Limpets

Limpets are so common on the rocky shores of Penwith, to such an degree that they are taken for granted and overlooked. Even if the tide is in, the limpets are the discarded shells you will certainly find. Limpets are very hardy species and are even found on the stormiest shores all year round.

Limpets are robust their cone shape shell is structurally strong, not many predators are capable of breaching this defence. Their conical shape means that there is a large surface area in contact with the rockface. This allows for a large foot, providing limpets with a phenomenal grip, to such an extent that they cannot easily be prised off by hand. If prised off, they are almost certain to die as their seal will have been compromised.

The seal reduces moisture loss, preventing the limpet from drying out when uncovered by the sea. The seal is an evolutionary success, as in the intertidal zone when uncovered by the tides more limpets can be seen out of the water than submerged in the rockpools.

When the tide uncovers the limpets, they tightly clinging to rock faces. At night they graze on algae. They feed by using an organ called the radula. The radula is extruded from the mouth, its rasp like surface is used to scrape off algae. As algae grow on hard surfaces, the radula is gradually abraded away, to make up for this, the radula is continually growing.

Limpets have an important affect on their environment, they keep it clear of algae (excluding Lithothamnia). If you look in rock pools, often the only algae seen growing is on the ungrazed backs of limpets.

This heavy grazing has the knock on affect of limiting the numbers of species which settle on the rocks such as barnacles or species that rely on the algae.

After the Torey Canyon oil spill, limpet numbers were decimated and the local rocky beaches became covered with slippery green weed. When limpet numbers recovered the weed receded.

Limpets display a homing instinct. While grazing, they produce a trail of slime. After grazing they follow the trail back to the same spot. Here they eventually gouge out a depression. These indentations are even seen in hard Cornish Rock. If the rock is hard, instead of wearing away the rock, the edge of the shell is abraded, either way a good fit is achieved.

When the limpet reaches its resting point. it orientates itself so that the shell’s edge snugly fits the rock face so providing a good seal.

Limpet Gouges in Hard Cornish Rock

Heavily Grazed Rock Pool

Reproduction

Unlike many gastropods with limpets there are separate sexes. Limpets start off as male but many change to female as they age. More unusal is that there may be two types of males, true males and temporary (protandric) ones.

Early in the year the females discharge their eggs while the males discharge their sperm into the sea. These eggs are fertilised and are carried by the currents. The larvae hatch and join other plankton. Here they feed on microscopic algae. Later in the year they settle and become miniature adults.

Physiology  What You Can See Without Dissection

When a limpet is removed from a rock and turned over its soft parts are exposed. The most obvious organ is the large disc shaped foot. This is what you would expect from the limpet’s powerful grip, strong enough for its shell to gouge its rocky resting place. As well as gripping, the foot is used for movement, which it does by the foot muscles producing a series of contractions and relaxations. This produces small contraction waves which ripple from the anterior to the posterior enabling the limpet to move forward.

Surrounding the foot is the mantle. This fleshy skin completely lines the shell, and encompasses all the soft parts of the limpet not on show. Around the edge of the mantle are fine tentacles, pallial tentacles. The head is almost completely retracted into its body only a small portion of it can be seen.

When grazing, the head with its two tentacles can be seen protruding from the shell.

Identification of Limpet Species

There are three species of true limpet found on the shores around Cornwall, the Common Limpet Patella vulgata, the China Limpet Patella aspera and the Black-footed Limpet Patella depressa a southern species. The common limpet is the most abundant by far.  It can be difficult to tell the species apart by just looking at one shell, even for the expert. This is eased when there are a number of them to highlight the differences.

Limpet species are already hard enough to identify without the problem of varying shell forms within the same species. Limpet shells tend to be flatter on unsheltered beaches or those with strong currents. This shell shape provides less resistance to the water flow, plus the base is larger in relation to its height, so the gripping power is proportionally improved.

To distinguish the species, the animal itself has to be observed, unfortunately to do this the limpet has to be dislodged, which is likely to damage the limpet. The following table attempts to distinguish between the species. It must be remembered that the indicators are not absolutes but only a tendency. Some specimens will display many of their species distinguishing features. However with the wide variation within each species, others will not, making it more difficult unless an expert or under a microscope to make a definite identification.

Common Limpet
Patella vulgata 

China Limpet
Patella aspera 

Black-footed Limpet
Patella depressa 

 Foot Colour     

greyish green to pale yellow

orange or yellow, with no hint of green.

black to dark grey or dark grey

 Pallial Tentacles 

transparent

opaque

opaque

 Shell Shape

The largest limpet, reaching over three inches across. Shell can be steeply conical.

Similar shape to the common limpet, but it is not so steeply concave and  does not grow so large.

Its shell tends to be flatter than the other two species, as it prefers exposed locations.

 Inner Shell

Smooth and greenish grey. Rays run a short way into the shell.

Tends to be white going to yellow or orange near the apex.

The inner shell has darker rays running into the shell.

 Shore Zone

Upper to Mid Shore

Mid Shore

Lower Shore

Limpet in Name Only

There are other gastropods that have been labeled with a common name that includes the word limpet but are not true limpets. The species that come in this category and that are found in Cornwall are; Blue-Rayed Limpet Helcion pellucidum, Slipper Limpet Crepidula fornicata, Cup and Saucer Limpet Calyptraea chinensis and the keyhole limpet Diodora apertura.

     Keyhole Limpet Diodora apertura  

       Blue - Rayed Limpet Helcion pellucidum

     Slipper Limpet Crepidula fornicata

        Cup & Saucer Limpet Calyptraea chinensis

Blue-Rayed Limpet Helcion pellucidum

An attractive, common, small oval shell of under an inch. It is distinctive as it has bright blue rays. Once the creature has died the rays wear off.

It is best found at spring tides on stipes and holdfasts of seaweed of the kelp forest. It feeds on seaweed, gradually gouging out a hollow in the seaweed until its almost buried. This in conjunction with others of the species, may weaken the stipe to such a degree that the stipe breaks off during a storm.

Slipper Limpet Crepidula fornicata

Only introduced about 100 years ago from America and has spread like wildfire. This foreign invader has a destructive affect on the environment, being one of the main reasons for the demise of the native oyster. It does not prey on the oyster but just out competes it for food and living space.

An identifying feature is an internal partition projecting halfway across the shell. When alive they are found in chains with the bottom one the oldest and female, those on top male. When the female eventually dies, the male nearest the bottom changes to female.

Keyhole Limpet Diodora apertura

Very similar in form to the true limpets, but has ribbing radiating from the apex and cross ridging.

Most distinctive feature of this shell is the hole at the apex.

Cup and Saucer Limpet Calyptraea chinensis

This species is not found in any number outside Cornwall. Even on Cornish shores it is not a common occurrence. Live specimens are only to be found at the low water of spring tides. It is a small shellfish reaching half an inch across. The shell is conical with a protuberance at its peak. The shell has an internal partition. Cup and Saucer Limpets are at first male, as they grow larger change sex to female.