internal differences from the rest of Anguilliformes. Notably, they have no symplectic bone, operclular bones, ribs, or swim bladders. Like many other eels, they lack scales and pelvic fins. 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.
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.
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
- Poulsen, Jan Y.; Miller, Michael J.; Sado, Tetsuya; Hanel, Reinhold; Tsukamoto, Katsumi; Miya, Masaki (2018-07-25). "Resolving deep-sea pelagic saccopharyngiform eel mysteries: Identification of Neocyema and Monognathidae leptocephali and establishment of a new fish family "Neocyematidae" based on larvae, adults and mitogenomic gene orders". PLOS ONE. 13 (7): e0199982. doi:10.1371/journal.pone.0199982. ISSN 1932-6203. PMC 6059418. PMID 30044814.
- Orton, Grace L. (1963). "Notes on Larval Anatomy of Fishes of the Order Lyomeri". Copeia. 1963 (1): 6–15. doi:10.2307/1441268. JSTOR 1441268.
- "Volume 1and 2: Orders Anguilliformes and Saccopharyngiformes and Leptocephali | Yale University Press". yalebooks.yale.edu. Retrieved 2018-09-25.
- Santini, Francesco; Kong, Xianghui; Sorenson, Laurie; Carnevale, Giorgio; Mehta, Rita S.; Alfaro, Michael E. (2013). "A multi-locus molecular timescale for the origin and diversification of eels (Order: Anguilliformes)". Molecular Phylogenetics and Evolution. 69 (3): 884–894. doi:10.1016/j.ympev.2013.06.016. ISSN 1095-9513. PMID 23831455.