The inability to limit heat loss while respiring through gills prevents most fish from maintaining body temperatures higher than ambient. However, among the order Scombroidei (tunas, billfishes and swordfish) the ability to retain metabolic heat and elevate tissue temperatures is widespread. Billfishes and swordfish employ a strategy of cranial endothermy warming the brain and eyes with a specialized thermogenic tissue called heater organ. These fish are able to dive into cold waters (~10^oC), but the cranial temperature is keeping constant at 25^oC. Derived from eye skeletal muscle, the heater organ expresses a unique thermogenic phenotype, with no contractile filaments, but increases all the cellular components involved in energy metabolism and Ca²⁺ mobilization, suggesting a mechanism of heat production involving Ca²⁺ fluxes from internal stores ("Ca²⁺ futile cycles"). The sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) maintains a low cytoplasmic Ca²⁺ concentration by pumping Ca²⁺ from the cytosol into the SR lumen at the expense of ATP.  Cloning data showed that heater organ express the same SERCA isoform found in skeletal muscle. The aim of this work was to investigate the activity of SERCA1 from blue marlin (Makaira Nigricans) heater organ at 10^oC and 25^oC, simulating the water temperature reached by these fishes in the ocean and compare its activity with the well-known SERCA1 from rabbit skeletal muscle. The results showed that as you decrease the temperature from 25^oC to 10^oC, the activity of both SERCAs deeply decrease. Measurements of the rate of ATP hydrolysis catalyzed by SERCA1 from marlin heater tissue and rabbit skeletal muscle were practically the same at both temperatures tested. However, the rate of Ca²⁺-uptake catalyzed by marlin SERCA1 was almost 3 times lower than in rabbit. Calculations of the ratio between Ca²⁺ uptake and ATP hydrolysis (coupling) at 10^oC catalyzed by the marlin SERCA was ~0,7 and as the temperature of the medium increases to 25^oC, this ratio decreases to ~0,3. For the rabbit SERCA1, this ratio was practically the same at both temperatures (~1,7). As a result, we observed that the SERCA1 pump from heater tissue is less coupled than rabbit, and also as the temperature of the medium increases to physiological values (25^oC) this ratio decreases. Taken together these results suggest that the inefficiency of the SERCA1 pump from blue marlin heater organ may be involved in thermogenesis. Supported by NSF and FAPERJ.