Limpet is a common name used for many kinds of saltwater or freshwater snails, specifically those that have a simple shell which is more or less broadly conical in shape, and which is either not coiled, or appears not to be coiled, in the adult snail.
The phrase "true limpets" is used only for marine limpets in the ancient order Patellogastropoda, subclass Eogastropoda, but as well as being applied to true limpets.
The common name "limpet" is also used for many widely different snails in the subclass Orthogastropoda including:
- The keyhole limpets - Fissurellidae
- The slipper limpets or slipper shells - Calyptraeidae
- The hoof limpets - Hipponix
- The opisthobranch notoaspidean limpets such as Tylodina and Umbraculum
- The pulmonate false limpets - Siphonaria
- The pulmonate river and lake limpets - Ancylidae
Most of the marine limpets have gills, whereas all the freshwater limpets and a few of the marine limpets have a mantle cavity which is adapted to breathe air and function as a lung; 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 which have independently evolved a shell of the same basic shape, see convergent evolution.
This article is specifically about "True limpets", which are marine gastropod mollusks in the order Patellogastropoda, for example, species such as Patella vulgata.
Introduction to true limpets
Limpets have flattened, cone-shaped shells, and the majority of species are commonly found adhering strongly to rocks or other hard substrates, looking like little bumps on the surface. In life, many limpet shells are often covered in microscopic growths of green marine algae, which can make them even harder to see, as they can closely ressemble the rock surface itself.
They attach themselves to the substrate using pedal mucus and a muscular "foot". They locomote using wave-like muscular contractions of the foot when conditions are suitable for them to graze. They can also "clamp down" against the rock surface with very considerable force when necessary, and this ability enables them to remain safely attached, despite the dangerous wave action on exposed rocky shores. The ability to clamp down also seals the shell edge against the rock surface, protecting them from desiccation during low tide, despite their being in full sunlight.
When true limpets are fully clamped down, it is impossible to remove them from the rock using brute force alone, and the limpet will allow itself to be destroyed rather than stop clinging to its rock. This survival strategy has led to the limpet being used as a metaphor for obstinacy or stubbornness.
Most limpets feed by grazing on algae which grows on the rock (or other surfaces) where they live. They scrape up films of algae with a radula, a ribbon-like tongue with rows of teeth. Limpets move by rippling the muscles of their foot in a wave-like motion.
In some parts of the world, certain smaller species of true limpet are specialized to live on seagrasses and graze on the microscopic algae which grow there. Other species live on, and graze directly on, the stipes (stalks) of brown algae (kelp).
Some species of limpets return to the same spot on the rock known as a "home scar" just before the tide recedes . In such species, the shape of their shell often grows to precisely match the contours of the rock surrounding the scar. This behaviour presumably allows them to form a better seal to the rock and may help protect from either predation or desiccation.
It is still unclear how limpets find their way back to the same spot each time, but it is thought that they follow a mucus trail left as they move, this trail contains pheromones. Other species, notably Lottia gigantea seem to "garden" a patch of algae around their home scar . They are one of the few invertebrates to exhibit territoriality and will aggressively push other organisms out of this patch by ramming with their shell, thereby allowing their patch of algae to grow for their own grazing. Also, where the limpets eat the algae off bare rocks, it creates places where other organisms can grow and thrive.
Predators and other risks
Limpets are preyed upon by a variety of organisms including starfish, shore-birds, fish, seals, and humans. Limpets exhibit a variety of defenses, such as fleeing or clamping their shells against the substratum. The defense response can be determined by the type of predator, which can often be detected chemically by the limpet.
Limpets can be long lived, with tagged specimens surviving for more than 10 years. If the limpet lives on bare rock, it grows at a slower rate but can live for up to 20 years.
Limpets found on exposed shores, which have fewer rock pools than sheltered shores and are thus in less frequent contact with water, have a greater risk of desiccation due to the effects of increased sunlight, water evaporation and the increased wind speed. To avoid drying out they will clamp to the rock they inhabit, minimizing water-loss from the rim around their base. As this occurs chemicals are released that promote the vertical growth of the limpet's shell.
Spawning occurs once a year, usually during winter, and is triggered by rough seas which disperse the eggs and sperm. Larvae are pelagic for a couple of weeks before settling onto a hard substrate. Because of this breeding in the home aquarium is unlikely.