Dataset: barnacle_mussel_survey

Field surveys of shores were carried out by teams of 2 to 4 people, all familiar with identification of the relevant species.  At each rocky shore location, average densities of barnacles were determined in the zone of maximum abundance, based on counts of individuals in multiple quadrats.  When densities were 100 m-2 or less, they were measured in 0.25 m2 quadrats.  When densities were greater than 100 m-2, they were measured in 0.01 m2 quadrats.   If no individuals were found in 30 minutes of searching, the species was reported as absent.  At each sedimentary shore location, average densities of polychaetes were determined in the zone of maximum abundance, based on counts of tube caps (Diopatra spp.) or fecal casts (Arenicola).  When densities were 0.1 m-2 or less, nearest neighbor distances were measured, and then converted to densities by the Clark and Evans (1954) relationship.  When densities were greater than 0.1 m-2, they were measured with 0.25 m2 quadrats.  Densities were converted to a log10 scale (ACFORN: Abundant, Common, Frequent, Occasional, Rare, Not seen).  For barnacles we used the scale of Crisp and Southward (1958) in which a density of 1 cm-2 or above is scored as Abundant.  For polychaetes we used a scale in which 100 m-2 or greater is scored as abundant.  For both groups, a species was scored as Rare when only a few individuals were found in 30 min of searching.

Mytilus mussels can only be identified to species by genotyping.  Individuals were collected from the intertidal zone during the field surveys described above.   Samples of the margin of mantle tissue were collected from each individual and preserved in 95% ethanol.  Total genomic DNA was extracted following the protocol of Rawson et al. (1999) and each individual was assayed at the single locus nuclear marker Glu-5’ (Inoue et al., 1995), which is diagnostically differentiated among allopatric populations of M. edulis and M. galloprovincialis (Rawson et al., 1996).