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 ce ntimeters. Variance in fertilization rates of the broadcast spawning C aribbean gorgonian Plexaura kuna Lasker, Kim, and Coffroth was examine d in the San Bias Islands, Panama in a series of field experiments tha t compared fertilization rates at set distances from spawning male col onies. Quadruplicate samples of water were collected from each of five positions around spawning male colonies during spawning events in Jun e and July 1994-1995. All samples were analyzed for their ability to f ertilize eggs. In 1995, sperm densities also were measured. The averag e fertilization was very low (15%), and 74% of the samples yielded les s 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) dur ing the experiments and were negatively correlated with fertilization rates. Samples collected from identical locations within seconds of ea ch other varied in fertilization rates by as much as 56%. Sperm densit ies in the quadruplicate samples differed by as much as two orders of magnitude. Average fertilization rates are most affected by current sp eed and rates of sperm release. However, finer scale variance is proba bly related to heterogeneities in mixing processes that occur on scale s of centimeters and seconds. Mean fertilization rates may be driven b y heterogeneity in dilution processes that produce patches of high spe rm density. (C) 1997 Elsevier Science B.V.