Evidence of veliger travel across the Atlantic was supplied by a study in
1971 by Scheltema.  In my discussion of Smaragdia viridis (Linnaeus, 1758) I
present some of the results of Scheltema's work when discussing whether or
not there is justification to separate Smaragdia viridis into an eastern
Atlantic subspecies and a western Atlantic subspecies.  In part, my entry
says:

"Another factor in this issue is a study by Scheltema (1971).  He studied
veliger dispersal of a number of different species of gastropod in the
Atlantic, including: Charonia variegata, Cymatium parthenopeum, Cymatium
nicobaricum, Tonna galea, Phalium granulatum, Thais haemastoma, Philippia
krebsii and Smaragdia viridis.  In 857 plankton tows taken throughout the
tropic and warm-temperate North Atlantic, he found veligers of each but two
of these species all the way across the Atlantic.  However, the veligers of
Phalium granulatum and Smaragdia viridis were grouped in the western
Atlantic and the eastern Atlantic with no evidence of intermediate travel.
Phalium granulatum is accepted as two subspecies, Phalium granulatum
granulatum in the western Atlantic and Phalium granulatum undulatum in the
eastern Atlantic.  The two populations of Smaragdia viridis show the same
geographic separation and lack of contact.  This added to the morphological
differences means there is probably sufficient justification for designating
two subspecies."

In the same study, Scheltema tested the "life-span" of the Smaragdia viridis
veliger.  He found it had a development stage of 25 days and a delay period
of 30 days.  The development stage is just that, the period in which the
veliger shows active feeding and growth.  The delay stage is the period the
veliger can remain in the plankton after becoming fully developed before
settling to the substrate.  The times given for the S. viridis veliger were
from specimens kept in a laboratory and were thought to be shorter than open
ocean numbers due to warmer conditions and a more concentrated food source.
Even if extended, this period seems a bit short for an Atlantic crossing by
ocean current alone.

Of course, all of this discussion on cross-Atlantic species is highly
dependent upon where the species in question are found.  The ocean currents
do support a crossing from the Caribbean to the Mediterranean areas and
back.  However, even though it is a much shorter distance, the currents do
not support such a crossing from Brazil to the west African area.  Finding
similar species in those areas would seem to be the result of recent
man-influenced introduction or the splitting of an original species by
continental drift.  It is all great stuff to consider!

Tom E.