Living in or near the intertidal zone of a rocky seashore usually requires being good at holding on. Organisms that live in this habitat commonly deal with crashing waves and strong underwater flows that exert varying forces in all directions.
To withstand these forces, many intertidal organisms attach to rocks and other surfaces using parts that function as flexible anchors. These anchoring structures commonly consist of a roughly disc-shaped base that gradually tapers and becomes more flexible as it joins the main body. Examples include holdfasts on seaweeds and the feet of intertidal snails. When forces impact the anchor, the base is firmly attached while the rest of the structure can bend and deform slightly. This flexibility prevents forces from being concentrated at a small area on the anchor, a situation that increases the chances of peeling and detaching.Edit Summary
“Loads that might cause peeling occur widely in nature. Neither the holdfast of a large marine alga nor the byssal thread of an intertidal mussel nor the foot of the wave-challenged snail can be assured of tensile forces perpendicular to its attachment surface. All respond with some tapering disk of attachment that’s increasingly flexible peripherally (fig. 21.1b)–the whole thing distorts a little to avoid stress concentrations at an edge. Thus, a good shape for a glued joint may resemble (and may also function as) a suction cup, even though the two mechanisms are physically distinct.” (Vogel 2003:425)