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Posted: Nov 11 2009

 Argopecten irradians amplicostatus (Dall, 1898), Texas Bay Scallop

                                                                     Argopecten irradians s.l. (Lamarck, 1819)
                                                                  Text and photos by Marlo F. Krisberg 2009

A. irradians has popularly been divided into subspecies. Using Abbott (American Seashells 1974) as a baseline the subspecies names were:

Argopecten irradians irradians (Lamarck, 1819)
Argopecten irradians concentricus (Say, 1822)
Argopecten irradians amplicostatus (Dall, 1898)

The distribution of these three shells is displayed in the below slide. Note that despite the occasional adventitious occurrence of an A. irradians between North Carolina and Palm Beach County, Florida, (see Lee 2009) living populations of the Bay Scallop have not been documented within this “Gap.”

Petuch introduced the following name for a subspecies based upon specimens from Rabbit Key Basin in Florida Bay near Key Largo, Florida Keys:

Argopecten irradians taylorae Petuch, 1987

In 1997a Marelli, et. al., analyzed the status of A. i. taylorae and determined that “Neither morphometric nor genetic evidence supports the proposed status of A. i. taylorae as distinct from Florida populations of A. i. concentricus.” They found the differences Petuch identified to be spurious and stated, “…we refute those differences and conclude that bay scallops from Rabbit Key Basin do not represent a subspecific taxon distinct from Florida populations of A. i. concentricus.”

However, in 1997b Marelli, et. al., also examined the subspecific status of A. i. concentricus and concluded that use of this name was “untenable.” They examined samples of A. irradians from three locations within the range for A. i. irradians, three locations in Florida for A. i. concentricus, and a sample from Core Banks, North Carolina (putatively A. i. concentricus). They found that the Core Banks shells were genetically closer to the northern A. i. irradians than to the Florida A. i. concentricus and should not be distinguished from A. i. irradians. It’s a little complicated, but Say’s application of the name concentricus was to specimens from New Jersey which he consider distinct from A. i. irradians. Based upon Marelli, et. al.’s findings, he was wrong. So, Say’s A. i. concentricus must be considered a junior synonym of A. i. irradians and is no longer a name for valid use. However, the name A. i. concentricus has been applied to the Florida populations, which Marelli, et. al.’s analysis established was significantly genetically distant from the four northern samples. Therefore, the Florida shells are essentially without a name. Marelli, et. al. reviewed the literature for instances where other names had been applied to the Florida shells. They found two; one being Petuch’s A. i. taylorae and Pecten circularis Sowerby, 1835. However, use of the latter name is questionable since its holotype compares favorably with the North Carolina A. i. concentricus. Marelli, et. al. also expressed the need for further exploration of the genetic compositions of populations in the transitional zone between New Jersey and Virginia (if such populations exist) before either A. i. taylorae or Pecten circularis are adopted or rejected. They commented “We advocate caution to those who might immediately apply the name Argopecten irradians taylorae for the Florida and eastern Gulf stocks.” In personal email correspondence on November 6, 2009 in discussing this issue, Dr. Marelli indicated that “My choice would be to refer to bay scallops as A. irradians, either with s.l. following or not.” The expression “s.l.” refers to the Latin "sensu lato" and in this context means within the broad sense of a taxon - including all its subordinate taxa and/or other taxa sometimes considered as distinct. Dr. Marelli suggests not making distinctions among the three geographically separate populations of Bay Scallops and, despite some genetic and morphometric differences that might justify subspecific nomenclature, to simply identify them all as A. irradians or A. irradians s.l.

Since the morphometric differences among the three populations of A. irradians are sufficiently distinct and consistent to be of interest to collectors, despite my personal distaste for the concept of subspecific names, in the instant case of such a popular and plentiful shell, it is useful to utilize names for the differing populations to facilitate discourse about them. Since we have commonly used and accepted names for the north Atlantic populations (A. i. irradians) and the western Gulf populations (A. I. amplicostatus), the issue becomes, “what to call the Florida/E. Gulf populations.” Although some have adopted A. i. taylorae (Lee 2009), others (Hartmann, Bivalve Seashells of Florida 2006; Mikkelsen & Bieler, Seashells of Southern Florida - Bivalves 2008) have identified the Florida Bay Scallop as simply Argopecten irradians. The approach suggested by Dr. Marelli and used by Hartmann and Mikkelsen & Bieler is the one I personally prefer. However, since my objective is to facilitate understanding and distinction between similar shells and doing so will be greatly facilitated by utilizing labels, for this presentation I have continued the use of the subspecific names for the north Atlantic and west Gulf populations and, as Marelli, et. al, did in their paper, simply refer to the Florida/E. Gulf Bay Scallops as:

Argopecten irradians of Florida.

The below slide portrays the current status of the names and distributions of A. irradians s.l. based upon the work of Marelli, et. al, which is the most current on the issue as of this time.


Abbott, R. Tucker. 1974 American Seashells, Second Edition, Van Nostrand Reinhold Company.

Hartmann, Trish. 2006. Bivalve Seashells of Florida. Anadara Press, Tampa, Florida.

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

Marelli, D. C., M.K. Krause, W.S. Arnold, and W.G. Lyons, 1997a. Systematic Relationships Among Florida Populations of Argopecten irradians (Lamarck, 1819)(Bivalvia: Pectinidae). The Nautilus 110(2): 31-41. Feb. 19.

Marelli, D. C., W.G. Lyons, W.S. Arnold, and M.K. Krause, 1997b. Subspecific Status of Argopecten irradians concenticus (Say, 1822) and of the Bay Scallops of Florida. The Nautilus 110(2): 42-44. Feb. 19.

Mikkelsen, P.M., and R.Bieler. 2008. Seashells of Southern Florida – Bivalves. Princeton University Press. Princeton, N.J.

Petuch, E. J. 1987. New Caribbean molluscan faunas. The Coastal Education and Research Foundation, Charlottesville, Virginia, 158 p.

Following is a photo discussion of Argopecten irradians amplicostatus (Dall, 1898) followed by a comparison of all three varieties.

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