Station 7: Phylum Vertebrata: Class Petromyzontida

Petromyzontida is a group of vertebrates that includes modern lampreys and their fossil relatives. The only surviving members of this lineage today are the lampreys, classified in the order Petromyzontiformes. Fossil evidence, such as the discovery of Priscomyzon from the Late Devonian, reveals that lampreys are among the most ancient known vertebrates. Phylogenetic analyses suggest that their lineage diverged early from that of other vertebrates, making them especially important for understanding vertebrate evolution.

Lampreys are found primarily in temperate coastal and freshwater habitats of the Northern and Southern Hemispheres, though they are absent from Africa. Some species are anadromous, meaning they migrate between freshwater and marine environments, while others are restricted to inland lakes. Their larvae, known as ammocoetes, are sensitive to high water temperatures, which likely limits the group’s distribution in tropical regions.

Unlike Myxini, lampreys possess rudimentary vertebral elements, meaning they have small, simple structures associated with the notochord that resemble the beginnings of a backbone. These structures, called arcualia, are made of cartilage and provide limited support but do not form a fully segmented vertebral column like those seen in jawed vertebrates.

During development, lampreys form somites, which are repeating blocks of tissue that develop along the length of the embryo. In most vertebrates, somites give rise to muscles and parts of the skeleton. One important part of each somite is the sclerotome, which is the portion that develops into vertebrae in animals with a fully formed backbone. In lampreys, somites are present, but the sclerotome is reduced, meaning it does not fully develop into distinct vertebrae.

This combination of features places lampreys in a critical evolutionary position. They show an intermediate condition between animals like hagfish, which lack vertebral elements entirely, and jawed vertebrates, which possess a fully developed backbone. As a result, lampreys provide important insight into how the vertebral column evolved from a simple notochord into the segmented structure seen in most vertebrates today.

After hatching, lamprey larvae—called ammocoetes—drift downstream until they reach areas of soft sediment and detritus. There they burrow into the substrate and take up a sedentary existence as filter feeders, drawing in microscopic algae, detritus, and microorganisms through the mouth and pharynx. Ammocoetes have poorly developed eyes that can only detect changes in light intensity, and they rely on ciliary currents and mucus to trap suspended particles.

The body of the ammocoetes is elongated and laterally compressed, with visible myotomes, gill pouches, and a continuous notochord. They lack the suctorial oral disc of adults, instead possessing an oral hood fringed with small papillae that help funnel water toward the pharynx. Ammocoetes can grow to 8–20 cm in length and may remain in this stage for several years, depending on species and environmental conditions.

During metamorphosis, which lasts several months, the larva undergoes dramatic anatomical changes. The eyes enlarge, the oral hood transforms into a circular mouth equipped with keratinized teeth, and the digestive system reorganizes. The result is a fully formed adult adapted to a very different mode of life.

Adult lampreys possess elongated, cylindrical, scaleless bodies that superficially resemble eels and range from 13 to 100 cm (5 to 40 inches) in length. They lack paired fins and have a single median nostril atop the head, seven gill openings on each side, and large lateral eyes.

The most distinctive feature of the adult lamprey is its circular, suctorial mouth equipped with concentric rows of keratinized teeth and a rasping tongue. This specialized feeding apparatus enables parasitic species to attach firmly to the bodies of fish hosts, using suction to maintain contact while the tongue abrades the skin and underlying tissue. The lamprey then secretes an anticoagulant, preventing the host’s blood from clotting as it feeds on blood and tissue fluids. Some species, such as the brook lamprey (Lampetra planeri), are nonparasitic as adults and do not feed after metamorphosis, surviving only long enough to reproduce.

Reproduction occurs in freshwater, where adults construct nests in gravelly or sandy streambeds. After spawning, the adults die, and the eggs hatch into larvae, continuing the life cycle.

While native lamprey species inhabit many North American rivers, the sea lamprey (Petromyzon marinus) is an introduced and highly destructive invader in the Great Lakes. Originally native to the Atlantic Ocean, the sea lamprey is an anadromous parasite that attaches to fish using its toothed oral disc and rasps away flesh to feed on blood and body fluids. In its native range, sea lamprey populations are kept in balance by coevolution with their host fish species and by natural barriers to spawning migration. However, human modifications to waterways altered that balance dramatically.

Introduction and Spread

The sea lamprey first entered Lake Ontario in the early 1800s, likely through the natural connection with the Atlantic Ocean via the St. Lawrence River. For decades, the species was unable to move farther inland because of natural barriers such as Niagara Falls. This changed with the construction of the Welland Canal, a ship passage built to bypass Niagara Falls and connect Lake Ontario with Lake Erie. By the 1920s, sea lampreys had invaded all five Great Lakes.

Without natural predators or effective environmental controls, the lampreys quickly established breeding populations in tributary streams. Each adult female is capable of laying up to 100,000 eggs. The larvae—ammocoetes—develop in stream sediments for several years before metamorphosing into parasitic adults that move into open lake waters to feed on large fish such as trout, whitefish, and salmon. Each feeding lamprey can kill multiple fish over its lifetime. By the 1940s, sea lamprey predation had caused catastrophic declines in native fish populations, including the near collapse of lake trout fisheries.

Ecological and Economic Impact

The invasion profoundly altered Great Lakes ecosystems. With the decline of top predators, food webs destabilized, leading to increases in smaller fish and invasive species that further disrupted the system. Economically, commercial and recreational fisheries suffered enormous losses—by the 1950s, annual lake trout harvests had dropped by over 90 percent in some areas. The invasion became one of the most severe biological disruptions in North American freshwater ecosystems.

Control and Management

Efforts to control the sea lamprey began in earnest in the 1950s through cooperative programs between the United States and Canada, leading to the creation of the Great Lakes Fishery Commission in 1955. This binational organization coordinates sea lamprey management, research, and fishery restoration.

Control methods focus on targeting lampreys at different stages of their life cycle:

• Lampricides – The most effective tool has been the use of selective chemical treatments such as 3-trifluoromethyl-4-nitrophenol (TFM) and niclosamide, applied to spawning tributaries. These compounds kill lamprey larvae (ammocoetes) while leaving most other aquatic organisms unharmed when used at controlled concentrations.

• Barriers and Traps – Physical barriers are installed in streams to prevent adult lampreys from reaching spawning grounds. Traps placed near these barriers capture migrating adults for removal before they can reproduce.

• Sterile Male Release – Some programs release sterilized males into streams to compete with fertile males, reducing successful reproduction rates.

• Pheromone Research and Genetic Methods – Ongoing studies explore the use of chemical cues and genetic interventions to disrupt spawning behavior or bias sex ratios.

Current Status

Sea lamprey populations in the Great Lakes are now maintained at a small fraction of their mid-20th-century levels, allowing native fish populations to recover and commercial fisheries to rebound. However, control requires continual effort—lampreys can recolonize rapidly if management lapses. The Great Lakes Fishery Commission continues to refine integrated pest management approaches, balancing chemical, biological, and physical control methods to minimize environmental impact while maintaining ecological stability.

Below is an overview of the problem that Sea Lamprey have posed in lake systems.