Estimating the size of molluscs' feet in situ by staining their foot-prints

• Measuring ability of molluscs to adhere to substrata requires foot-size estimates.
• Existing methods are problematic, because foot may not fully attach to surfaces.
• A new in situ technique was developed to stain mucous “foot-prints” of limpets.
• Technique provided better estimates of foot-size than previously obtainable.

Measuring the ability of gastropod molluscs to adhere to substrata often requires estimates of their foot-size. Existing methods use dimensions of shell-aperture as indirect measures, or acquire more direct measurements of the area of foot by anesthetising animals or by allowing them to re-attach to clear synthetic sheets of material in laboratory. Such methods are problematic, because the foot may not always be fully attached to its substratum when in natural habitats. Alternatively, gastropods may attach more or less firmly to unnatural surfaces in laboratories. To reduce these problems, a new in situ technique was developed to stain (with Gentian Violet) the mucous “foot-prints” of individual limpets on rocky shores and seawalls. Analyses of photographs of stained foot-prints and measurements of shells of limpets provide estimates of the area of foot that is actually in contact with the substratum, relative to the size of the animal. This method was used to measure the feet of two patellogastropods (Cellana tramoserica, Holten, 1802; Patelloida latistrigata, Angas, 1865) and one pulmonate (Siphonaria denticulata; Quoy and Gaimard, 1833) limpet on rocky shores (horizontal and vertical) and seawalls (sandstone and concrete) in 3 locations in SE Australia. There were consistent linear relationships of foot-size based on estimates from shell-dimensions and direct measure of stained foot-prints across habitats and locations, but these differed among species. The new technique provided more accurate estimates of the size of foot than previously obtainable. The relationship between the foot-print and the shell-size of limpets was not 1:1 with the commonly-used method using estimation as an ellipse. Instead the shell-size was often twice the area of the foot-print. Results are discussed in relation to providing more accurate and ecophysiologically relevant in situ estimates of the ability of mobile gastropods to adhere to and move about in natural habitats.