Small-scale heterogeneity of fertilization success in a broadcast spawning octocoral

Fertilization rates of many broadcast spawning marine taxa are highly variable and exhibit marked variation even on scales of seconds and centimeters. Variance in fertilization rates of the broadcast spawning Caribbean gorgonian Plexaura kuna Lasker, Kim, and Coffroth was examined in the San Blas Islands. Panama in a series of field experiments that compared fertilization rates at set distances from spawning male colonies. Quadruplicate samples of water were collected from each of five positions around spawning male colonies during spawning events in June and July 1994- 1995. All samples were analyzed for their ability to fertilize eggs. In 1995, sperm densities also were measured. The average fertilization was very low (15%), and 74% of the samples yielded less than 20% fertilization rates. Fertilization rates dropped by 44% 1 m from a spawning male. Current speeds ranged from 1.9-5.5 cm s^-1 during the experiments and were negatively correlated with fertilization rates. Samples collected from identical locations within seconds of each other varied in fertilization rates by as much as 56%. Sperm densities in the quadruplicate samples differed by as much as two orders of magnitude. Average fertilization rates are most affected by current speed and rates of sperm release. However, finer scale variance is probably related to heterogeneities in mixing processes that occur on scales of centimeters and seconds. Mean fertilization rates may be driven by heterogeneity in dilution processes that produce patches of high sperm density.