Limpets

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:
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 (and in some cases has been again adapted to absorb oxygen from water) all these various kinds of snail are only very distantly related. 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.
http://en.wikipedia.org/wiki/Limpet

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. Cape false-limpet uses chemical weapons as defence; Giant limpets use brute force in territorial battles; Ducks foot limpet develops and tends gardens of algae; Goat�s eye limpets slam their shells down like a guillotine, dismembering ferocious predators, Kelp limpet�s parachute from the canopy when kelp breaks loose in storms before being cast ashore. The Limpets� 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 scare 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 radular (toung) rasp out the rock creating a tight fit for their shell. (ref Currents of contrast)

The Duck�s foot limpet Scutellastra longicosta
Establish gardens of Ralfsia which they defend from other grazers buy pushing other limpets away, the duck�s foot prune this� algae by eating furrows stimulating growth within a manageable territory. The symbiosis is mutual as the algae receives waist products (fertilizer) from the limpet.��

�

The Kelp limpet Cymbula compressa� 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 but fern-like epiphytes that attach to the kelps stem, which when not controlled create hydro drag often breaking the kelps holdfast and so 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 and 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 receiving a protein rich meal, these attacks can be survived unless vital organs are penetrated.

Pear Limpets Scutellastra cochlear is very slow growing� living 25 years living in dense aggregations also cultivates algae a coralline and a 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 here this bird flourishes far from human influences. A favourite diet of the Oystercatcher is the pear limpet and by approaching from behind, being the fat end of the pear (out of the limpets vision) the oystercatcher is able to prize the limpet of the rocks before it has time to react, through natural selection a few Pear limpets with a more oval shape shell have been able to survive and reproduce, and become the dominant shape of Pear limpets on the islands.

Goats eye limpet Cymbula oculus
� The juveniles have a flecked iridescent green in their shells and are usually found under boulders starting life as a male becoming female in the second or third year. As fully grown adult females they are able to aggressively counter attacking 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 goats eye limpets will in limpet terms flee on sight of danger.

Source Ref books
Two oceans a guide to the marine life of southern Africa
Currents of contrast life in southern Africa�s two oceans
