Optimal resource allocation between growth and reproduction in clams: Why does indeterminate growth exist?

1. That many animals and plants continue to grow after reproduction begins is a theoretically challenging evolutionary problem. We compare and extend the theories predicting intermediate resource allocation patterns. Our population dynamics model is developed for freshwater clams, which typically show considerable growth after maturity. 2. In the simulations, we used an age- and size-structured discrete-time resource allocation model, and applied dynamic programming algorithms to determine the resource allocation strategies that maximize fitness, measured as expected lifetime production of offspring, R(0). 3. High levels of mortality select for early maturity and high reproductive effort. 4. Some adult growth is optimal in cases of size-dependent survival and production rates. However, this is not a sufficient explanation of the levels of adult growth observed. 5. We consider two trade-offs related to the mass of the offspring: one between relative glochidia mass (ratio of glochidia weight to the weight of the rest of the body) and production weight, and the other between relative glochidia mass and survival. In both cases, substantial adult growth is optimal, and the growth curves produced by the model are similar to those observed in clams. We discuss how these trade-offs are likely to arise in freshwater clams.