The point contact method is efficient for measuring the abundance of spatially common organisms, but it does not adequately represent rare or spatially uncommon organisms such as mobile invertebrates. The abundance of mobile invertebrates is measured using 50 x 50 cm quadrats. Each transect is divided into three regions, representing the high, mid and low zone biology. Quadrats are randomly placed within each of the three zones on each transect line resulting in 33 plots. See diagram below.
Mobile invertebrates in the quadrat, such as snails, limpets and crabs, are identified, counted and recorded on a data sheet. Organisms living within the interstices (the spaces between the substrate and the sessile organisms such as mussels) are not recorded because in order to obtain an accurate count the organisms creating the interstices would have to be removed.