Petromyzoniformes

(Lampreys)

Class Cephalaspidomorphi

Order Petromyzoniformes

Number of families 3

Evolution and systematics

Modern vertebrates are classified into two major groups, the Agnathans (jawless vertebrates) and the Gnathostomes (jawed vertebrates). The Agnathans are classified into two groups, myxinoids (hagfishes) and petromyzonids (lampreys). The Gnathostomes include all other living vertebrates, including the bony and cartilaginous fishes and the tetrapods.

There are approximately 40 species of lampreys, which belong to the order of Petromyzoniformes. This order is divided into three families: the Petromyzonidae, the Northern Hemisphere lampreys (also referred to as the Holarctic species), and the two Southern Hemisphere families, Geotriidae and Mordaciidae. The Petromyzonidae consists of six genera: Ichthyomyzon, Petromyzon, Caspiomyzon, Eudontomyzon, Tetrapleurodon, and the Lampetra. The genus Lampetra is further divided into three subgenera: Entosphenus, Lethenteron, and Lampetra. The Geotriidae and Mordaciidae each consist of only one genus, Geotria and Mordacia, respectively.

The phylogeny of lampreys is based primarily on dentition and is justified by other shared anatomical traits, such as the proportional measurements of body parts, size of the adult, snout shape, eyes, and dorsal fins. The species of Ichthyomyzon are thought to be the most ancient of the lampreys because their simple teeth are arranged into rows throughout the entire oral disc. Of these species, the silver lamprey (I. unicuspis) is considered the most primitive.

Lampreys are representatives of the oldest lineage of vertebrates, the Agnathans. The agnathans probably arose as the first vertebrates about 550 million years ago, immediately after the evolutionary explosion of multicellular organisms in the Cambrian period. Paleontological analysis of extinct agnathans suggests that lampreys are more closely related to gnathostomes (the jawed vertebrates) than either group is to the hagfishes, although recent molecular analysis groups the hagfishes together with the lampreys in a single clade. Definite fossil records date back to the Upper Carboniferous, about 280 million years ago. Like hagfishes, lampreys are an important linkage between invertebrates and vertebrates and thus their anatomy and physiology are of interest to evolutionary biologists because they may retain characteristics of ancestral extinct species that are common to their closest relatives, the primitive fossil fishes.

Physical characteristics

Lampreys are scaleless, eel-like fishes that have skeletons of cartilage instead of bone. They have a notochord, but lack vertebrae. They also lack true fin rays and paired fins, but have one to two dorsal fins. Lampreys lack jaws but have teeth on the oral disc and tongue. Adult lampreys range in length from 7.9 in to 47.2 in (20 to 120 cm).

Lamprey species may be parasitic or nonparasitic. With a few exceptions, the nonparasitic species appear, based on characters and distribution, to have evolved from an extant parasitic lamprey. The four species of lampreys described in this chapter are parasitic lampreys: the sea lamprey (Petromyzon marinus), the silver lamprey (I. unicuspis), the pouched lamprey (Geotria australis), and the short-headed lamprey (Mordacia mordax). The sea lamprey and silver lamprey belong to the Northern Hemisphere family; the pouched lamprey and short-headed lamprey belong to the Southern Hemisphere family. Members of Petromyzonidae have the highest number of chromosomes (164–174) among vertebrates. Adult lampreys are distinct in their sex, either male or female. Lampreys spawn only once in their lifetime, after which they die. Parasitic lampreys are generally anadromous.

Distribution

Lampreys occur mainly in temperate zones. Parasitic species are generally of one of two types: those that are anadromous and feed at sea, and those that are restricted to river systems. There is little information on the marine distribution of anadromous species, although it is thought that the larger species move farther away from the coastline than the smaller species. The larger species may in turn give rise to forms that feed in lakes, such as the landlocked sea lamprey of the Great Lakes. Nonparasitic species are restricted to fresh water, most commonly to creeks and smaller rivers. The sea lamprey is found in coastal waters on both sides of the North Atlantic and in the western Mediterranean, and also in fresh waters of the Atlantic coasts of Europe and North America. The land-locked sea lamprey is found in the Great Lakes of North America. The silver lamprey is found in and around the states and provinces of the Great Lakes region. The distribution records of Southern Hemisphere lampreys are less well known due to lack of systematic investigations, but it is known that the pouched lamprey occurs in New Zealand, Western Australia, and Tasmania, and on both coasts of South America; the short-headed lamprey occurs only in southeast Australia and Tasmania.

