The genus Mercenaria is restricted to the Gulf of Mexico and the Atlantic coast of the United States and is considered to be represented by two species: Mercenaria mercenaria and Mercenaria campechiensis. The former occupies a more northerly range from New England to the Carolinas. The latter is described as ranging from Cape Hatteras into the Gulf of Mexico, although there are limited records for collections as far north as offshore New Jersey. The two species are generally differentiated by external ribs and a thickened shell in M. campechiensis versus a smoother shell in M. mercenaria, although this feature is not definitive.
Historical records describe a fishery for M. campechiensis in North Carolina in the 1950s (no longer active) with individual clams up to 140 mm shell length and 4 pounds (1.81 kg) total weight. Four specimens were collected off the intracoastal waterway at Redington Beach, Boca Ciega Bay, Florida in December 1964 measuring 149.5, 155.5, 160 and 168 mm SL. The last of these weighed 6.5 pounds (2.95 kg). No age data is reported. The last three all exceeded the largest specimen know at that time; a 153 mm SL in the U.S. national Museum. More recently an “old clam”, nicknamed “Lincoln”, was reported in the popular press (https://www.usatoday.com/story/news/nation/2023/03/03/aber-clam-lincoln-214-years-old-florida/11389421002/) at 6 inches (152 mm) SL and 2.6 pounds (1.18 kg). This animal was described as Arctica islandica, but based on images in that article, we believe it was in fact M. campechiensis. An age estimate of 214 years was made for “Lincoln” based on external shell banding sculpture. The specimen was returned to the sea. Despite these records of large individuals of M. campechiensis we can find no records for age at length and terminal values of both, although an extraordinary thin section image published by Moss et al. in Paleobiology (2021) provides intrigue.
Herein we focus on six sets of articulated valves of M. campechiensis collected in June 2023 at a site 43.5 km offshore of Virginia Beach, USA, in sandy substrate in 18-42 m depth. Individual clams ranged from 80.2 to 111.6 mm SL. For each individual animal we estimate age at length based on annuli in both the chondrophore and exposed edge of valves sectioned from the growing edge to the hinge and compare age at length estimators from annuli signatures to external shell banding. Finally, we discuss the possible history of these individuals in relation to their collection location north of Cape Hatteras.
This work was supported by Dominion Virginia Energy and an NSF Research Experience Award to Carpenter as the presenting author.