ammocoetes

Ammocoetes are the larval form of lampreys (order Petromyzontiformes). They resemble small, eel-like organisms and inhabit freshwater streams and rivers, living buried in soft sediments for extended periods before metamorphosis into adults.

During the larval stage, ammocoetes are transparent or milky, with a slender, worm-like body up to several centimeters long. They lack jaws; instead they possess a circular mouth surrounded by a fleshy oral hood. They feed as filter feeders, drawing water through their pharynx to trap detritus and microscopic organisms in the sediment.

Lifecycle and metamorphosis: Ammocoetes typically reside for several years (often 3–7) buried in sediments, gradually developing eyes and a more robust digestive tract. They eventually metamorphose into adult lampreys, which may be parasitic and feed on fish, or non-feeding, depending on species.

Habitat and distribution: Ammocoetes occur in temperate freshwater habitats worldwide, particularly in rivers and streams with soft sediments. Their populations are affected by pollution, sedimentation, and barriers to migration such as dams, which can impede progression to the adult stage.

Significance: As a member of an ancient lineage of jawless vertebrates, ammocoetes provide insight into early vertebrate evolution and development, and their larval stage has informed studies of filter-feeding mechanisms and lamprey life cycles.