In mammalian cardiac myocytes, a small entry of extracellular Ca triggers a much larger release of Ca from the sarcoplasmic reticulum (SR) by a process called Ca-induced Ca release. Contraction of the fish heart is only weakly or moderately sensitive to ryanodine, a specific blocker of the SR, suggesting that SR Ca stores provide relatively little Ca for contraction in fish cardiac myocytes. To this end, Ca storing capacity and calsequestrin (CASQ) content of the fish cardiac SR were determined by caffeine-induced Ca releases in single myocytes and immunoblotting of cardiac proteins with an antibody specific to the fish cardiac CASQ, respectively. The Ca storing capacity of the fish cardiac SR is in many fish species larger than that of the mammalian SR and the Ca-buffering protein, CASQ, is abundantly expressed in the fish hearts. Thus, the apparently small contribution of SR Ca to contractile activation is not limited by the large SR Ca stores, but sooner must be due to the release mechanism. One contributing factor to the weak release seems to be the low Ca-sensitivity of the fish cardiac ryanodine receptors. On the other hand, the usage of SR Ca stores in excitation-contraction coupling is enhanced by acclimation to low temperatures, suggesting that SR Ca stores are physiologically significant under changing temperature regimes.