Lottia alveus, a gastropod limpet once found only on the blades of the eelgrass Zostera marina from Labrador to New York in the western Atlantic Ocean, is the first marine invertebrate known to have become extinct in an ocean basin in historical time. The last known specimens were collected in 1929, immediately prior to the catastrophic decline of Zostera in the early 1930s in the North Atlantic Ocean. The brackish water refugium of Zostera throughout the decline was apparently outside of this gastropod's physiological range, and the limpet became extinct. Few marine invertebrates have habits as specialized and ranges and tolerances as narrow as did L. alveus. The fact that most marine invertebrates have large effective population sizes may account for their relative invulnerability to extinction.