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Among 67 hard clams Mercenaria mercenaria collected during spawning on 4 June 1982 from Core Sound, North Carolina (USA), gonadal mass increased significantly with body size (shell length and internal volume). Clam age did not explain a significant amount of the residual variance in the linear regression of log gonadal mass on log shell length. Because this collection contained 11 individuals over 24 yr of age, including the oldest M. mercenaria ever reported at 4 1 and 46 yr, these results imply that gamete production in M. mercenaria continues into old age at a quantitative level predicted simply by the power curve relating gonadal mass to body size. There was no suggestion of even partial reproductive senility in gamete production at old age. Exponents in the 2 power curves relating gonadal mass to shell length (3.1) and shell volume (1.1) did not differ significantly from those expected under isometric growth of gonadal mass. Knowledge of how reproductive output in any organism changes with age is critical to completing the agespecific fecundity function (m,) of its life table and necessary for a better understanding of the general incidence and character of reproductive senility among different groups of species. In marine invertebrates, reproductive output (gamete production) generally increases as a function of increasing body size (reviewed by Spight & Emlen 1976, Hughes & Roberts 1980). In such studies of the allometry of gamete production with changing body size, organism age is usually an unknown factor. Because individual growth rate declines rapidly with increasing age and quickly approaches zero in solitary marine invertebrates (e.g. Hallam 1967) and because the oldest age classes usually represent a small fraction of any population, the existence of reproductive senility at some particular age would rarely be detectable in such allometric studies. Average gamete production would still increase monotonically with body size. Although the presence of reproductive senility might be detected by a significant increase in the variance in gamete production at the largest size classes (Vahl 19851, an analysis that incorporates age information directly is a O Inter-ResearchIPrinted in F. R. Germany more direct and preferable means of testing for reproductive senility. Many bivalve molluscs can be aged by counting growth bands incorporated into their shells (Rhoads & Lutz 1980), thus providing one group of marine invertebrates amenable to a ready test of how age affects reproductive output. I suggested previously (Peterson 1983) that in addition to looking for absolute reproductive senility (zero reproductive output after some age) one might also test for quantitative (= partial) reproductive senility, defined as the degree to which reproductive output after some age falls short of that expected from the curve relating reproductive output to body size alone among all adults younger than that initial age of the onset of partial senility. The hard clam Mercenaria rnercenaria (L.) in 2 collections from Back Sound, North Carolina, exhibited no evidence of even partial reproductive senility (Peterson 1983), but Vahl (1985) has since discovered a substantial decline in gamete production of the Iceland scallop Chlamys islandica in the oldest age classes. Because my previous test for quantitative reproductive senility in M. mercenaria only included individuals ranging up to age 19 (Peterson 1983) despite the known occurrence of substantially older clams nearby (Peterson et al. 1983), I extend my tests of how age affects gamete production in M. mercenaria by analyzing here a collection that includes far older clams ranging up to age 46. Methods. To test how gamete production varies with body size and age among relatively old Mercenaria rnercenaria, I collected M. mercenaria on June 4 , 1982 from the Johnson Creek area of Core Sound near Cape Lookout, North Carolina (USA). Johnson Creek (described in detail and charted in Peterson et al. 1983) is a shallow tidal creek that drains Spartina alterniflora marshes bordering the barrier island of Core Banks. The collection area was in shallow (0.1 to 0.5 m water Mar Ecol Prog. Ser. 29: 93-97, 1986 coverage at low tide) subtidal waters and was characterized by bottom surface (0 to 5 cm) sediments comprised of about half fine sands and half silts and clays by weight. Clams were gathered by both rake and suction dredge with no attempt at preserving natural size-frequency distributions. I chose the early June sampling date to fall after the reported season of gametogenesis but before most gamete release (Porter 1964) to enable me to use the dry mass of all gonadal material as a relative measure of size-specific fecundity (one possible measure of reproductive effort: Vahl 1981). All 67 clams collected were transported to the laboratory in dry buckets and then frozen at -8 "C for 1 to 5 wk before dissection. Clams were removed from the freezer in small groups and measured individually to