Decadal demographic trends of a long-lived temperate encrusting sponge

Demographic data play a central role in determining the overall dynamics of species and forecasting the effects of global change. We examined a decadal pattern of population dynamics and its underlying mechanisms in a population of the encrusting sponge Crambe crambe. A photographic series monitored yearly for 14 yr (1993 to 2007) was analyzed. A total of 133 genets and 157 ramets were individually identified and followed over 14 yr to record their fates: survivorship, partial mortality, recruitment via larvae, asexual events (fission and fusion), and growth, The number of genets varied little over time; their survival was high and differed significantly with size class (52, 82, and 100% for size classes 0-250, 250-700, and >700 mm(2), respectively), while recruitment via larvae was very low. Overall, the population was characterized by the inputs and outputs of asexual ramets. Patterns of partial mortality were (1) significantly related to size and (2) did not differ significantly among years, and (3) no patch >500 mm(2) suffered any type of mortality. We give the first evidence that net growth over more than a decade was close to zero for the larger specimens, while the smallest specimens exhibited the highest growth rates, quadrupling in size over 14 yr. Low mortality combined with the slow growth of large specimens indicates that C, crambe may live for several decades. This study provides rigorous baseline information necessary to uncover general patterns in the demography of marine sessile species and better predict the long-term fate of populations.