SEASONAL ALLOCATION OF RESOURCES TO GROWTH OF SHELL, SOMA, AND GONADS IN MERCENARIA MERCENARIA.

Abstract

Thirteen monthly measurements of individually marked juvenile (16 mm long) and adult (60 mm long) specimens of Mercenaria mercenaria (L.) from field plots in North Carolina demonstrated similar seasonal patterns in size-adjusted monthly growth rates in shell volume: a large absolute maximum occurred in spring (April or May) with smaller relative maxima in mid summer and late autumn. The ratio of juvenile to adult size-adjusted growth rates in shell volume was nearly constant for ten months but then increased eight-fold in December and January. This growth anomaly between size classes could not be explained by examining dry weights of soma and gonads from additional marked juvenile and adult Mercenaria that were sacrificed monthly. Juveniles differed from adults by possessing negligible gonadal mass on all dates. However, knowledge of monthly changes in adult gonadal mass did not explain a significant amount of the residual variation in the regression of monthly juvenile volumetric growth on monthly adult volumetric growth. Seasonal changes in growth of adult gonadal mass and quarterly examinations of gonad histology both suggested a winter period of negligible gametogenesis followed by a spring burst of intense reproductive activity. The best explanations for the anomalously high volumetric growth of juveniles relative to adults in December and January are: (1) winter availability of a food source accessible only to juveniles or (2) biochemical storage of energy during winter by adults in preparation for the process of rapid gametogenesis in spring. If the latter explanation is correct, adult Mercenaria exhibit a large seasonal change in the allocation of resources between somatic growth and reproduction with maximal allocation to reproduction in winter months before gonad histology and growth of gonadal mass indicate reproductive effort.