Barnacles have the longest penis length, relative to body length, among all organisms. It is common for their penis length to be several times that of their bodies¡¦. One reason is that barnacles are immobile and they rely on penis to reach mates. Thus the longer the penis the more mates there may be. How do barnacles balance mate numbers and penis cost? In this investigation, we studied the possible relationship between local densities and penis lengths of barnacles. Our goal is to explain the high intraspecific variation in penis lengths of barnacles. First, we measured the Nearest Neighbor Distance(NND) and Morphological Index (MI), both indices of local barnacle densities, of individuals of Amphibalanus amphitrite at Tainan, Tetraclita kuroshioensis at Keelung and Kenting, and Tetraclitella chinensis at Penghu. The cirrus lengths were used as indices of body sizes. A high percentage of penis length variation could not be explained by body sizes in these barnacles. In A. amphitrite and T. kuroshioensis, a positive correlation between residue penis length, i.e., after deducting the effect of body size, and MI was found. Assuming no sperm competition, we found, through simulation, that the optimal penis length, is not related to local densities, whereas in the presence of sperm competition, the longer penis length the lower fitness.In lab experiment, however, penis length of A. amphitrite was found to increase with increasing number of neighbors.Under high densities, the shell base become relatively small (high MI) and the shell becomes elongated. Investment in eggs may be limited by space available for brooding, thus more energy is available to invest in male functions. Under the assumption of no sperm competition, penis length remains the only option for effective investment. The preliminary result here suggests that the penis length of barnacles is plastic and is influenced by the local density. In Tetraclitella chinensis, no correlation between local densities and penis lengths was found; the very flat shell morphology of the species may have contributed to this phenomenon.