IN-SITU RATES OF FERTILIZATION AMONG BROADCAST SPAWNING GORGONIAN CORALS

Fertilization rates among marine benthic taxa have implicitly been ass umed to be uniformly high in most analyses of life history evolution, but in situ fertilization rates during natural spawning events are rar ely measured. Fertilization rates of the Caribbean gorgonians Plexaura kuna and Pseudoplexaura porosa were measured at a site in the San Bia s Islands, Panama, by collecting eggs downstream of colonies during sy nchronous spawning events during the summer months in the years 1988-1 994. Eggs collected by divers were incubated, and the proportion of eg gs that developed was determined. Proportions of eggs developing sugge st fertilization rates that vary from 0% to 100%. Monthly means ranged from 0% to 60.4%. Failure of gametes to develop can be attributed to sperm limitation, as eggs collected during spawning had higher fertili zation rates if incubated with an excess of sperm. Plexaura kuna ferti lization rates were highest during the July spawning events. Fertiliza tion of Plexaura kuna eggs was usually lower during the first two nigh ts of the 4-6 night spawning event. The proportion of eggs being ferti lized when collected from a given place and time was highly variable, with one peak in the frequency distribution at or below 20% fertilizat ion, and a second group of samples with greater fertilization rates. H igh variance in fertilization rates is evident at all levels of analys is: between replicate samples, times within nights, and among nights a nd months. This variance can be attributed to a combination of the eff ects of heterogeneity in the water column as gametes are diluted, spaw ning behavior of the gorgonians, and the current regime. Fertilization rates are often low and may represent a limiting step in recruitment during some years. Low fertilization rates may also be an important co mponent of the life history evolution of these species.