Lampreys are eel-like fish that belong to the order Petromyzontiformes. Together with hagfish, they constitute the infraphylum Agnatha, a group of jawless fish. They are among the most primitive vertebrates, characterized by a cartilaginous skeleton and a toothed oral disc used for sucking on prey.
These fish inhabit both freshwater and marine environments in temperate regions, except Africa, with some species migrating between the two habitats. Unlike most fish, lampreys lack scales, paired fins, and bones and instead possess a unique set of seven pairs of gill pouches for respiration. Their counter-shaded bodies, with dark backs and pale undersides, provide natural camouflage against their predators.
Their lifecycle starts off as filter-feeding larvae in freshwater before undergoing metamorphosis into adults. While some species are parasitic, feeding on the blood and body fluids of other fish, others are non-parasitic and rely solely on their larval nutrition.
The largest species, the sea lamprey, measures around 3.9 ft (1.2 m), while the smallest predatory lamprey, the Miller Lake lamprey, is around 3 to 6 in (7.6 to 15.2 cm) long.
These fish have scaleless, elongated bodies, just like eels, with no paired fins. They have a single nostril on top of their head and seven gill openings or pores on each side of their body.
As jawless fish, they are devoid of jaws and have an internal skeleton composed of cartilage.
Their gonads lie in the abdominal cavity.
The buccal cavity lies anterior to the gonads and comprises a round sucking-type mouth or the oral disc. The tongue is piston-like and bears three horny plates (laminae) on the tip. One plate is transverse, while the other two are positioned longitudinally. Additionally, a flap-like muscular structure called the velum is located near the mouth, helping to direct water currents toward it.
The teeth are composed of keratin and other proteins and possess a hollow core that provides room for new teeth to grow and replace the older ones.
Their heart is composed of a sinus, a single atrium, and a ventricle covered by pericardial cartilage.
Unlike most fish with gill slits leading to gill arches, lampreys possess seven pairs of tiny gill pouches on each side of their body. They undergo unidirectional tidal ventilation, in which water moves in and out of the gill pouches without passing through the mouth.
These fish lack lungs.
They do not have a true stomach, and instead, the food passes directly from the esophagus to the intestine, lying behind the notochord. Unlike in most vertebrates, their intestine is simple, straight, and tubular. In addition to digestion, it plays a crucial role in osmoregulation by reducing the salinity of ingested water and ensuring it reaches an iso-osmotic state relative to the blood.
Lampreys are the only extant vertebrates that possess four functional, well-developed eyes: a pair of regular eyes and two parietal eyes (a pineal and one parapineal). In juveniles, the eyes resemble spots (poorly developed) and are covered in non-transparent skin.
Members of the family Mordaciidae have a single type of rod-like photoreceptor cells, while those in the family Petromyzontidae possess two types of photoreceptor cells (both rod-like and cone-like). Lampreys of the family Geotriidae have as many as five types of photoreceptors.
The brain of lampreys is compartmentalized into diencephalon, mesencephalon, telencephalon, cerebellum, and medulla.
The name lamprey derives from the Latin word lampetra, a combination of two words: lambere, meaning ‘to lick’ and petra, meaning ‘stone.’
These fish are members of the subphylum Vertebrata (hence vertebrates) and are placed under the infraphylum Agnatha, along with the other group of jawless fish, the hagfish. According to the 5th edition of Fishes of the World, lampreys are placed under the class Petromyzontida and order Petromyzontiformes.
About 38 extant species of lampreys are divided into 3 families and 10 genera. Currently, there are 7 extinct lamprey species.
Lampreys are more closely related to hagfish (order Myxiniformes) than jawed vertebrates. Together, both groups form the superclass Cyclostomi. However, they also differ from hagfish in a number of aspects. For instance, lampreys have well-developed eyes and a complex cartilaginous skeleton, whereas hagfish are almost blind and have a simpler skeletal structure.
The oldest lamprey, Priscomyzon, dates back to around 360 million years ago (Late Devonian Period, South Africa). Other old stem-group lamprey fossils include Pipiscius, Mayomyzon, and Hardistiella, which were unearthed from Carboniferous fossils of North America. These old lampreys were smaller than modern ones and lacked the specialized toothed discs.
Yanliaomyzon from the Middle Jurassic Period (around 163 million years old) was the first lamprey to possess the toothed oral disc.
Though views vary, the last common ancestor (LCA) of all extant lampreys is thought to have existed either around 175 million years ago in the Middle Jurassic Period or later in the Late Cretaceous Period. Most modern forms are believed to have originated in the Cenozoic Era, especially in the last 10 to 20 million years.
These fish inhabit both coastal and freshwater environments in the temperate region of the world. The two families, Geotriidae and Mordaciidae, are abundant in the Southern Hemisphere, while Petromyzontidae is restricted to the Northern Hemisphere. However, lampreys are not found in Africa.
Some species, like the sea lamprey, have established their population (as invasive species) in landlocked water bodies, like the Great Lakes in North America.
While lamprey larvae feed on microscopic organic matter in the water, adults consume a variety of prey. Many species, like the sea lamprey and the Pacific lamprey, live off the tissues, blood, and other bodily fluids of host fish and marine mammals. Their preferred fish varieties include salmon, trouts, sturgeons, pikes, burbots, whitefish, cods, and walleyes.
Some species, such as the Australian brook lamprey, do not feed as adults; instead, they survive on the nutrients stored from their larval stage.
Parasitic lampreys typically attach themselves to their prey using the suction generated with the help of their disc-like mouth. They then use the horny plates on their tongue to scrape through the surface tissues and reach the body fluids.
These fish also produce anticoagulants from their buccal glands to prevent blood clotting while they feed.
They are excellent swimmers, moving by creating low-pressure zones around their bodies. This pressure difference helps them glide smoothly along the current by pulling their bodies forward in the water.
The lifespan of these fish varies by species. For example, the sea lamprey typically lives for 8 to 9 years but can survive up to 20 years. The Pacific lamprey has a lifespan of 7 to 11 years, while the European brook lamprey lives up to 7 years.
Adult lampreys spawn in freshwater environments, typically in nests made of sand, gravel, and pebbles. These eggs hatch into larvae called ammocoetes that drift downstream with the current to settle on fine sediment, such as silt beds. These larvae are filter-feeders that grow from 3 to 4 in (8 to 10 cm) to about 8 in (20 cm). They may remain in the larval stage for anywhere between a year to as many as eight years, after which they metamorphose into adults. Modern lampreys spend most of their life as ammocoetes.
During metamorphosis, which typically lasts for 3 to 4 months, the larvae lose their gallbladders and biliary tracts, and their endostyle forms the thyroid gland. Additionally, they develop
eyes, teeth, and a sucking mouth. The larvae do not usually feed during this phase.
Adults may migrate to marine habitats (anadromous) or remain in freshwater environments (non-anadromous) to spend the rest of their lives.
These adults migrate to the sea and switch to a carnivorous diet by preying on fish or marine mammals. They spend up to four years in the sea and return to freshwater for spawning. The females lay numerous eggs (sometimes up to 100,000), which the males fertilize externally. Shortly after the eggs are fertilized, both the males and the females die (semelparity).
They continue to remain in freshwater, stop feeding, and die shortly after metamorphosis.
These fish are preyed upon by water birds, aquatic mammals like otters, and other fish, such as pikes, bass, and catfish.
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