Tumbling under the surf: wave-modulated settlement of intertidal mussels and the continuous settlement-relocation model

Abstract

For many mussel species, the model of planktonic development followed by meta- morphosis and settlement in the benthic habitat is complicated by the existence of planktonic post-metamorphic stages and/or pediveliger benthic stages that can relocate after initial settle- ment. This has led to the long-standing hypothesis of 'primary' settlement from the plankton onto intertidal algal substrate followed by 'secondary' relocation to mussel beds. Here, we investigate settlement of the intertidal mussels Perumytilus purpuratus and Semimytilus algosus in central Chile to test this hypothesis and explore physical drivers. Our results indicate that: (1) these spe- cies do not have planktonic post-metamorphic stages, (2) larvae typically arrive to the intertidal zone at a size 80-150 µm larger than the largest planktonic larva, which based on growth rates, corresponds to a 3-20 d delay, (3) there are no differences in pediveliger sizes between different algal substrates, mussel beds, or artificial collectors, and (4) there is no evidence that larvae meta- morphose in the intertidal and grow in alternative habitat before relocation to mussel beds. In 2 summers, daily settlement of both species was tightly and positively associated with wave height, despite large inter-annual variability in wind conditions. Our results reject the primary-secondary settlement hypothesis and support a new settlement model in which, after metamorphosis beyond the surf zone, the negatively buoyant settlers become semi-benthic and readily sink to the bottom. There, they can be transported onshore through the surf-zone by wave-driven near-bed transport. The process of tumbling under the surf may take from a few hours to several days, with larvae arriving at the shoreline in a wide range of sizes at any given time. For some larvae, relocation continues in the intertidal zone for months.