Energy balance, gonad development and biochemical composition in the clam Ruditapes decussatus

This study evaluates the influence of different states of energy balance, positive and negative, on the accumulation and utilisation of energy reserves during the period of gonad development in the clam Ruditapes decussatus (L.). Our results show a clear sexual differentiation in the biochemical composition of the clams. The differences were associated with gonad development and were particularly evident in proteins and lipids. These findings imply that separation of the sexes for studying variations in the biochemical composition of sexually developed specimens is essential, and thus questions the results of previous studies in which both sexes were analysed simultaneously. Gonad development occurs under conditions of both positive and negative energy balance, and permits the development of a model of the utilisation of reserves during gonad development, which is closely linked to the clam’s energy balance. Under conditions of positive energy balance, the amount of food ingested provides sufficient energy for both metabolic consumption and the accumulation of reserves. The absolute value of all biochemical components increases, in particular for lipids in the case of female clams, with no evidence of consumption of glycogen and other carbohydrates or their conversion into lipid reserves. Conditions of negative energy balance lead to a moderate degree of organic weight loss in the clams. Glycogen and other carbohydrates are rapidly consumed, falling to almost a quarter of initial values, whilst protein and lipid levels remain constant in females. Lipid values decrease slightly in males. Finally, under situations of extreme nutritive stress (when energy reserves are scarce) there is a loss of all biochemical components in a specific order of priority. Most of the glycogen and other carbohydrates are consumed initially (a decrease of 61 and 74%, respectively), followed to a lesser extent by lipids in males (40%) and then proteins (between 20 and 25%). In females, there was no lipid loss until the majority of the carbohydrates and part of the proteins had been consumed, indicating that lipids constitute the reproductive reserve par excellence in this species.