Hagfishes and lampreys: evolutionary relationships
Ancestor–descendant relationships of jawless fishes are fraught with controversy. Just about every conceivable permutation on relationship among hagfishes, lampreys, and jawed fishes has been proposed at some time, including hagfishes, lampreys, or gnathostomes as ancestral to the other groups (see Hardisty 1982). The possible ancestors of jawless fishes are well represented in the Silurian and Devonian. But the extinct groups are very different from one another, as are extant jawless fishes from presumed ancestral groups and from each other.
Although traditionally treated as related orders in the subclass Cyclostomata (“round mouths”) – an hypothesis supported by recent molecular studies (see Nelson 2006) – similarities in the body morphology of modern hagfishes and lampreys are thought to reflect convergent evolution. It is probably wisest to deal with them individually and independently, and appreciate them for the unique yet primitive organisms that they are. Similarly, the “Agnatha”, previously given superclass status, is now recognized as being paraphyletic; the term is still used as an informal adjective for jawless fishes. Hagfishes are considered a more primitive, separate, non-vertebrate group in their own superclass, the Myxinomorphi, constituting the sister group of vertebrates and the basal craniate taxon (Nelson 2006).
Lampreys are placed in the infraphylum Vertebrata with all six superclasses of extinct and extant jawless and jawed fishes except the hagfishes. Vertebrates possess essential traits in common, especially dermal skeletal elements (secondarily lost in lampreys) and neural crest tissue (the embryonic nerve cord tissue that develops into gill arches, connective tissue, and bone), among others (see Subphylum Craniata, Infraphylum Vertebrata). Lampreys and hagfishes share a host of anatomical, physiological, and biochemical traits but have an even greater number of differences. Although both groups are scaleless, lampreys lack the mucus-producing capability of hagfishes. Lampreys have one or two dorsal fins supported by radial muscles and cartilage, whereas hagfishes have a single continuous caudal fin. Lampreys have a terminal mouth, hagfishes a subterminal mouth. Lampreys have a larval stage, hagfishes have direct development. In adult lampreys the external opening of the nasohypophysis is dorsal and the tract ends internally in a blind sac above the branchial region; in hagfishes, the external opening is terminal and the internal opening is into the pharynx. Lampreys have two semicircular canals, hagfishes only one. Lampreys have a pineal organ and functional eyes, hagfishes possess neither.
A major similarity between the two groups involves their immune responses. All gnathostomes, including fishes, have immune systems that involve immunoglobulin-type antigen receptors that produce pathogen-specific antibodies in response to infectious agents such as microbes. Lampreys and hagfishes also produce pathogen-specific defensive substances, but instead of antibody proteins, jawless fishes produce different kinds of proteins called variable lymphocyte receptors. Hence, “two strikingly different modes of antigen recognition . . . have evolved in the jawless and jawed vertebrates” (Alder et al. 2005, p. 1970).
Among the differences between the groups, lampreys possess lateral line neuromasts that are touch sensitive; these are lacking in hagfishes. All lampreys have seven gill openings, hagfishes vary between one and 16. Although the tongue possesses keratinous (horny), replaceable teeth in both groups, it is anatomically and functionally different. Myxinoids use the tongue for biting and tearing, whereas lampreys use it for rasping and suction. These and other differences in embryological, skeletal, neuromuscular, respiratory, cardiovascular, endocrine, osmoregulatory, chromosomal, and reproductive features all point out the disparate nature of the two groups (Hardisty 1982; Fernholm 1998; Nelson 2006).