Posted: Jul 20 2022

Ensis megistus Pilsbry & McGinty, 1943.  Minor Jackknife Clam

(This shell found along the Atlantic coast was previously accepted by authors as Ensis minor Dall, 1899, which in 2015 was found to be limited to the Gulf of Mexico.)

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In the early days the Jackknife Clam found all along the Atlantic coast was reported as E. directus Conrad, 1843 or E. minor Dall, 1899 [later updated to E. minor (Chenu, 1843)].   E. minor is also found in Europe.  More recently researchers recognized that E. directus is a fossil from the Pliocene and that the Jackknife Clam found all along the Atlantic coast sufficiently differed from the European E. minor that it should be treated as a separate species (Lee, 2009; Huber, 2010).  In 1943 Pilsbry and McGinty identified Gulf of Mexico specimens they considered to be a subspecies of E. minor which they named E. megistus.  So, when Lee and Huber ascertained that the Atlantic coast Jackknife was a species distinct from E. minor, they compared it to E. megistus and, as Huber commented, “was unable to draw a line” between them.  As a result, as Huber commented, “Ensis megistus is the valid name to represent this well-known North Atlantic ‘minor jackknife’.”  However, Lee and Huber continued to study the Atlantic coast and Gulf of Mexico taxa and in 2015 Huber published an update and recognized that the two differed sufficiently that the Jackknife Clam found all along the Atlantic coast was a distinct species.  Unable to find another available name, Huber proposed Ensis leei for this species. 

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Short version:   There is no E. minor in the western Atlantic (it’s an European species), E. directus is a fossil from the Pliocene, E. megistus is limited to the Gulf of Mexico, and the species we find all along our Atlantic coast and rarely on west Florida is E. leei. 

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Huber M. (2010). Compendium of Bivalves. Harxheim: ConchBooks.

Huber M. (2015). Compendium of Bivalves 2. Harxheim: ConchBooks.

Lee, Harry G.  2009 Marine Shells of Northeast Florida.  Jacksonville Shell Club, Jacksonville, Florida.

Pilsbry, H. A. & McGinty, T. L. (1943). Ensis minor megistus n. subsp., a west Florida razor clam. The Nautilus. 57(1)