LARVAL DISPERSAL AS A MEANS OF GENETIC EXCHANGE BETWEEN GEOGRAPHICALLY SEPARATED POPULATIONS OF SHALLOW-WATER BENTHIC MARINE GASTROPODS

¹ Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543

1. Ten species of prosobranch gastropod veligers collected from the open waters of the North Atlantic Ocean have been identified by comparison of their larval shells with the protoconchs of identifiable juvenile or adult museum specimens. The larvae described are those of Cymatium parthenopeum (von Salis), Cymatium nicobaricum (Röding), and Charonia variegata (Lamarck) belonging to the family Cymatiidae; Tonna galea (Linné) and Tonna maculosa (Dillwyn) belonging to the family Tonnidae; Phalium granulatum (Born) belonging to the family Cassidae; Thais haemastoma (Linné), a muricid; Philippia krebsii (Mörch), an architectonicidae; Smaragdia viridis (Linné), a neritid; and Pedicularia sicula Swainson belonging to the family Ovulidae.

2. The geographical distribution of the veligers of these ten gastropod species has been determined in the North and tropical Atlantic from approximately eight hundred and fifty plankton tows. The relationship between the North and Equatorial Atlantic circulation and the dispersal of gastropod veliger larvae can be seen from these data (Figs. 5-12). Charonia variegata, Philippia krebsii, and Pedicularia sicula were found in all three trans-Atlantic currents sampled, namely, the eastwardly moving North Atlantic Drift and the westwardly flowing North and South Equatorial Current. Cymatium parthenopeum and Tonna galea were found throughout the North Atlantic gyre, but only from scattered records in the South Equatorial Current. Phalium granulatum and Thais haemastoma were found in the North Atlantic Drift and South Equatorial Current. These seven species are regularly dispersed in either direction across the North Atlantic barrier. Smaragdia viridis was found in the western half of the North Atlantic Drift and in the eastern half of the South Equatorial Current; it is probably less frequently transported across the Atlantic. Cymatium nicobaricum veligers were found only once in mid-ocean in the North Atlantic Drift; all other records were restricted to the tropical and warm temperate Western Atlantic. Tonna maculosa veligers were found only in the Gulf Stream. The adults of all ten species are amphi-Atlantic in their geographical distribution and occur in the tropical and warm-temperate shelf waters. Adults of Cymatium nicobaricum and Tonna maculosa, however, are known from only a few records in the eastern tropical Atlantic.

3. The duration of pelagic larval development has been estimated for the same ten species of gastropods. Six species, Charonia variegata, Cymatium parthenopeum, Cymatium nicobaricum, Tonna galea, Tonna maculosa, and Phalium granulatum have a period of pelagic development of over three months. Pedicularia sicula and Smaragdia viridis can probably reach the settling state in less than two months. It is possible that the latter four forms have settling responses and can delay metamorphosis. A comparison between the duration of larval development and the velocity of the North and tropical Atlantic surface currents shows that transoceanic dispersal of the first six mentioned species is possible even without a delay in settlement.

4. The frequency of long-distance dispersal across ocean basins is chiefly dependent upon (a) the drift coefficient, that is, the probability that larvae will be carried off-shore into the major ocean surface currents rather than retained in the coastal waters of the parent population, and (b) the size of the parent population from which the larvae originate. There may be considerable variation in the frequency of long-distance dispersal related to larval mortality.

5. The lower limits of sensitivity obtained by using a conventional plankton net are such that it is possible for trans-oceanic dispersal to occur and yet go completely unnoticed. The concentration of only one larva per tow reoccurring at each station along a transect across the Atlantic may represent a significant amount of trans-oceanic dispersal.

6. If pelagic larvae are important in maintaining genetic continuity, then the degree of morphological differentiation between eastern and western Atlantic populations of gastropod species having amphi-Atlantic distributions would be expected to bear an inverse relationship to the frequency with which the veliger larvae of these species were found in the plankton of the open sea. The evidence from the gastropod species considered here seems to support this hypothesis (Table V).

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