Pisaster ochraceus arm length conversion using center or madreporite

Conversion for Pisaster ochraceus Arm Length MeasurementsTo enable P. ochraceus size comparisons between California and Oregon and to allow other researchers to easily convert between arm lengths using different published methods, we measured arm lengths of 25 sea stars ranging in arm length from ~5-200mm at each of 6 sites in Oregon (see manuscript) using both measurement approaches. To determine whether site influenced this conversion, we first ran a linear model with madreporite measure as the response variable and site and center measure as the interacting factors. No interacting effect of site and arm measurement from center to arm tip was found (F5,138 = 0.64, P = 0.672), suggesting this metric is consistent geographically, at least within Oregon. We had no data for California. So, we used a simple linear regression (R2 = 0.97, P < 0.001) to convert madreporite measures to center measures for all our data. Conversion formulas in centimeters are: [madreporite arm length = 4.074 + (1.060*center arm length)] or [center arm length= -0.729 + (0.914*madreporite arm length)].