An experimental assessment of measures of mussel settlement: Effects of temporal, procedural and spatial variations

• Field experiments highlight sources of variation in mussel settlement.
• There are differential, size specific settlement patterns among intertidal mytilid mussel species.
• Influx and efflux of Perna canaliculus secondary settlers can occur at short temporal scales (days).
• Analyses show that temporal variations in sampling regimes can affect data interpretation.

Much research has focused on linking larval abundance and dispersal data to observed patterns of settlement. Consequently, measures of settlement have become increasingly important in benthic marine ecology and in understanding benthic-pelagic linkages. Potentially, measures could be confounded by complex settlement behaviors and physical variables that may impact on the abundance of larvae. The effects of timing or duration of deployment and data standardization (to a mussel per day rate) on measures of mussel settlement made using standardized artificial units of habitat (Tuffies™) were tested in three seven day trials. In addition, variations in methodological deployment procedure and position on the shore were assessed. The day of deployment and duration of sampling had species- and size-specific effects on measures of mussel settlement. The abundance of settlers of large (> 500 μm) Perna canaliculus was highly variable over short temporal scales (one day), and longer deployment durations indicated significant peaks and falls in the number of settlers per Tuffy over seven days. By contrast, the abundance of small (< 500 μm) Xenostrobus pulex increased linearly and measures taken over longer time-periods well reflected overall settlement. These data suggest that samples should, where possible, be sorted into species and size-classes because different settlement patterns may mask underlying settlement processes.