Order PETROMYZONTIFORMES (Hyperoartii) (2)—lampreys. Two semicircular canals; seven pairs of external lateral gill openings; eyes well developed in adult, lateral (except in Mordacia); single median nostril (nasohypophyseal)
opening between eyes with pineal eye behind; body naked, eel-like; no bone; no paired fins; one or two dorsal fins present; tail diphycercal (isocercal) in adults, hypocercal in ammocoete larvae; barbels absent; teeth on oral disc and tongue (except in fossil form); dorsal and ventral nerve roots separated; nasohypophy-seal sac with external opening only; spiral valve and cilia in intestinal tract; small cerebellum; sexes separate; eggs small, not yolky, occurring in the hundreds (Mordacia praecox) to thousands; larval stage (ammocoete) undergoes radical metamorphosis in freshwater. All lampreys die shortly after spawning.
Lampreys are either parasitic or nonparasitic, and both life-history types characterize individuals of closely related species. It is believed that nonparasitic species have been independently derived from a parasitic species. The parasitic phase, after metamorphosis from the ammocoete larvae but before reproducing, goes through a period of feeding on blood from other fishes (very rarely on other animals) by rasping through their skin. The nonparasitic phase reproduces, without feeding, after metamorphosis. It is always confined to freshwater, whereas the parasitic form may be freshwater or anadromous. No parasitic freshwater lampreys are known from the Southern Hemisphere. Maximum length of larvae about 22 cm and parasitic adult about 1.2 m.
The sister group of the petromyzontiforms, previously thought to be myx-inids, Jamoytius kerwoodi, or anaspidiforms, is now postulated to comprise all the following taxa (including the ostracoderms, i.e., all of the jawless and jawed vertebrates), following Donoghue et al. (2000). They were placed in the Class Cephalaspidomorphi in Nelson (1994).
The phylogenetic study of Gill et al. (2003) found a trichotomy between a monophyletic Northern Hemisphere clade (Petromyzontidae) and the Southern Hemisphere Geotriidae and Mordaciidae, and recommended that all three be treated as separate families. This recommendation is followed here. In the previous edition, all four lineages were recognized as subfamilies within the one family, Petromyzontidae.
Four families, one known only from fossils, and 10 genera with 38 extant species (Renaud, 1997; Gill et al., 2003; Kullander and Fernholm, 2003). Of the 38 species, 29 are confined to freshwater, and 18 feed parasitically as adults (and are generally said to be parasitic, but this usage is correctly understood by ichthyologists as not referring to them as parasites).
Family PETROMYZONTIDAE (2)—northern lampreys. Anadromous and freshwater; cool zones of the Northern Hemisphere, generally north of 30°N.
Three or four lateral circumoral teeth on each side of oral aperture (five or more in other lampreys); dorsal fins continuous or contiguous in mature adults (separate in other lampreys). (Gill et al., 2003, gave four unique characters.)
The following recognition of subfamilies is based on the cladogram of Gill et al. (2003). The subgenera recognized in LLampetra in Nelson (1994), with the exception of Okkelbergia,, are recognized as genera following Renaud (1997) and Gill et al. (2003). The number of species follows Renaud (1997).
Eight genera as follows with 34 species.
Subfamily Petromyzontinae. Median velar tentacles absent (one or two in other lampreys). Two genera as follows.
Ichthyomyzon. Freshwater; eastern North America; three pairs of species (i.e., six species), each pair with an ancestral parasitic species and a nonparasitic derivative.
Petromyzon marinus. Anadromous (landlocked in Great Lakes region); Atlantic drainages of Canada, United States, Iceland, and Europe (including the Mediterranean); parasitic.
Subfamily Lampetrinae. Tuberculated or papillose velar tentacles in most (smooth in other lampreys); 60 to 70 trunk myomeres in most (usually fewer than 60 or more than 70 in other lampreys).
Six genera as follows. According to the cladogram of Gill et al. (2003), Caspiomyzon is sister to the other five genera and Tetrapleurodon is sister to a clade comprising the remaining four genera, in which several nominal species exist that are of uncertain status and are not recognized here; these could be recognized in three sequenced tribes.
Caspiomyzon wagneri. Caspian Sea basin; probably parasitic. Tetrapleurodon. Freshwater; Rio Lerma system of southern Mexico; nonparasitic and parasitic; two species, T. geminis and T. spadiceus.
Entosphenus. Anadromous and freshwater; coastal regions of North Pacific in North America and Asia; parasitic and nonparasitic; seven species. Eudontomyzon. Freshwater; Black Sea drainage (primarily Danube basin), China, and Korea; parasitic and nonparasitic, four species. Lampetra. Anadromous and freshwater; coastal regions of Europe and North America; parasitic and nonparasitic; seven species (this includes the nonparasitic L. aepyptera, southeastern United States, recognized in the subgenus Okkelbergia in Nelson, 1994).
Lethenteron. Anadromous and freshwater; circumarctic drainage basins, western Pacific coast south to Japan, coastal regions of western Alaska, eastern North America, and Adriatic Sea basin; parasitic and nonparasitic; six species.
Family GEOTRIIDAE (3)—southern lampreys. Anadromous; Southern Hemisphere, southern Australia, Tasmania, New Zealand, Chile, Argentina, and the Falkland and South Georgia islands.
Teeth on oral disc are spatulate-shaped (pointed or rounded in other lampreys); supraoral lamina (= supraoral plate) with two large centrally located teeth flanked by two lateral flanges; transverse lingual lamina strongly trident, bident at maturity; velar tentacles 23-32; two well-developed diverticula in midgut of ammocoetes; caudal and second dorsal fins well separated in the immature (continuous or contiguous in other lampreys); dorsal fins separate from each other in mature adults; approximately 180 mainly acrocentric chromosomes (Gill et al., 2003, gave 10 unique characters). Parasitic.
One species, Geotria australis (e.g., Hubbs and Potter, 1971; Kullander and Fernholm, 2003).
Family MORDACIIDAE (4)—southern topeyed lampreys. Anadromous and freshwater; Southern Hemisphere, southeastern Australia, Tasmania, and southern Chile.
Two discrete supraoral laminae (= supraoral plate); transverse lingual lamina incurved, largest cusps are median and at each lateral edge; velar tentacles fewer than 5; one well-developed diverticulum in midgut of ammocoetes; dorsal fins separate from each other in mature adults; eyes dorsolateral in immature and dorsal in mature (lateral to dorsolateral in other lampreys); 76 metacentric and submetacentric chromosomes (Gill et al., 2003, gave 10 unique characters). Parasitic and nonparasitic.
One genus, Mordacia, with three species (e.g., Hubbs and Potter, 1971; Kullander and Fernholm, 2003).
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