The Petromyzoniformes with Particular Reference to Paired Species

Abstract

The distribution of living lampreys (Petromyzoniformes) is antitropical, a feature almost certainly related to the fact that the ultimate incipient lethal temperature of ammocoetes lies between ~28 and 32 °C. In general, the species that attain the largest body size have the widest distribution. The holarctic species are placed in the Petromyzonidae, while those of the Southern Hemisphere are assigned either to the Geotriidae or Mordaciidae. Reference is made to the techniques used in lamprey systematics, placing particular emphasis on defining the terminology of the components of the dentition and the ways in which body proportions can be measured. Using dentitional characters, the Petromyzonidae can be arranged in a logical sequence from Ichthyomyzon and Petromyzon to either Caspiomyzon or to series consisting either of Eudontomyzon and Tetrapleurodon or the subgenera of Lampeira, i.e. Entosphenus, Lethenteron, and Lampetra. A list is given of the 17 parasitic and 22 nonparasitic lampreys described from the Northern and Southern hemispheres. The evolution of nonparasitic lampreys has involved an extension of larval life and a reduction in the postmetamorphic period, a process which has led to a marked reduction in the size of the mature adults. The more conspicuous differences between nonparasitic and parasitic species start to appear after the onset of metamorphosis. These include the rapid enlargement and maturation of gonads in nonparasitic species. However, significant differences have been found between the larvae of nonparasitic lampreys and those of their parasitic ancestor in such features as the number of oocytes, trunk myomeres, and size at the onset of metamorphosis.Key words: lampreys, life cycles, parasitic, nonparasitic, distribution, dentition, phylogeny, Petromyzonidae, Geotriidae, Mordaciidae