LARVAL ABUNDANCE, RECRUITMENT AND EARLY MORTALITY IN PARACENTROTUS-LIVIDUS (ECHINOIDEA) - INTERANNUAL VARIABILITY AND PLANKTON-BENTHOS COUPLING

The temporal patterns of larval abundance, recruitment and early morta lity in the sea urchin Paracentrotus Lividus (Lamarck) in the northwes tern Mediterranean were studied by combining planktonic and benthic su rveys. The abundance of planktonic stages was monitored by weekly plan kton hauls from 1992 to 1996. The presence of juveniles was studied by collecting epibenthic samples at monthly intervals from 1992 to 1995. The size distribution of the same population was studied in 1993. A m ain recruitment episode in late spring-summer occurred every year, alt hough small recruitment events had been observed in autumn and winter. Interannual variability in larval abundance and recruitment was high, both being most intense in 1992, when up to 33.47 larvae m(-3) were r ecorded in plankton samples, and up to 23000 recruits (sea urchins <2 mm) m(-2) were found in the benthic samples. We estimated that only ab out 12.7% of larvae (from the a-armed stage onwards) survived the plan ktonic Life, and that 0.5 to 0.7% of the settlers survived until they reached a diameter of 2 mm, while 0.040% of settlers survived the firs t year of benthic life and only 0.028% of settlers may attain reproduc tive size. The main bottlenecks in the dynamics of this species occurr ed, therefore, during the planktonic phase and the first year of benth ic existence, particularly during the early post-settlement phase. The benthic samples provided a more robust depiction of events than plank tonic data, which featured a higher variability. There was a significa nt relationship between the interannual variation in the late winter p hytoplankton bloom and the spring peak of larvae of this species, indi cating that changes in larval abundance are coherent with changes in p lanktonic primary production. There was also an exponential negative r elationship between the abundance of larvae and abundance of recruits relative to larvae over the years studied, indicating density-dependen t mortality rates in the planktonic and early benthic stages. Successf ul cohorts of this species can be traced to years with high larval abu ndance, which in turn are related to episodes of high planktonic prima ry production.