Sea lamprey play important roles in their multiple ecosystems. During the larval stage, sea lamprey are a significant portion of a freshwater stream biomass and provide a food source for predatory fishes, birds and some mammals. When migrating downstream sea lamprey juveniles also provide an important food source for river, estuarine, and ocean predatory fishes. Following spawning and death, adult lamprey carcasses provide a pulsed marine-derived nutrients to the local rivers and streams. Nevertheless, during the adult parasitic stages, particularly in the Great Lakes, they can pose a threat to fish stocks. As a result, large sums of money are spent every year for control of these freshwater population in North America. On the other hand, anadromous populations are threatened or endangered in the western coast of Europe due to loss of habitat and overfishing.

 

Sea lamprey are the earliest known vertebrate that have adopted an osmoregulatory strategy in which they maintain constant internal salt concentrations irrespective of their environmental salinity. Their fellow Agnathans, the hagfish, are osmoconfomers (keep the same internal salt concentration as their external environment) and are restricted to marine habitats. The evolution of an osmoregulatory strategy by lampreys has allowed them to utilize both freshwater and ocean habitats. While in freshwater, sea lamprey must counteract the passive loss of ions to the environment, which they do by actively taking up salts across the gill using specialized cells known as ionocytes, while the kidney is responsible for the creation of a dilute urine in order to remove passive water gains. While in seawater, the sea lamprey passively loses water to the environment and gains ions. To overcome water loss the sea lamprey drinks seawater, and both water and salt are taken up by the intestine. The excess sodium and chloride is then actively secreted by the seawater type ionocytes in the gill and divalent ions removed via defecation and renal secretion.