Qualitative and quantitative histological and histochemical techniques were used to study seasonal dynamics of energy reserves associated with gonad development and oocyte quality in the horsemussel Modiolus capax in Bahía de La Paz during the period February 2013 to January 2014. Thirty adult mussels were collected on a monthly basis and evaluated for gonad developmental stages and changes in oocyte features (theoretical diameter, nucleus/cytoplasm ratio, lipid index, and carbohydrate index). The overall somatic coverage area versus gonad coverage area and energy reserves in the gonad (carbohydrates and neutral lipids) was also analyzed. In summer (26.5 ± 1.4°C and 2.3 ± 0.7 mg m^-3 chlorophyll a), oocytes achieved greater sizes that were significantly correlated with higher lipid reserves in the gonad. This suggests that female gonads have superior breeding capacity in summer, compared with those in the autumn and winter periods, when oocyte performance and ovarian energy reserves were minimal. The evidence indicates that M. capax follows a conservative strategy to sustain gametogenesis relying on energy reserves previously stored in somatic tissue between autumn and winter. By contrast, optimal reproductive performance between April and August associates more with decreasing somatic coverage area and increasing gonad coverage area; this reflects a period of energy (lipids) transfer from storage tissues to the gonad to fuel gametogenesis. These results help establish the biological basis for gonad conditioning and maturation of M. capax to ensure continuous production of spat at the laboratory.