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Posted: Apr 15 2007

Smaragdia viridis (Linnaeus, 1758), Emerald Nerite

A Conch-L discussion about S. viridis:

David Kirsh asked:

"Is it a certainty that the Mediterranean Smaragdia viridis (Linnaeus, 1758) and its Caribbean counterpart are conspecific?

Is Smaragdia viridis viridemaris Maury, 1917 then an acceptable name for the Caribbean item?"

Harry Lee wrote:

"The type locality for the Linnean species is Jamaica. What does Tom E. think about the taxonomy?"

Tom Eichhorst elaborated:

"I treat them as conspecific, although the Mediterranean and Western
Atlantic/Caribbean populations of Smaragdia viridis have not interbred for
many, many years (widening of the Atlantic and a slight change in the
currents). During the Miocene the Atlantic was only about 4,000 km wide.
With a drift speed 3 km/hour, a Smaragdia viridis veliger could have made it
across the Atlantic in 6.5 to 25 weeks. Smaragdia viridis have been shown
to have an average life span in veliger form of 8 weeks before settling to
the substrate or dying. This means a veliger, during the Miocene, could
have made the crossing in either direction (the currents reverse depending
upon latitude and depth). Today, however, the distance is about 6,500 km
and the drift time required is 9 to 28 weeks. Thus a veliger, even with the
best currents possible, would not live long enough to make the trek. This
has been born out in a study that took 857 plankton tows across the
Atlantic. They found groupings of Smaragdia viridis in the Western Atlantic
and the Eastern Atlantic, but none in the Central Atlantic. So today the
populations are separated, making a case for subspecies status. Now things
get complicated...

As for the taxonomy, the name Smaragdia viridemaris was originally applied
to a Miocene fossil, and not the Recent Caribbean/Mediterranean species. If
the Miocene S. viridimaris is to be considered the same as the recent S.
in the Caribbean, then we have a problem as the S. viridis name is
senior. Furthermore, S. viridis is also reportedly found in late Miocene
deposits. So S. viridimaris is either a valid Miocene fossil or a junior
synonym of the Recent species. As a last wrench into the works, there are
at least four other named Pliocene fossils that are obviously closely
related to S. viridis and may or may not be forms of this species. Linnaeus
named S. viridis in 1758 (as Nerita viridis) and gave the "type locality" as
"Minorcam, Jamaicam," or Minorca and Jamaica. If there is a subspecies
involved, it will have to be determined which population would be S. viridis
viridis. The Miocene fossil S. viridimaris may be a precursor to S. viridis
and it may be this species that made the Miocene trek across the Atlantic.
In fact, the shell morphology of S. viridimaris is pretty much right between
the Atlantic and Mediterranean population of S. viridis. So our Miocene
populations (of S. viridis, S. viridimaris, or both) crossed the Atlantic
and are responsible for our modern forms, which have diverged in shell
structure and color/pattern in the millions of years they have been
separated. Today if you dump a dozen specimens from the Mediterranean and a
dozen from the Caribbean into a single pile, they can quite easily be
separated morphologically (they differ in both shell shape and
color/pattern). The first one to attempt to split the Linnaeus species
into two species was Schröter with Nerita subviridis in 1779. The latest
was Russell with Smaragdia viridis weyssei in 1940 for the Caribbean
population. I just counted and found 17 synonyms used at one time or
another for Smaragdia viridis.

So, I treat them as conspecific, though I believe a good case can be made
for subspecies status, the viridimaris name is not available for the Recent
species, and it all is really a wonderful project if someone with some time
to really dig into this issue."

Paul Callomon Noted:

"In the ANSP collection, all the Caribbean material is classified as S. viridis weyssei, with S. viridis viridis used for specimens from elsewhere."

Erick Staal informed us:

" Interesting: in the original description by Linnaeus he mentions the locations 'ad Minorcam (=Menorca, Mediterranean?), Jamaicam (= Jamaica, Caribbean?)'. It looks like Linnaeus was aware of the amphiatlantic character of the species."

1/31 To which
Tom replied:

"Smaragdia viridis weyssei is probably the best choice for a subspecies name if one is going to designate a subspecies for this taxon. In my write up for Smaragdia viridis (sent separately to those who asked for it) I state this viewpoint."

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