The lamprey life cycle includes a prolonged larval stage, scientifically known as Ammocoetes, which is profoundly different from the adult form. This unique, non-parasitic phase is an extended juvenile period, sometimes lasting many years, necessary for the species to reach reproductive maturity. Ammocoetes are characterized by a sedentary, filter-feeding existence in freshwater sediments, contrasting sharply with the often-migratory and parasitic adult lamprey. The transition between these two disparate forms involves a significant transformation observed in the vertebrate world.
Identifying Ammocoetes and Their Habitat
The Ammocoetes larva possesses a slender, worm-like body shape, reaching lengths between 8 and 20 centimeters depending on the species. Unlike the adult, the larva lacks the distinctive oral sucking disk, having instead a semicircular oral hood surrounding the mouth. Their eyes are rudimentary and covered by thick skin, making them functionally blind, though they detect changes in light intensity. The absence of teeth and a continuous dorsal fin further differentiate the larva from the adult lamprey.
The habitat requirements for Ammocoetes are highly specific, centering on soft, fine sediment in freshwater streams and rivers. After hatching in gravel nests, the larvae drift downstream until they find areas with silt, fine sand, and detritus where they burrow. They remain sedentary within U-shaped or crescent-shaped burrows, often protected from strong currents and predation.
These nursery areas typically feature low water flow, generally less than 0.8 meters per second, which allows the fine substrate to accumulate. The larvae live entirely within this substrate, with only the oral hood exposed to the water column. This burrowed existence ties their survival directly to the availability of stable, soft-bottom habitats.
The Unique Filter-Feeding Stage
The core function of the Ammocoetes phase is to accumulate energy reserves through specialized filter feeding. The larvae are microphagous suspension feeders, relying on microscopic particles suspended in the water column. Their diet consists mainly of detritus, algae, diatoms, and various microorganisms, including bacteria.
The feeding mechanism involves a low-velocity, continuous pumping of water through the pharynx, generated by a muscular velum located behind the oral cavity. A specialized lining within the pharyngeal cavity produces mucus, which acts as a sticky net to trap suspended food particles. The food-laden mucus is then transported to the digestive tract for absorption.
This larval period is remarkably long, lasting anywhere from three to over ten years, depending on the species, temperature, and food availability. This extended duration is necessary to reach the minimum size and acquire the lipid reserves needed to fuel the subsequent non-feeding metamorphosis and the energetic demands of the adult life stage.
The efficiency of this filter feeding is high, despite the rate of water flow generated by the Ammocoetes being one of the slowest documented among suspension feeders. The unique pharyngeal structure and reliance on a mucus filter allow effective particle trapping, even at slow rates. The filter-feeding activity of the Ammocoetes can also have an ecological benefit by reducing the concentration of suspended bacteria in the water column.
The Process of Metamorphosis
Metamorphosis marks the irreversible transition from the sedentary, blind larva to the free-swimming, sighted juvenile form. This transformation begins when the Ammocoetes reach a species-specific size threshold, often triggered by environmental cues like seasonal temperature changes. The entire process is a non-trophic, or non-feeding, period lasting several months. The larva must rely entirely on the energy reserves accumulated during the filter-feeding phase.
Virtually every organ system undergoes a profound reorganization during this transformation. Externally, the most apparent changes include the development of large, functional eyes from the rudimentary ones. The oral hood is restructured, forming the characteristic suctorial oral disk and, in parasitic species, sharp, horn-shaped teeth.
Internally, the larval pharynx and the gill structure are altered to support the adult’s respiration. The endostyle, a glandular structure involved in larval feeding, transforms into the thyroid gland. The digestive system changes as the larval esophagus closes off, preparing for the adult’s specialized diet (parasitic feeding on other fish or, in non-parasitic species, no feeding at all). The completion of metamorphosis results in a juvenile that leaves the sediment, migrating downstream towards larger bodies of water.