NO CHANGE IN THE RATE OF SHELL PRECIPITATION WITH AGE IN THE ATLANTIC SURF CLAM, SPISULA SOLIDISSIMA: ABANDONING THE ONE-DIMENSIONAL INTERPRETATION OF GROWTH

Fairly well engrained in the mindsets of ecologists and paleoecologists alike is that the growth rate of most bivalves decreases logarithmically with age. The major influence on this commonly held belief resides in the method of interpretation of annual growth bands. Whether one slices the clams to reveal internal bands or simply examines the outer shell surface, the linear distance between each subsequent pair of growth bands continues to decrease throughout ontogeny. The implication here is that growth rate decreases in the same fashion. This, however, is a one-dimensional observation. Clams, like most species, are three-dimensional creatures, not one; therefore, one should instead consider their growth in the third dimension.

Here, we present evidence that the shell growth rate of Spisula solidissima, a clam of great economic importance, is in fact constant with age. We measured individuals from 3 populations of the surf clam totaling 869 specimens collected from Corson’s Inlet, Brigantine Beach, and South Longport Beach, New Jersey, and generated a growth curve from linear measurements between the umbo and each successive annual growth band in a subset of sectioned clams. The relationship between age and shell size is very similar to others that have been published. The mass of each clam, as a proxy for volume, is then plotted against age, revealing the amount of shell produced per unit time over a wide range of shell sizes. The relationship between mass and age is linear (r²=.96) over the first 15 years of ontogeny. Data beyond this appear to have a lower slope, but are constrained by very few shells. The overall results suggest that Spisula solidissima continues to produce shell material at a constant rate until at least the age of 15.

The linear growth curve of Spisula is not unlike that for many other bivalves, and so by inference this pattern of constant shell growth with age is likely to be characteristic of a number of other taxa as well. This realization forces us to reconsider some of the more typical explanations for a reduction in growth rate with age (e.g., redirection of metabolic energy at the onset of sexual maturity, attainment of a size refuge from predators).