Lampreys, an ancient group of jawless fish, have a distinct larval stage called the ammocoete. These larvae are remarkably different from their adult counterparts, both in appearance and lifestyle. Ammocoetes are unique burrowing, filter-feeding creatures, representing a significant portion of the lamprey’s life cycle. Early biologists even mistakenly classified ammocoetes as a separate species due to their pronounced differences from adult lampreys. This stage is integral to lamprey development, enabling growth and preparation for transformation.
The Ammocoete Stage
Ammocoetes are worm-like and generally blind, with rudimentary eyes covered by thick skin. Unlike adult lampreys, they lack a true mouth or buccal funnel, instead having a hood-like upper lip and a short lower lip. Their elongated, eel-like bodies feature a continuous dorsal fin that extends into a caudal fin. These larvae typically measure between 8 to 20 centimeters in length.
Ammocoetes burrow in soft freshwater stream sediments, preferring fine silts and sands. They often create U-shaped burrows in areas with slow-moving water and abundant organic detritus. While they can move between habitat patches, they generally remain in these burrows, emerging primarily at night to shift feeding grounds. Their preferred habitats often include eddies, alcoves, pools, and stream margins where fine sediments accumulate.
These larvae are specialized filter feeders, drawing water through their mouths and expelling it via gill slits. They use a velar pump and a mucus-based system within their pharynx to trap microscopic organisms. Their diet primarily consists of diatoms, algae, and various detritus particles. This unique feeding mechanism allows ammocoetes to extract food while remaining largely concealed within the substrate.
From Larva to Adult
The ammocoete stage is prolonged, varying significantly among species and environmental conditions. Larvae typically remain in this form for several years, commonly ranging from 3 to 7 years. Some species, like the Arctic lamprey, may spend only 1 to 2 years as larvae, while others, such as certain sea lamprey populations, can extend this stage for as long as 17 years.
After this extended larval phase, metamorphosis occurs. This involves significant physiological and morphological changes, preparing the lamprey for adult life. During metamorphosis, the previously rudimentary eyes develop fully, and a distinct circular oral disc with teeth begins to form. The body also elongates, and the digestive system undergoes substantial remodeling, including the loss of structures like the gallbladder and biliary tract in some species.
Metamorphosis from a blind, burrowing filter feeder to an eyed, free-swimming adult is complex and can last several months, typically with a non-feeding period. The changes include shifts in organ systems, such as the kidney, to adapt to new environments like saltwater for anadromous species. While some lamprey species remain non-feeding as adults, others develop into parasitic forms, relying on their newly formed oral disc and teeth to feed on other fish.
Role in Aquatic Ecosystems
Ammocoetes play a role in aquatic environments through their filter-feeding activities. As primary consumers, they process organic matter and microscopic organisms from the water column and sediments. This continuous filtering helps cycle nutrients within freshwater ecosystems. Their burrowing behavior also contributes to sediment aeration and can influence the physical structure of stream beds.
During their long larval period, ammocoetes serve as a food source for aquatic organisms. Fish, birds, and even some mammals may prey upon these larvae, integrating them into the broader food web. Despite their small size, their abundance and prolonged presence in freshwater systems mean they contribute to the energy flow within these habitats.
Understanding the ammocoete stage is important for assessing freshwater system health and biodiversity. Their presence and density can indicate stable and suitable habitat conditions. Conservation efforts for lampreys often focus on protecting and restoring stream habitats required by these larvae, recognizing their contribution to the ecosystem before they transform into adults.