Production, settlement, and survival of plexaurid gorgonian recruits

Observed recruitment rates among benthic invertebrates reflect a cascade of events that starts with the production of gametes and includes fertilization, settlement and the mortality that occurs shortly after settlement. Although recruitment rates are commonly measured, studies that include the steps leading to recruitment are relatively rare. Fecundity, fertilization rates, larval supply, post-settlement mortality and annual recruitment of the clonal gorgonian Plexaura kuna were determined for a population in the San Bias Islands, Panama. Fecundity of colonies was determined by enumerating the eggs contained within polyps of female colonies and from the number of eggs released in aquaria by colony explants. On average, colonies released 23.2 eggs cm(-1) of branch length. Polyps on second order branches contained more eggs than those on primary or tertiary branches. Extrapolation of egg release data as well as the density of eggs in the water column during spawning suggests that genets, which may contain 10(2) colonies, can produce 10(6) to 10(7) eggs annually. Only 1 planula was observed in a total of 40 settlement traps that were sampled weekly during June and July 1989. To assess post-settlement survival, planulae of P. kuna were reared and allowed to settle on 40 seasoned Plexiglas plates which were then returned to the reef. Mortality during the first 2 wk on the reef was 60%. Survivorship was greatest on the most protected section of the PIexiglas plates, and when the entire settlement plate was floated above the substratum and away from benthic grazers. Monthly recruitment of P. kuna was monitored in 2 m(2) areas at 3 sites between June 1986 and July 1988. Few recruits were observed and all died within several months of their appearance in the survey areas. Annual recruitment was followed in 230 m(2) during 1984 to 1996. At one site, P. kuna recruits were not observed, and at the other 2 sites, recruitment rates were 0.04 and 0.002 m(-2) yr(-1). Extrapolations of colony production rates, clone size, and post-settlement survival suggest that recruitment is most sensitive to survival of larvae in the water column and survival of newly settled individuals. The 3-fold difference in mortality rates on different areas of the settlement plates leads to a 100-fold difference in recruitment. Although recruitment rates are low when considered on an annual basis, clonal propagation, which extends genet lifespan over many decades, dramatically increases the likelihood of successful settlement over the Lifespan of colonies and genets.