Often minute differences among shells within populations and in the same shell from different geographic locations give rise to arguments about speciation. Collectors, dealers and researchers all contribute to fostering differentiation and create the ambiguities for their own differing reasons, be they fame, fortune or the search for truth. However, there are those among us who believe within-species genetic variability, habitat, diet, and other environmental factors drive differentiation within species and that such differences should not be utilized to justify speciation in the absence of comparison to a reliable basis for measurement. Reliability in this circumstance is defined to mean where the thing measured is consistently present in populations and the technique for measuring it is objective, repeatable and dependably consistent. For me, DNA testing is the current state of the technology technique that meets the requirement for reliability.

Source:

Adamkewicz, S. Laura and Harasewych, M. G. 1996. Systematics and biogeography of the genus Donax (Bivalvia: Donacidae) in eastern North America. American Malacological Bulletin, Vol. 13(1/2): 97-103.

Findings:

A&H set out to confirm or simplify the recognized six taxa of the western Atlantic species of Donax established by Morrison (Morrison, J. P. E. 1971. Western Atlantic Donax. Proceedings of the Biological Society of Washington. 83(48): 545-568). Morrison identified two groups as follows:

GROUP 1 - Fairly large (15-20 mm), flattened, elongated triangular shells, and occupying in the middle intertidal zone, migrating actively with the tide.

Donax variabilis variabilis Say, 1822
Donax variabilis roemeri Philippi, 1849

GROUP 2 - Smaller (5-8 mm), smoother more inflated, and occupying the intertidal to subtital zone, little tendency to migrate with the tide.

Donax fossor Say, 1822
Donax parvulus Philippi, 1849
Donax dorotheae Morrison, 1971
Donax texasianus Philippi, 1847

A&H acknowledged that the morphological distinctions among these six taxa were few and largely subjective leading to controversy regarding the validity of these species differentiations. Using randomly amplified polymorphic DNA markers they found no distinctions that would separate the two shells in the first group (D. variabilis = D. roemeri), the first two in the second group (D. fossor = D. parvulus), or the second two in the second group (D. dorotheae = D. texasianus). Therefore, A&H found that there were only three taxa involved and that by priority the names that apply are:

 

Donax variabilis Say, 1822

(Entire Gulf coast and E coast of Florida north to mouth

of Chesapeake Bay)

Donax fossor Say, 1822

(East central coast of Florida north to about New York)

Donax texasianus Philippi, 1847

(Gulf coast from Florida panhandle west to about

Veracruz, Mexico)

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A&H emphasized that because D. variabilis migrates with the tide and mixes with D. fossor and D. texasianus, and because D. variabilis, the younger sister species, retains the morphology of its ancestors at a comparable shell size, it is virtually impossible to distinguish these species as young and reliance should be placed upon comparison of adults.

However, Harry Lee suggests otherwise. Harry found that:

D. variabilis "has more conspicuous radial sculpture and posterior ridge;" "rayed shells in populations of D. variabilis and NOT D. (fossor);" and "a difference in the shape of the hinge ligament. (not described)"