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Posted: Oct 2 2009

 Geukensia granosissima (G. B. Sowerby III, 1914), Southern Ribbed Mussel

On October 6, 2009 I made the following post on Conch-L:

"G. granosissima has been treated as a subspecies of G. demissa, but more recently they've been treated as separate. I was never sure and still am not. However, I finally got my hands on some G. granosissima, did a little reading and corresponding and have concluded that the Geukensia on Florida's east coast (G. demissa) is morphologically different from the Geukensia on Florida's west coast (G. granosissima).

I would certainly like to hear from anyone who has found these shells on either coast of Florida that contradict the geographic separation I have presented. More photos of west coast Geukensia would also be appreciated."

Following are some of the responses and photos offered by Conch-Lers.

John of Lancaster, PA pointed out:

"Mikkelsen & Bieler, 2008 Seashells of Southern Florida (Bivalves) - which is likely the current authority on the subject, says under G. granossissima: "Florida, Gulf of Mexico, Caribbean, Central America, South America (Venezuela)...Formerly known as demissa Dillwyn, 1817 (eastern Canada to northern Florida)...."

Marlo replied:

"It’s interesting that you note Paula’s comment. I was unsure of its intent and send an inquiry, but have not yet received a reply. However, I suspect what was meant was that G. g was not “Formerly” separated from G. d as a distinct species. Just below the comment you quote, she describes an adjacent picture with the words, “… such as these Geukensia demissa (Dillwyn, 1817) from the mid-Atlantic …” This certainly does not indicate the prior language meant to treat the two as synonyms.

However, I remain hopeful to hear from Paula."

Rüdiger Bieler (coauthor with Paula) repied to my inquiry and explained that they used the words "formerly known as" to "distinguish between ... common prior misidentifications." "So we meant to say that G. granosissima was formerly called G. demissa in the Keys. The "live" image shows what we consider the real northern G. demissa and labeled it as coming 'from the mid-Atlantic coastline of the eastern United States,' thus not from the Keys."

José H. Leal, Ph.D., Director and Curator Bailey-Matthews Shell Museum wrote:

"Actually the molecular work on Geukensia was done partially by Taehwan Lee and Diarmaid Ó Foighil (below). They used Ischadium recurvum, Geukensia demissa, and G. granosissima as outgroups in molecular studies of species of Brachidontes. Both combined (28S and ITS1) nuclear ribosomal (left) and mitochondrial COI (right) datasets suggested to these authors that G. granosissima is more closely related to Ischadium recurvum than it is to G. demissa. One of the possible interpretation of their results, from the taxonomic standpoint, is that Ischadium recurvum could be just another species of Geukensia.

Lee, T. and D. Ó Foighil. 2004. Hidden Floridian biodiversity: mitochondrial and nuclear gene trees reveal four cryptic species within the scorched mussel, Brachidontes exustus, species complex. Molecular Ecology 13, 3527–3542 (doi: 10.1111/j.1365-294X.2004.02337.x).


Harry Lee then provided the above links for the papers cited by José, and commented:

"I understand Dr. Ó Foighil was a Bailey-Matthews Shell Museum Visiting Curator not long ago. Perhaps you have a better sense of the zoogeography of Geukensia demissa and of G. granosissima as a result of that association.

Can you share?"

José responded:

"Actually I don't have much more to add to the pot than what has been discussed. The information in Marlo's site is consistent with my view of the problem, G. granosissima W coast, G. demissa E coast, not sure about the boundaries or whether species could hybridize. Marlo's images are absolutely stunning and show well the difference, especially in shell surface sculpture. We have been using G. granosissima as a standalone species in the Museum web site since its inception.

Taehwan and Diarmaid's papers are not focused on Geukensia but on Brachidontes; however they show, via analyses of two independent gene groups, that Ischadium recurvum is nested within the two species of Geukensia. I recall Diarmaid pointing the results to me and commenting on the strange relationship among these species and how I. recurvum would actually be a Geukensia."

Following are photos to illustrate the species José mentions:

Geukensia granosissima Key Largo.JPG
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