Variation in recruitment: differentiating the roles of primary and secondary settlement of blue mussels Mytilus spp.

摘要

Recruitment has often been cited as a key factor regulating population abundance and community structure in benthic marine ecosystems, where the life cycle of many species includes a dispersive planktonic phase. Still, the patterns and causes of temporal heterogeneity in the recruitment process, from daily to annual scales, are poorly understood for most taxa. We conducted weekly (2008) and biweekly (i.e. every 2 wk; 2008-2009) assessments of settlement of the marine mussels Mytilus spp. in the St. Lawrence maritime estuary, and differentiated between primary (metamorphosis) and secondary (post-metamorphosis movements) settlers. At a biweekly temporal resolution, recruitment in 2008 occurred over a 2 mo period with a single peak in August. A more complex pattern of recruitment involving multiple peaks of primary and secondary settlement was, however, revealed at a weekly resolution. In 2009 the biweekly settlement rates were an order of magnitude lower, with again only a single peak. In both years, secondary settlement was observed throughout most of the season and contributed as much or more (50-81 %) than primary settlement for several peaks. Based on prodissoconch II size and estimated larval growth rates, the mean planktonic larval duration was estimated to be 38 d, but may have ranged from 27 to 67 d depending on the actual growth rate. The importance of secondary settlement increased over time and was a major contributor to local recruitment dynamics. Variation in primary and secondary settlement as well as inter-annual variations could strongly affect estimates of recruitment rates, local dynamics and the spatial scales of connectivity among coastal populations, and thus our understanding of local population regulation and metapopulation dynamics.