limpet

True limpets
Scientific classification
Kingdom: Animalia Phylum: Mollusca Class: Gastropoda (unranked): clade Patellogastropoda Lindberg, 1986
Superfamilies and families
See text

The name Limpet is used for many kinds of mostly saltwater but also 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.

Thus the word "limpet" is an inexact term, which is fairly often used as part of the common name of a wide variety of different marine and freshwater gastropod species, 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 which have such a shell are not at all closely related to one another.

The phrase "true limpets" is used only for marine limpets in the ancient clade Patellogastropoda. This article is primarily about the patellogastropods, the true limpets.

General taxonomy

In the latest taxonomy the true limpets, the Patellogastropoda, have become an unranked taxon as a separate clade. Families that are exclusively fossil are indicated with a dagger †:

• Superfamily Patelloidea
  • Family Patellidae
• Superfamily Nacelloidea
  • Family Nacellidae
• Superfamily Lottioidea
  • Family Lottiidae
  • Family Acmaeidae
  • Family Lepetidae
• Superfamily Neolepetopsoidea
  • Family Neolepetopsidae
  • † Family Daminilidae
  • † Family Lepetopsidae

As well as being applied to true limpets, the common name "limpet" is also used for many other widely different gastropods, including:

Marine limpets:

The fossil fissurellid or "keyhole limpet", Diodora italica, from the Pliocene of Cyprus.

• The keyhole limpets - Fissurellidae
• The hydrothermal vent limpets – Neomphaloidea and Lepetodriloidea
• The nerite limpets – Phenacolepadidae
• The slipper limpets or slipper shells - Calyptraeidae
• The hoof limpets - Hipponix

• The opisthobranch notaspidean limpets such as Tylodina and Umbraculum

• Two different groups of pulmonate false limpets - Siphonariidae and Trimusculidae

Freshwater limpets:

• The pulmonate river and lake limpets - Ancylidae and Acroloxidae

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 (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 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. 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 resemble the rock surface itself.

The majority of limpet species have shells that are less than 3 in (8 cm) in maximum length and many are much smaller than that.

Until quite recently a west Mexican limpet species, the giant limpet, Patella (Ancistromesus) mexicana, was known to grow as large as 8 in (20 cm). This species is now in serious danger of extinction, as it is slow to reach maturity, and has suffered from overcollecting, both as a food item, and by shell collectors and dealers.

Distribution

Representatives from the order Patellogastropoda, the true limpets, live on the rocky coasts of all oceans.

Habitat

Various different species live throughout the intertidal zone, from the high zone (upper littoral zone) to the shallow subtidal.

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.

Life habits

Feeding

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).

Homing behaviour

Limpet shell

Limpets in the intertidal of Cornwall, England.

Common limpets in Pembrokeshire, Wales.

Some species of limpets return to the same spot on the rock known as a "home scar" just before the tide recedes.^[1] 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 pheromones in the mucus left as they move. Other species, notably Lottia gigantea seem to "garden" a patch of algae around their home scar ^[2]. 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.

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.^{[citation needed]}

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.

Reproduction

Spawning occurs once a year, usually during winter, and is triggered by rough seas which disperse the eggs and sperm. Larvae float around for a couple of weeks before settling onto a hard substrate.^[1]

Human use

Larger limpet species are, or were historically, cooked and eaten in many different parts of the world. For example, in Hawaii, limpets (Cellana species) are commonly known as ‘opihi,^[3] and are considered a delicacy; the meat sells for $30-40 a pound (454g). In Portugal, limpets are known as lapas and are also considered to be a delicacy. Within Gaelic Scotland, a limpet is known as a 'báirnach', and Martin Martin recorded (on Jura) limpets being boiled to use in substitute of breast milk.

References

External links

• University of Hawaii Educational page from Christopher F. Bird, Dep't of Botany. Photos and detailed information distinguishing the different varieties.
• Lottia gigantea: taxonomy, facts, life cycle, bibliography
• Acmaeidae at the NCBI taxonomy website