Habitat

Larval lampreys are wormlike filter-feeding fishes that bury in the sand or mud of rivers. Toward the end of the larval phase, lampreys undergo an extensive metamorphosis in which they become free swimming and migrate to oceans or lakes, where they become parasitic. After one to three years in the parasitic phase, lampreys return to freshwater streams with sand, gravel or pebble substrates to spawn.

Behavior

Lampreys spawn only once in their lifetime, after which they die. The parasitic lampreys begin their lives as freshwater ammocoetes (larval lampreys), which are blind, filter-feeding larvae. After approximately three to seven years in freshwater streams, metamorphosis occurs, and the ammocoetes become free-swimming, sexually immature lampreys, which migrate to the sea or lakes. The actual time for the parasitic phase is not known for all species, but is generally thought to be one to two years. After this period, lampreys return to freshwater streams and undergo the final maturational processes resulting in mature eggs and sperm. The lampreys carry out specific spawning behaviors, including nest building and fanning behavior, after

which they die. Prior to metamorphosis, the parasitic and non-parasitic lampreys are indistinguishable. After metamorphosis, the two are distinguished based on size, feeding behavior, and gonad structure, among other traits.

Feeding ecology and diet

During their larval phase, lampreys feed on microscopic plankton, algae and detritus filtered from the mud. During the parasitic phase, they attach to a host fish and extract the blood and/or muscle tissue. Lampreys do not feed in the final spawning phase. Natural predators of the nonparasitic and immature parasitic lampreys include a variety of species of fishes (e.g., eel sand trout) and birds (e.g., gulls).

Reproductive biology

The gonad in both sexes of lamprey sexes is unpaired and median, and is suspended from the dorsal wall of the body cavity by means of a mesentery containing connective tissue. Lampreys are among the few vertebrates, including teleost fishes, that have no intraperitoneal genital ducts. After hatching, for periods varying from six months to over two years in the larval phase, the undifferentiated gonad shows comparatively little further development. Throughout this stage, the germ cells divide only slowly, if at all, remaining solitary or arranged in small groups of slightly advanced cells. After this period, these cells continue to develop into primary oocytes, which occur in all ammocoete gonads regardless whether the lamprey is to become a male or female. Just before metamorphosis, the lampreys undergo sexual differentiation. In lampreys destined to become females, the gonad will continue with the process of oogenesis. In males, the oocytes undergo degeneration and atresia, and the remaining germ cells develop into nests of primary spermatogonia either shortly before, or during, metamorphosis. In the parasitic sea lamprey, sexual maturation is a seasonal, synchronized process. During the parasitic sea phase, which lasts for approximately one to three years, the development of the gametes progresses. In males, spermatogonia proliferate and develop into primary and secondary spermatocytes; in females, vitellogenesis occurs. After this period, lampreys return to freshwater streams and undergo the final maturational processes that result in mature eggs and sperm, and finally spawn, after which the lampreys die. Both sexes of lampreys develop secondary sexual characters during the final weeks of reproduction and spawning activity.

Conservation status

No species of Petromyzoniformes are included on the IUCN Red List.

Significance to humans

Lampreys are important species in the ecosystems in which they reside, whether in streams as filter-feeding organisms

