The investments male animals make in reproduction have profound effects on mating systems and individual behaviors. Sperm generally are considered inexpensive relative to eggs, and hence relatively little attention has been devoted to their optimal allocation, or the possibility that they become limiting. In `cheap sperm` species (e.g. broadcast spawners), huge numbers of sperm are produced. Nevertheless, adverse physical factors (e.g. dilution) often lead to sperm limitation and poor fertilization success. Among `expensive sperm` species that invest heavily in mechanisms to increase the reliability of sperm transfer (e.g., courtship, internal fertilization), sperm limitation is more likely to be caused by factors that hinder successful mating. These biological factors, acting before sperm are emitted, are subject to natural selection and provide evidence of evolutionary principles. To understand how individual behaviors of males affect population-level processes, the PIs propose to study an `expensive sperm` species, the blue crab (Callinectes sapidus). They have chosen two widely-separated study sites in the crabs' latitudinal range, which impose very different conditions on the mating system. High-latitude (Chesapeake Bay) females have only short reproductive seasons and, unless they live very long, can produce only a few broods. Low-latitude (Florida) females can produce as many as 11 broods/year in long reproductive seasons. The probability of sufficient sperm being available to realize full potential fecundity in these two environments probably is quite different. Males are subject to disproportionate fishery pressure, especially in Chesapeake Bay. They thus are participants in a large-scale experiment that we propose to exploit to better understand sperm allocation and limitation at the population level. The PIs have strong preliminary data indicating that in Chesapeake Bay (high latitude, heavily fished), female spermathecae receive significantly less sperm, and male vasa deferentia are significantly more depleted, than in Florida where fishing pressure is lower. At Chesapeake Bay and Florida sites, biotelemetry will verify where pairing and mating occur; field sampling will reveal the ratios of mature males to receptive females in mating habitats and how much sperm those males have to transfer. Laboratory and field experiments will test the effects of male size and mating history on amount of sperm transferred and on competition for mates, whether a subset of sperm-depleted males might dominate mating, and how many eggs/broods can be fertilized by a given amount of sperm.
