The name limpet is most often applied to members of the clade Patellogastropoda, the true limpets, which are all marine; however, the feature of a simple conical shell has arisen independently many times in gastropod evolution, in many different lineages, some of which have gills and some of which have a lung. The name is given on the basis of a limpet-like or "patelliform" shell, but the several groups of snails that have a shell of this form are not at all closely related to one another.

Goat's Eye Limpet on the rocks
📷 Goat's Eye Limpet on the rocks

Most of the marine limpets have gills, whereas all the freshwater limpets and a few of the marine limpets have a mantle cavity that is adapted to breathe air and function as a lung. In other words, the name limpet is used to describe various extremely diverse groups of gastropods that have independently evolved a shell of the same basic shape.

When it comes to limpets, southern Africa is the world's foremost biodiversity and biomass hotspot, with some areas having densities of 2,600 individuals per square meter!

Limpets at first glance are static, even boring creatures, but a closer look reveals an evolutionary masterpiece:
The Cape false-limpet uses chemical weapons as defence;
 Giant limpets use brute force in territorial battles;
The Duck's foot limpet develops and tends gardens of algae;
 Goat's eye limpets slam their shells down like a guillotine, dismembering ferocious predators;
Kelp limpets parachute from the canopy when kelp breaks loose in storms before being cast ashore.

The limpet's shell shape and amazing adhesion has allowed them to become the dominant species on rocky shores exposed to heavy wave action. Each individual limpet has what is called a home scar when the shell grows to fit the particular rock it inhabits. There are some limpets that produce an acidic mucus which softens the rock, and with their iron-tipped radula (tongue) rasp out the rock creating a tight fit for their shell.

Duck's Foot Limpet (Scutellastra longicosta)

The Duck's foot limpet establishes gardens of Ralfsia algae which they defend from other grazers by pushing other limpets away. The duck's foot prunes this algae by eating furrows, stimulating growth within a manageable territory. The symbiosis is mutual, as the algae receives waste products (fertiliser) from the limpet.

Kelp Limpet (Cymbula compressa)

The Kelp limpet has a concave opening fitting the stem of Sea bamboo (Kelp) Ecklonia maxima. The kelp limpet also defends its territory—the entire plant—from intruders. Its diet is not the kelp itself, but fern-like epiphytes that attach to the kelp's stem, which when not controlled create hydro drag, often breaking the kelp's holdfast and washing the plant ashore. This is not necessarily the end for the kelp limpet, as it can sense the change in pressure when the kelp dislodges, releases itself from the kelp, and floats to the seabed to find another unoccupied kelp. This is a perilous journey with many predators like Whelks that drill holes in the shell of the limpet for a protein-rich meal.

Pear Limpet (Scutellastra cochlear)

Pear Limpets are very slow-growing (living 25 years in dense aggregations) and cultivate a coralline and faster-growing red algae to sustain larger individuals. The shape of the pear limpet has been influenced on islands off the coast by the African Black Oystercatcher, where this bird flourishes far from human influences. A favourite diet of the Oystercatcher is the pear limpet: by approaching from behind (the fat end of the pear, out of the limpet's vision), the oystercatcher is able to prise the limpet off the rocks before it has time to react. Through natural selection, a few Pear limpets with a more oval-shaped shell have been able to survive and reproduce, becoming the dominant shape of Pear limpets on the islands.

Goat's Eye Limpet (Scutellastra granularis*)

The juveniles have a flecked iridescent green in their shells and are usually found under boulders, starting life as a male and becoming female in the second or third year. As fully grown adult females, they are able to aggressively counter-attack predators such as whelks and even spiny starfish by quickly guillotining predators' appendages foolish enough to probe under the shell! The predator soon retreats and is often mortally wounded. Smaller goat's eye limpets will, in limpet terms, flee on sight of danger.