helping to recycle nutrients, or as food for predatory fishes and birds in streams and oceans. In certain parts of the world, such as New England in the United States, efforts are being made by state and federal agencies to maintain or increase the populations for this reason. However, in the Great Lakes Region and Lake Champlain, sea lampreys are considered a major deterrent to fish populations because of their parasitic phase, during which they feed on other fishes with their suctorial mouth, extracting body fluids, and often causing high mortalities. The extraordinary amount of damage to the fishery of the Great Lakes caused by the invasion of the sea lamprey has resulted in one of the largest and most intensive efforts to control a vertebrate predator ever attempted. The lampreys are believed to have invaded the Great Lakes beginning with the opening of the Erie Canal in 1819, and the Welland Canal in 1829, which allowed the movement of fishes from Lake Ontario into the Upper Great Lakes from the Atlantic Ocean. By the 1930s, the lampreys had established themselves in all the Great Lakes. The Great Lakes Fishery Commission (GLFC) was established in 1955 by a treaty between Canada and the United States. The two major responsibilities of this Commission were, and continue to be, to develop coordinated programs of research in the Great Lakes, and to formulate and implement programs to eradicate or minimize sea lamprey populations in the Great Lakes.

While lampreys are not presently regarded as food fishes, they were highly prized by both classical and medieval consumers of sea food.

Species accounts

List of Species

Pouched lamprey

Geotria australis

family

Geotriidae

taxonomy

Geotria australis Gray, 1851.

other common names

English: Wide-mouthed lamprey; Spanish: Anguila blanca, lamprea de bolsa.

physical characteristics

Total length generally around 19.7 in (50 cm), but fishes up to 24.4 in (62 cm) have been reported. Eel-like, scaleless, lack jaws, have funnel-like mouths and cartilaginous skeletons. Grayish in color with bands of blue-green or brown, depending on stage of reproductive development. Gonad in both sexes is unpaired and median, and is suspended from the dorsal wall of the body cavity by means of a mesentery containing connective tissue.

distribution

Coastal waters of continents of the Southern Hemisphere; also upstream within freshwater tributaries.

habitat

Heads of freshwater streams in coastal areas. Lives in open waters for approximately two years, then returns to fresh waters to spawn.

behavior

Anadromous; returns to fresh waters to reproduce, during which time it carries out spawning behaviors, including nest building and fanning behavior.

feeding ecology and diet

Larvae feed on microscopic plankton, algae, and detritus filtered from the mud. During the parasitic phase, this lamprey attaches to a host fish and extracts blood and/or muscle tissue. Does not feed during migration upstream to spawn in fresh water.

reproductive biology

The spawning run lasts for approximately 16 months and takes place during the night, particularly during heavy rains and on nights with a dark moon. The female releases her eggs, which are fertilized by released sperm from the male. The adult lampreys die shortly after spawning.

conservation status

Not listed by the IUCN.

significance to humans

Research on this species can provide insight into human biology and perhaps yield medicinal applications.

Short-headed lamprey

Mordacia mordax

family

Mordaciidae

taxonomy

Mordacia mordax Richardson, 1846.

other common names

English: Australian lamprey, Murray lamprey.

physical characteristics

Length generally around 20 in (50 cm). Eel-like, scaleless, lack jaws, have funnel-like mouths and cartilaginous skeletons. Body grayish brown in color, but turns blue during upstream spawning migration. The gonad in both sexes is unpaired and median, and is suspended from the dorsal wall of the body cavity by means of a mesentery containing connective tissue.

distribution

Coastal waters of southeastern Australia and Tasmania, as well as upstream within freshwater tributaries.

habitat

Heads of freshwater streams of coastal areas of southeastern Australia and Tasmania. Lives in the open waters around southeastern Australia, then returns to fresh waters to spawn.

behavior

Anadromous; returns to fresh waters to reproduce, during which time it carries out spawning behaviors, including nest building and fanning behavior.

feeding ecology and diet

Larvae feed on microscopic plankton, algae, and detritus filtered from the mud. During the parasitic phase, adult attaches to a host fish and extracts blood and/or muscle tissue. Does not feed after migrating upstream to spawn in fresh water.

reproductive biology

Female releases her eggs, which are fertilized by released sperm from the male. The adults die shortly after spawning.

conservation status

Not listed by the IUCN.

significance to humans

Research on this species can provide insight into human biology and perhaps yield medicinal applications.

