In order to describe physiological and ecological aspects of the hard clam Eurhomalea exalbida in Ushuaia Bay (Beagle Channel), individual age, growth, mortality, production, bioenergetic and reproductive characteristics were studied.

The pattern of internal growth-bands, confirmed by a year-long mark-recapture experiment and by the d18 O isotope profiles of shells, presents the formation of one translucent and one opaque band every year, corresponding to periods of low and high growth-rates respectively. A strong pattern of seasonal growth was observed, with differences between juvenile and adult growth-rates throughout the year and in the growth period. This suggests that the growth is influenced not only by temperature but also by differences in the distribution of energetics resources related to the gametogenic activities of adults. The parameters for the von Bertalanffy growth function were Height=73.98 mm, K=0.180 y^-1, t0=0.147 y, reaching a maximum age of 70 years. The 56 years of mean relative annual growth increments analyzed showed the existence of strong fluctuations in long-term shell growth. As a result of these variations, clams older than 30 years of age have a different morphology from the younger clams. Thus animals of the same size but of very different ages were found. The use of a physiological marker such as the pigment lipofuscin was analyzed as a method for age-determination in E. exalbida. A positive exponential relationship was found between age and the % of fraction-area of lipofuscin in the connective tissue surrounding the intestine. This study is the first to show a relationship between age and the concentration of morphological lipofuscin in a bivalve.

Maximum individual production reached 2.742 g shell-free wet mass (SFWM) at 49.5 mm of shell height (SH). Individuals between 40 and 70 mm SH contributed most to somatic production (P) in the population, reaching 133.89 g SFWM m^-2y^1. Mean annual biomass (B) reached values of 1122.69 g SFWM m^-2y^-1. The annual ratio (P/B) and the mortality rate (Z) were estimated at 0.119 y^-1 and 0.141 y^-1 respectively. Analysis of the reproductive cycle showed a spawning peak in November, with mature gonads during the rest of the year. First maturity is reached at the age of 4. The variation detected in the energetics content and in the relative condition index for the Gonad-Digestive-Foot group during the experimental period was related to the reproductive stage of the individuals, while the low variation in the energetics content and relative condition index in the adductor muscle and the Gills-Mantle-Siphons groups throughout the year indicates the lack of energetics transfer between organs and the absence of an energetics reservoir. Hence, that the energy needed for different metabolic processes is extracted directly from the ingested nutrients.

The slow growth rate, the longevity of individuals and the low rate of renovation make this population of E. exalbida poorly suited to sustainable commercial exploitation.

E-mail:lomovask@mdp.edu.ar