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 be gins is a theoretically challenging evolutionary problem. We compare a nd extend the theories predicting intermediate resource allocation pat terns. Our population dynamics model is developed for freshwater clams , which typically show considerable growth after maturity. 2. In the s imulations, we used an age- and size-structured discrete-time resource allocation model, and applied dynamic programming algorithms to deter mine the resource allocation strategies that maximize fitness, measure d as expected lifetime production of offspring, R(0). 3. High levels o f 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 relate d to the mass of the offspring: one between relative glochidia mass (r atio of glochidia weight to the weight of the rest of the body) and pr oduction weight, and the other between relative glochidia mass and sur vival. In both cases, substantial adult growth is optimal, and the gro wth curves produced by the model are similar to those observed in clam s. We discuss how these trade-offs are likely to arise in freshwater c lams.