Occurrence of teleplanic veliger larvae of sublittoral species in major currents of the tropical Atlantic Ocean supports the hypothesis that passive dispersal of larvae contributes importantly to the biogeography of shoal-water molluscan species. Rafting may play a secondary role. Criticisms of passive larval dispersal as a factor in molluscan biogeography are: (1) that larvae have a fixed life span too short to account for long-distance dispersal: both laboratory and field data show this assertation to be mistaken; (2) that larvae after a time lose their competence to metamorphose: whenever tested, teleplanic larvae have been shown to retain their ability to metamorphose; (3) that dispersal is random and cannot explain congruent nonrandom associations among widely differing taxa: dispersal is largely accounted for by advection along major ocean currents which provide quasi-permanent or seasonally reoccurring corridors for the transport of planktonic larvae; therefore various taxa necessarily are dispersed over similar routes, leading to congruent geographic distributions; and (4) that some species lacking long planktonic larval stages nevertheless have wide geographic ranges and consequently there is little relationship between mode of reproduction and geographic range of species: those species lacking extended planktonic larval stages yet having wide geographic ranges are mostly sessile epibenthic forms that can attach to hard substrata, and consequently are preadapted for passive dispersal by rafting. Dispersal by human agencies also has contributed within historic time to the geographic distribution of marine mollusks. Such alternative modes of dispersal do not negate the importance of larval transport. Despite demonstrable evidence for widespread passive larval dispersal and in some instances also for the transport by rafting of molluscan species, ecological constraints place restrictions upon where and when new colonists can survive and reproduce. Geotectonics and sea-floor spreading have been significant in controlling the pattern and extent of passive dispersal over geologic time. The closing of the Tethys Seaway during the Oligocene and Early Miocene resulted in the isolation of the southwestern tropical Pacific Ocean from the Mediterranean Sea and tropical eastern Atlantic Ocean. Similarly, the closing of the corridor between North and South America divided the tropical eastern Pacific Ocean from the Caribbean Sea. Continental drift and sea-floor spreading resulted in the initial formation and subsequent enlargement of the Atlantic Basin and has led subsequently to the ''mid-Atlantic barrier'' which today acts as a filter between the tropical eastern Atlantic Ocean and Caribbean Sea. Both tectonic events and sea-floor spreading have placed important constraints on the dispersal between tropical faunas. Faunal studies of the amphi-Atlantic distributions of marine benthic mollusks further support the hypothesis of passive dispersal. Low endemism on oceanic islands suggests that initial colonization must be largely accomplished by teleplanic larvae. Available evidence shows that although no single process can completely explain the present composition of the Caribbean molluscan fauna, passive dispersal of planktonic veliger larvae must have played and continues to play an important role both in colonization and in maintaining genetic continuity between widely disjunct regions of the tropical Atlantic Ocean. Recent new techniques now available which reveal enzyme variation and mitochondrial DNA polymorphism can revolutionize the study of bio-geography. They can make possible, for example, the identification of closely similar larvae in instances where morphological characteristics are inadequate and can allow measurements of genetic exchange or gene flow and the genetic relationships between widely separated populations. Cladistic analysis, although revealing little about the processes leading to present geographic distributions, can help reconstruct large-scale geographic relationships as related to the evolution of taxa.
