GIVING AND RECEIVING - THE TROPICAL ATLANTIC AS DONOR AND RECIPIENT REGION FOR INVADING SPECIES

After the middle Pliocene uplift of the Central American seaway (3.1 t o 3.6 million years ago), the western Atlantic fauna became isolated f rom that of the eastern Pacific, but connections with the tropical Ind o-West-Pacific and eastern Atlantic were maintained. By analyzing the distibution, fossil record, and relationships of shallow-water shell-b earing molluscs (those living in less than 100 m depth) in die western Atlantic, we ascertained the extent to which the western Atlantic has served as a recipient and as a donor region for invading taxa. At lea st 33 species in the western Atlantic are late Pliocene or Pleistocene invaders from the Indo-West-Pacific (17 species) or eastern Atlantic (16 species), whereas at least 39 species dispersed eastward across th e Atlantic from die Americas to West Africa. Eleven species derived fr om the Indo-West-Pacific are included in both tallies, because they pr obably first dispersed westward from the Indian Ocean around southern Africa to Brazil and the Caribbean region before spreading eastward ac ross the Atlantic to West Africa. Most of this dispersal is probably b y means of planktonic larvae, but some species could have been spread as rafting adults. Oceanic currents and prior extinction histories det ermine the pattern of interchange among tropical marine biotas. Within the tropics, the western Atlantic suffered the greatest molluscan ext inctions since the early Pliocene (about 60 to 70%); it is also the re gion in which the great majority of immigrants have become common and geographically widespread. Extinction in die eastern Atlantic, eastern Pacific, and Indo-West-Pacific has been much less, and immigrants to these regions often have restricted geographical distribution there, a nd could be represented by populations that are not self-sustaining.