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,ex- tinct 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.