Living exclusively in trout streams which are very poor in nutrients, freshwater pearl mussels are physiologically adapted to the low food supply by a reduced metabolism. Longevity of these mussels spans decades and life-time fecundity of females is very high (up to 2*108 larvae). Their resource allocation policy favours survival over current reproduction. Reproductive effort (per reproductive period) falls within the lowest range of values reported for molluscs (0.8-5.3% of the total organic substance). The soft parts (without larvae) of reproducing females are heavier compared to pausing ones, indicating that only those females reproduce which exceed a threshold value of body weight. The more they exceed this value the more larvae they produce. Surplus energy (the amount above the threshold) is not completely devoted to reproduction but is in part invested in somatic functions. At the population level this allocation system results in largely size independent fecundity values, varying considerably between individuals and in a variable percentage (5-54%) of females taking part in reproduction every year. The evolution of this system must be attributed to the reduced metabolism and growth. This provides the basis for a very long life accompanied with many spawning periods. Accordingly fitness can be improved by extending longevity.