Lampreys are primitive vertebrates that have a distinctive suckered mouth, rather than jaws. 

River lamprey Lampetra fluviatilis has been designated as a qualifying feature in Carmarthen Bay and Estuaries SAC as it has been considered an important coastal migration route or feeding ground for this species, and as it is adjacent to an important freshwater site for the species (River Tywi SAC). There may be other relevant rivers that contribute to the SAC population. Overall, river lamprey in the SAC and relevant upstream spawning rivers are thought to be common and widespread. 

Sea lamprey Petromyzon marinus has been designated as a qualifying feature in Carmarthen Bay and Estuaries SAC as it has been considered an important coastal migration route or feeding ground for this species, and as it is adjacent to an important freshwater site for the species (River Tywi SAC). There may be other relevant rivers that contribute to the SAC population. Overall, sea lamprey numbers in the River Tywi are generally in the thousands, with much of the catchment suitable for spawning and being easily accessible.