SPERM ECONOMY IN A CORAL-REEF FISH, THALASSEMIA-BIFASCIATUM

In many coral reef fishes, males face the evolutionary problem of how to allocate sperm among frequent, daily matings in order to maximize the number of eggs they fertilize. A method is developed and tested for collecting and counting the number of sperm and eggs released during separate spawns of the coral reef fish Thalassoma bifasciatum. The method was used to examine the pattern of sperm allocation for pair- and group-spawning males. The number of sperm released by pair-spawning males varied positively with the number of eggs released by their female mates and with female body size. The data suggest that males economize on sperm release by providing the minimum amount of sperm needed to fertilize the egg clutch of the female partner. Fertilization efficiency (the number of eggs fertilized by a given number of sperm) was higher with large females than it was with small females. Males differed significantly among themselves in sperm output per spawn, with some males consistently releasing more sperm than other males with same-sized females. Male-male differences were not due to differences in male body size, quality of the spawning site, or the apparent degree of water movement. The number of sperm released per spawn did not decline throughout the daily spawning period, a pattern that disproved one but not all possible patterns of sperm depletion. Tn group spawns, the total number of sperm released and the number released per male were respectively 50 and 6 times the number released in pair spawns, on average. Both of these measures increased significantly with the clutch size of the spawning female. Overall, sperm production is probably sufficiently costly that males have been selected to allocate sperm carefully among their frequent daily spawns.