Field measurements were performed in three separate beds of the California sea mussel, Mytilus californianus, in the rocky intertidal zone adjacent to Hopkins Marine Station, Pacific Grove, California. The beds comprised two layers of mussels: a basal layer attached to the rock and a surface layer attached to the basal layer. Measurements were conducted in summer and early autumn of 2008.

Tenacity measurements:
We used the method of Denny (1987) to measure the attachment strength of individual bed mussels in the surface layer. We attached a recording spring scale to a mussel by placing a hook through a hole drilled in the exposed posterior lip of the shell. We then pulled the mussel evenly and swiftly perpendicular to the substratum (i.e., in the direction of lift) until it was dislodged. We divided the recorded force by the mussel’s planform area to calculate tenacity. Planform areas were estimated using shell height and width as the axes of an ellipse, which closely approximated the shape of mussels’ cross section. We measured shell height and width to the nearest millimeter using calipers. Sampled mussels were separated by at least 10 cm to ensure that each sample had no effect on subsequent samples.

We also tested groups of contiguous mussels in the surface layer. To pull multiple mussels simultaneously, we implemented an expansion of the procedure used to measure individuals. We attached groups of 2-6 contiguous mussels to a spring scale via hooks and lines, and adjusted line lengths so that each mussel was in approximately equal tension when the group was subsequently dislodged. Due to the increased force required to dislodge several mussels at once, we used a rigid, robust aluminum tripod to support the spring scale, and a winch to apply the normal force, FG, required to dislodge the group. We measured the width and height of the individuals within each group, calculated their planform areas, and summed these areas for use in calculating whole-group tenacity:
TG(n)=FG(n)/sum(Ai)
We repeated each measurement 20 times; that is, we sampled 20 individual bed mussels, 20 groups of two mussels, 20 groups of three mussels, etc.

muss_tenacity.csv