Sexual structure of a highly reproductive, recovering gorgonian population: quantifying reproductive output

Abstract

A population of the Mediterranean red gorgonian Paramuricea clavata has exhibited unexpected resilience after being impacted by 2 anomalous mortality events in 1999 and 2003. To understand the recovery mechanisms, we examined the population reproductive structure and reproductive output based on data collected via non-destructive sampling techniques. The overall population sex ratio was balanced, though the spatial distribution of sexes was significantly segregated. Dividing the population into 14 size classes on the basis of their measured average annual growth revealed a decreasing monotonic trend of abundance of larger classes. The Recruitment class was consistently dominant. The minimum size at first reproduction was 8.5 cm in height, corresponding to an age of ~3 yr. The percentage of fertile colonies increased with size, reaching 90% in size Class 9. Polyp fecundity increased with colony size and did not differ significantly between healthy and damaged colonies. As the number of mature oocytes produced by a colony is a function of polyp fecundity and of the number of reproductive polyps, colony reproductive output increased exponentially with size. The population reproductive output (145 × 103 mature oocytes m−2 yr−1) was one-fifth of that measured in stable, undamaged populations and came mainly from the medium size classes. After the catastrophic mortality, the population has been recovering, albeit with reduced reproductive output. Moreover, it has exhibited a 2-fold increase in recruitment rate, 3-fold greater than that measured in other, undisturbed populations. Our findings are consistent with a strict density-dependent recruitment control operating in crowded, stable P. clavata populations