Dataset: mussel size vs. byssal count

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.

Byssal thread allocation:
We examined byssal thread allocation (interlaminar vs. intralaminar) in the same beds used for tenacity measurements. Because mussels were tightly packed, it was necessary to dissect both the surface and basal layers to accurately ascertain the per capita number of intra- and interlaminar threads anchoring surface-layer mussels. In the surface layer, we counted for each individual the total number of byssal threads produced (n_tot) and the number of threads that were attached to other surface-layer mussels (n_s). The difference between the average of n_tot and the average of n_s is an estimate of the average number of interlaminar threads produced by each surface-layer mussel. To obtain a second estimate of the average number of interlaminar threads produced by surface-layer mussels, we examined the basal layer. For each individual we counted the number of intralaminar threads extended to neighboring shells (n_b) and the total number of byssal threads attached to each mussel’s shell (n_sh). The difference between the average of n_b and the average of n_sh is a second estimate of the average number of interlaminar threads extending downward from each surface-layer mussel. We then averaged the two estimates of interlaminar thread number per surface-layer individual for comparison to the average number of intralaminar threads per surfacelayer individual. The planform area of each individual mussel was calculated from the width and height of its shell.