Silver lamprey

Ichthyomyzon unicuspis

family

Petromyzonidae

taxonomy

Ichthiomyzon unicuspis Hubbs and Trautman, 1937.

other common names

None known.

physical characteristics

Total length 15.3 in (39 cm). Eel-like, scaleless, lack jaws, have funnel-like mouths and cartilaginous skeletons. Body grayish brown in color. Gonad in both sexes is unpaired and median, and is suspended from the dorsal wall of the body cavity by a mesentery containing connective tissue. Considered the most primitive Ichthyomyzon species.

distribution

Hudson Bay and Great Lakes regions, as well as the St. Lawrence river system.

habitat

Heads of freshwater streams around the Great Lakes and Hudson Bay regions, as well as the St. Lawrence.

behavior

Anadromous; returns to fresh waters to reproduce, during which time it carries out spawning behaviors, including nest building and fanning behavior.

feeding ecology and diet

Larvae feed on microscopic plankton, algae, and detritus filtered from mud. During the parasitic phase, adult attaches to a host fish and extracts blood and/or muscle tissue. Does not feed after migrating upstream to spawn in fresh water.

reproductive biology

Female releases her eggs, which are fertilized by released sperm from the male. The adults die shortly after spawning.

conservation status

Not listed by the IUCN.

significance to humans

Research on the species can provide insight into human biology and perhaps yield medicinal applications.

Sea lamprey

Petromyzon marinus

family

Petromyzonidae

taxonomy

Petromyzon marinus Linnaeus, 1758.

other common names

English: Eel sucker, Green sea lamprey, lamprey eel; French: Lamproie marine; German: Große lamprete; Spanish: Lamprea de mar.

physical characteristics

Total length 47.2 in (120 cm). Eel-like, scaleless, lack jaws, have funnel-like mouths and cartilaginous skeletons. Body grayish brown in color. Gonad in both sexes is unpaired and median, and is suspended from the dorsal wall of the body cavity by a mesentery containing connective tissue.

distribution

Coastal waters on both sides of the North Atlantic, the western Mediterranean, also fresh waters of the Atlantic coasts of Europe and North America: landlocked in the Great Lakes of North America.

habitat

Immature fishes can be found in the mouths of freshwater streams of eastern North America, Northern Europe, and

western regions of the Mediterranean. Mature fishes live in the open waters of the Atlantic Ocean and Mediterranean Sea.

behavior

Anadromous; returns to fresh waters to reproduce, during which time it carries out spawning behaviors, including nest building and fanning behavior.

feeding ecology and diet

Larvae feed on microscopic plankton, algae, and detritus filtered from mud. During the parasitic phase, adult attaches to a host fish and extracts blood and/or muscle tissue. Does not feed after migrating upstream to spawn in fresh water.

reproductive biology

Female releases approximately 200,000 eggs, which are fertilized by released sperm from the male. The adults die shortly after spawning.

conservation status

Not listed by the IUCN. Considered a critical species in their natural ecosystems and efforts are being made by state and federal agencies to maintain or increase populations there. In the Great Lakes, where the species has been introduced, authorities are working to control their populations because of their detrimental impact on native fishes.

significance to humans

Very destructive to fish populations in the Great Lakes Region and Lake Champlain. During the parasitic phase, feeds on other fishes with its suctorial mouth, extracting body fluids and often causing high mortalities.

Resources

Books

Fulton, W. "Tasmanian Freshwater Fishes." In Fauna of Tasmania Handbook. No. 7, edited by A. M. M. Richardson. Tasmania: University of Tasmania, 1990.

Hardisty, M. W., and I. C. Potter. The Biology of Lampreys. New York: Academic Press, 1971.

Sower, S. A., and A. Gorbman. "Agnatha." In Encyclopedia of Reproduction. Vol. 1, edited by E. Knobil and J. D. Neill. New York: Academic Press, 1999.

Periodicals

Hardisty, M. W., I. C. Potter, and R. W. Hillard. "Gonadogenesis and Sex Differentiation in the Southern Hemisphere Lamprey, Geotria australis Gray." Journal of Zoology 209 (1986): 477–499.

Smith, B. R., ed. "Proceedings of the Sea Lamprey International Symposium." Canadian Journal of Fisheries and Aquatic Sciences 37 (1980): 1,585–2,215.

Stacia A. Sower, PhD

Matthew R. Silver, BS