Sperm allocation, sperm production and fertilization rates in the bucktooth parrotfish

Males and females of pelagically spawning marine fishes face different problems in maximizing reproductive success. Males must allocate sperm over frequent, successive spawns. Females must ensure successful fertilization of their egg clutches. This study examined sperm allocation between individual spawns, the daily 'standing crop' of sperm just prior to spawning, gonadal investment (testis weight/body weight) in males and fertilization rates of females of the coral reef fish Sparisoma radians. Males released a median of 8.94 x 10(6) sperm per pair spawn, with the number increasing with increasing clutch size and body size of the female mate. Standing crop of sperm correlated with male spawning frequency, and testicular investment correlated with degree of sperm competition by streaking males. Fertilization efficiency (number of eggs fertilized by a given number of sperm) increased with the body size of the female partner. Fertilization rate, i.e. the proportion of the clutch successfully fertilized, was over 90% in most spawns, but significantly increased with the number of sperm released by the male. The probability that a spawn was joined by a second, competing male ('streaker') increased with the body size of the spawning female. Overall, the sperm allocation pattern can be explained both as a form of sperm economy in which males release the minimum number of sperm needed to fertilize most of the eggs in clutches of different size, and as a response to an increased probability of sperm competition with females of increasing size. (C) 1996 The Association for the Study of Animal Behaviour