the nearest 0.1 mm with vernier calipers in each of 3 perpendicular, linear dimensions (length, height and thickness). After thawing, all dissections were carried out by the same person over a 5 wk period to achieve uniformity in a somewhat subjective process. For each clam, gonadal tissues were scraped away from the remaining soft somatic tissues. Each fraction was placed into a separate aluminum weighing dish and dried to constant weight (3 to 4 d) at 65 "C. After dissection, 1 valve from each clam was sectioned and aged by the process described and tested for North Carolina in Peterson et al. (1983). This procedure produced 3 necessary pieces of information on each clam: its size, age, and gonadal mass near the beginning of the spawning season. To test for possible quantitative (partial) reproductive senility, I performed single linear regressions on log,,-transformed variables to determine which independent variable, size or age, explained more of the variation in gonadal mass and multiple linear regressions on log,,transformed variables to test whether age explained any significant proportion of the residual variation around the regression of gonadal mass on clam shell size (length). The use of a single linear regression between 2 log-transformed variables is equivalent to fitting a power curve: the exponent in the power curve becomes the regression coefficient (slope) in the linear regression. The handling, dissection, and analytical procedures are identical to those used in Peterson (1983) and permit a contrast between the new (4 June 1982) results incorporating older clams from Johnson Creek and the previous results from 22 to 29 May 1980 and 3 to 11 June 1981 collections from nearby Back Sound, North Carolina. Results. The 1982 collection from Johnson Creek contained relatively old Mercenarja mercenaria. Eleven of the 67 clams were 25 yr of age or older, while the 2 oldest were 41 and 46. The single linear regressions between the double log-transformed variables (Fig. 1) E 0.01 ? l 2 4 6 8 10 15 g Length (cm) , . 2.0 r 0.01 1 I 1 1 1 1 1 1 t 1 1 l 2 4 6 8 10 15 20 30 4050 Age ( y r ) Fig. 1. Mercenaria mercenaria. Gonadal mass on 4 June 1982 as a function of shell length (top) and age (bottom), as estimated by growth lines. Regression equations and fits given in Table 1. In the top graph, ages of all clams 25 yr old or older are plotted as the data points to illustrate the lack of effect of old age on the fit revealed that clam length (r2 = 0.53) was a better predictor of gonadal mass than clam age (r2 = 0.11). In the multiple linear regression among log-transformed variables (Table l), clam age did not contribute significantly to explaining variance in gonadal mass although length was highly significant (p < 0.001). The multiple linear regression using length and age as independent variables was able to explain only 1 % more of the variance in gonadal mass (54 % vs53 % : Table l ) than could be explained by clam length alone. Age also did not explain a significant (a = 0.05) proportion of the residual variance around the single linear regression of gonadal mass on clam length. To illustrate how old age failed to contribute significantly to the shape of the Peterson: Gamete production by old Mercenaria rnercenaria 95 Table 1. Mercenaria mercenaria. Outcomes of single and Table 2. Mercenaria rnercenaria. The contrasting allometry of multiple linear regressions among loglo-transformed varigamete production as a function of body size in 3 separate ables of dry gonadal mass (G) in g on shell length (L) in cm, collections. Presented are the regression coefficients (A95 % clam age (A) in yr and internal shell volume (V) in cm3 for Cl) from the linear regressions of log gonadal mass (G) on log 1982 collection from Johnson Creek. 95 % Cl for each regresshell length (L) and on log internal shell volume (V). Data for sion coefficient is given in parentheses. Sample size is 67 1980 and 1981 collections come from Peterson (1983). The flrst individuals for all regressions set of regressions (a) was performed on the complete data sets, whereas the second set (b) was performed on only those clams > 6 cm to remove the possible influence of changing sizefrequency distributions among collections. n = numbers of clams entering into the analysis Regression equation r2 (1) Log G = -3.26 + 3.14 (f 0.73) log L"' 0.53* (2) Log G = -0.79 + 0.35 (k0.25) log A" 0.11 (3) Log G = -3.44 + 3.53 (k0.91) log L' " 0.16 (f 0.22) log ANS 0.54 (4) Log G = -2.35 + 1.12 (k0.28) log V"' 0.50 " p < 0 . 0 1 ; ' "p<0 .001; NS:p>0.05 a Age did not explain a significant proportion of residual variance in the single linear regression of log G on log L relation between clam size and gonadal mass, ages of all clams older than 24 yr of age are plotted instead of points in their appropriate positions around the regression line of gonadal mass on clam length (Fig. 1). Of these 11 oldest clams in the collection, 5 fell above the line a

Quantitative allometry of gamete production by Mercenaria mercenaria into old age