The species found on kelp holdfasts have darker shells with obscure ribbing. It typically lives under boulders or among the holdfasts of seaweeds. It favours crevices and shady gullies and sometimes adheres to "Laminaria" holdfasts, stones and shells. Usnea scabrida is a foliose lichen that grows from holdfasts on trees. Other organisms, such as sea urchins, feed on the holdfasts, which can kill the algae. The spurs primarily serve as holdfasts during mating. Species are usually rhizinate on the lower surface, occasionally with holdfasts, rhizohyphae, or a hypothallus. It may also have had a series of holdfasts along the bottom of the thread. Many species use shells and holdfasts to provide stability against strong wave actions. Holdfasts are often the only evidence for a weapon that survives. Holdfasts vary in shape and form depending on both the species and the substrate type.