For many mussel species, the model of planktonic development followed by metamorphosis 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 settlement. 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 species 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 metamorphose 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.

No abstract

Upwelling intensity modifies coastal primary production and influences individual traits of habitatforming species. Along the Peruvian coast, beds of the mytilid Perumytilus purpuratus provide structurally complex habitats that harbour many organisms. We predict that in the nutrient-rich system of Central Peru, the modification of structural complexity would have stronger effects on the Perumytilus community than nutrient addition. We experimentally examined the effects of nutrient addition on the Perumytilus-dominated assemblage and we evaluated the potential effect of varying shell size on the Perumytilusdominated assemblage. Nutrient addition to the mussel bed with slow-release fertilizers caused no changes in the total macro-and microalgal biomass and did not affect abundances or composition of the assemblage. To explore the effect of structural complexity on the Perumytilus assemblage, we manipulated mussel size with experimental bags containing small and large individuals. Predators, grazers and mobile organisms were more abundant among smaller mussels, with smaller gap volume, whereas the barnacle Jehlius cirratus was more abundant on larger mussels. In conclusion, point-source nutrient addition to the mussel bed did not enhance primary production. However, the modification of structural characteristics related to mussel size induced changes in the faunal assemblage. Thus, it seems that in this nutrient-rich system, nutrient enhancement would not significantly affect Perumytilus and its assemblage, whereas structural habitat seems to play an important role in shaping this community.

RESUMENSe evaluó mensualmente los cambios y la magnitud del impacto de "El Niño" (EN), sobre el mecanismo del asentamiento larval de algunos invertebrados marinos bentónicos, en sustratos artificiales filamentosos (fibra nylon), entre enero 1996 y julio 1998, en una estación fija, a 10m de profundidad, situada en el lado oriental de la Isla Independencia, en Bahía Independencia, Durante 1996, en la fase fría "La Niña" (LN), el número de especies presentó un pico en abril y la densidad en junio. En la fase cálida EN 1997 98, la densidad total y el número de especies, presentaron un primer pico en marzo de 1997, luego, un segundo pico, en febrero y julio de 1998, respectivamente. Un primer grupo de especies que intensificó su asentamiento durante la fase fría LN 1996, estuvo constituido por el bivalvo Hiatella solida, el turbelario Notoplana sp. y el gastrópodo Caecum chilense. El segundo grupo intensificó su asentamiento en la etapa temprana de la fase cálida EN 1997-98, Y lo formaron el braquiópodo Discinisca lamel/osa, el equinodermo Ophíactís kr6yerí y bivalvos de la Familia Mytllidae, Un tercer grupo, mostró una intensificación del asentamiento larval, en la etapa tardía de la fase cálida EN 1997-98, a inicios de 1998, y fue formado por el bivalvo Argopecten purpuratus y un gastrópodo turriforme. La aparición de larvas recién asentadas de especies tropicales, como el bivalvo Ptería stema y el gastrópodo Epitoníum sp., tuvo lugar en la etapa tardía de la fase cálida EN 1997-98.Palabras claves: Asentamiento larval. invertebrados marinos, El Niño, bentos, Bahía Independencia. ABSTRACTThe changes and magnitlJde of "El Niño" (EN) impact, on larval settlement mechanisms, of some benthic marine invertebrates, on artificial lilamentous substrates (nylon fiber) were monthly evaluated, lrom January 1996 to July 1998, in a 10m depth station located eastern of Independencia Island in Independencia Bay. During 1996, in "La Niña" (LN) warm phase, the number 01 species showed a peak in April and the density showed one in June, The total density and species number showed a firsl peak in March 1997, then, a second peak in February and July 1998 in the warm phase EN 1997-98, respectively. The bivalve Hiatel/a solida, turbellaria Notoplana sp., and gastropod Caecum chilense constituted a first group 01 species that intensilied its settlement during cold phase LN 1996. The second group intensified lts settlement in earlier warm phase of EN 1997-98, and its was conformed by the brachiopod Discinisca lamel/osa, echinoderm Ophiactís kr6yeri, and the bivalves 01 Mytilidae family. At the beginning 01 1998, a third group showed an intensiflcation 01 larval settlement in later phase 01 the EN 1997-98 warm phase, and il was lormed by the bivalve Argopec/en purpura/us, and a turrilorm gastropod, The appearance 01 recently settlemenl larvae 01 tropical species as the bivalve Pteria stema, and the gastropod Epitonium sp. appeared in later warm phase 01 ENSO 1997-98.

No abstract