Adaptive Radiation of Lampreys

Abstract

Lampreys, together with hagfishes, are the sole survivors of the jawless (agnathan) stage in vertebrate evolution. The 34 species of northern hemisphere lampreys are placed in the Petromyzontidae, whereas each of the four species of southern hemisphere lampreys are separated into either the monospecific Geotriidae or monogeneric Mordaciidae. Eighteen species feed parasitically as adults in either marine or fresh waters, whereas 20 species are nonparasitic and have apparently evolved from parasitic species. This account of adaptive radiation is based on the hypothesis that the basal stock of contemporary lampreys possessed many characters that were similar to those of contemporary Ichthyomyzon and Petromyzon species, and that the Mordacia species and Geotria australis have been separated for a long period. Most of the morphological differences among the adults of northern hemisphere lampreys (i.e., variations in dentitional arrangement and size of buccal glands and velar tentacles) are related to whether the species feeds mainly on blood, as with Ichthyomyzon and Petromyzon species, or predominantly on flesh, as with the more derived Lampetra species. Mordacia species have apparently remained blood feeders, while G. australis has become a flesh feeder. The eyes of the adults of southern hemisphere lampreys are unique amongst lampreys, with those of G. australis possessing two types of cone photoreceptor (as well as a rod photoreceptor) and an irideal flap, while those of Mordacia mordax possess only a rod photoreceptor but contain a tapetum. These differences represent adaptations to life in bright surface waters by G. australis and to diurnal burrowing and nocturnal activity by Mordacia species. The restriction of larval intestinal diverticula to Mordacia species and G. australis may represent adaptations for increasing the ability to digest the particularly tough plant material that is found in southern hemisphere rivers. The bile duct of the larvae of southern hemisphere lampreys enters the apex of the above intestinal diverticula and thus, unlike the situation in northern hemisphere lampreys, does not play a role in forming the endocrine pancreas of adult lampreys during metamorphosis. This accounts for the absence of intermediate and caudal pancreatic cords in the adults of southern hemisphere species. The amino acid sequences of intestinal molecules in different lamprey taxa support the view that the lampreys are monophyletic and that polyploidy occurred early in or just prior to their evolution.