by Dr. Douglas S. Jones
The regular spacing of external growth rings on shells and their progressive crowding as the animals grow older continue to prompt people to interpret them as age markers, analogous to annual rings in trees.

Actually, for many decades the growth patterns in molluscan shells have been the subject of serious biological and paleontological inquiry. Early workers concentrated on growth structures visible on the external surfaces of shells, such as ribs, frills, and concentric rings on the bivalve shells. These structures often were considered (usually without much evidence) to form annually, so that by counting the number of rings, a clam's age could be determined. Despite extensive study, however, the use of external shell growth patterns in most species has been found to be rather limited. The inability to distinguish true periodic structures on shell surfaces from random disturbance marks (e.g., those induced by severe storms), and the difficulty in obtaining accurate ring counts because of extreme crowding toward the edge of old shells, have contributed to skepticism concerning the reliability of external shell rings as age indicators.

Within the last few decades, studies of molluscan growth structures have focused upon periodic patterns within shells. These studies have dramatically changed our ideas about the lifespans of typical bivalve species. For instance, longevity estimates of some common coastal bivalves, once thought to be fairly ephemeral creatures, have been extended to 50 years or more, while certain offshore species are known to live for over 100 years.

Internal shell growth patterns are best viewed in cross-sections taken from the umbo to the growing shell margin. This technique is readily applied to bivalve shells, where a straight cut by a rock-cutting saw equipped with a diamond blade reveals the entire growth history of the animal in the sectioned shell. The majority of shells that have been examined in cross sections possess annual patterns of growth increments, typically recognizable as large white bands alternating with thinner dark bands or rings. The combination of one dark and one white increment represents an annual cycle of shell growth. The dark rings correspond to the rings on the shell's exterior (where there may also be spurious disturbance marks) and can form in response to reduced shell calcification rates resulting from a variety of factors, including cold winter or warm summer temperatures, annual spawning cycles, etc.

Because of this potential variability in timing and cause of ring formation, the periodicity of supposed yearly increments in a given bivalve species should be adequately documented before age assessments are made. This is frequently accomplished by mark-and-recovery experiments where live bivalves are marked, released for a known interval of time such as a year or two, recaptured and sectioned. The number of rings that formed in the elapsed time, and hence the periodicity of growth ring formation can then be determined. Other documentation techniques include monthly collection and analysis of specimens from local populations to assess seasonality of growth increment formation as well as a variety of sophisticated chemical/isotopic approaches.

Studies of internal shell growth increments confirm that annual rings are generally the most ubiquitous and useful periodic growth feature in bivalves, directly analogous to annual tree rings. However, they are not the only periodic shell structures. About 25 years ago, R.M. Barker suggested that a whole hierarchy of periodicities is preserved as alternating light and dark increments in the bivalve shell. These range in size from a few microns to a few centimeters and are thoughts to reflect periodicities such as sub daily tidal cycles, daily light-dark cycles, fortnightly tidal cycles, and annual (seasonal) temperature cycles. Most of these smaller scale increments can be studied only with the aid of a microscope.

In almost every instance, bivalve ages that are based upon annual, internal growth increments are far more accurate than other estimates. In addition the number of annual increments suggests lifespans substantially greater than had previously been imagined. For example, consider the case of Spisula solidissima, one of the largest and most common bivalves from Canada to Cape Hatteras. On the basis of external shell rings this clam was once thought to live about seventeen years. Counts of documented yearly growth increments from shell cross-sections, however, reveal that individuals may actually live over 30 years.

Equally famous among seafood lovers is the New England Softshell or Steamer Clam, Mya arenaria. Once they were thought to live for only a few years, but growth increment analysis has extended the known life span of this clam to at least 28 years. The story is much the same for Mercenaria mercenaria, the Hard Clam or Northern Quahog (perhaps more familiar as cherrystone, little neck or chowder clam -- depending on its size). Scientists recently reported finding two live specimens that had been tagged 33 and 36 years ago and that possessed 33 and 36 growth increments, respectively. Large specimens from Rhode Island that lived over 50 years, occasionally approaching ages of 75 years, have been documented through growth increment analysis.

Soviet marine biologists have employed these age determination techniques with similar results. A study of lifespans of common bivalves from the far eastern seas of the USSR reported that over half the species had lifespans in excess of 20 years. Many were found to live beyond fifty years. Although the idea of clams living for 20, 50 or 75 years seems a bit difficult to accept, the most surprising result of this entire research effort was the discovery of several bivalve species with over 100 annual shell growth increments. Naturally, substantial independent corroboration was sought to verify these age assessments, and the results largely have been upheld. A decade ago, Margaritana margaritifera, the European Freshwater Mussel, was considered to be the longest-living invertebrate, with an estimated lifespan of 100 years. Today it is but one of many bivalve centenarians and definitely not the longest lived.

For example, specimens of Panope generosa [now known as Panope abrupta (Conrad, 1849)] the geoduck (pronounced gooey duck) clam, harvested commercially from the West Coast of the United States, have been found to possess 120 annual increments. On the other side of the Pacific, Crenomytilus grayanus from Peter the Great Bay is reported to have the greatest lifespan encountered among mollusks from the USSR, 150 years. Perhaps the slowest-growing centenarian bivalve, however, comes from the deep sea. The diminutive Tindaria callistiformis grows extremely slowly in the cold dark world it inhabits, attaining a size of only 8.4mm in 100 years.

The Ocean Quahog, Arctica islandica, currently holds the longevity record for bivalves as well as for all non-colonial invertebrates, and may, in fact, be the longest-lived animal. Individuals dredged from the middle Atlantic continental shelf often show over 150 annual growth increments. One specimen had 220! Because of these unusually high age estimates, mark-and-recovery experiments were supplemented by radiometric dating techniques to test the yearly periodicity of the internal growth rings. The results verified the annual nature of the rings and confirmed the conclusions regarding age. The longest-lived animal on earth may well be a bivalve!

Originally published in the March 1989 issue of American Conchologist (Vol 17, No. 1, pp. 12-13).