Patellogastropoda (True Limpets)

(True limpets)

Phylum: Mollusca

Class: Gastropoda

Order: Patellogastropoda

Number of families: 5

Thumbnail description
Cap-shaped snails typically found on wave-exposed rocky shores

Evolution and systematics

There are numerous limpet taxa in the Paleozoic fossil record, however none possesses unique characteristics that convincingly place these taxa within the order Patellogastropoda. The earliest patellogastropod verified by shell microstructure is from the Triassic of Italy, but it is in the Late Cretaceous and Tertiary periods that many of the living higher taxa have their first occurrences in the fossil record.

Phylogenetic analysis places the patellogastropods at the base of the gastropod tree as the sister taxon to all other gastropods. Although all living patellogastropods are limpets with cap-shaped shells, it is likely that like other living limpets they are descended from coiled snails. However, possible ancestors have not yet been identified in the fossil record.

The patellogastropods are divisible into two major suborders, the Patellina and Acmaeina. The Patellina includes the families Patellidae and Nacellidae. The Acmaeina includes the families Lepetidae, Acmaeiadae, and Lottiidae. Because of their simple shell morphology and anatomy, classifications of the patellogastropods have tended to underestimate their diversity. The application of molecular techniques is providing help in resolving the evolutionary history of this group, and systematic and nomenclatural revisions are certain.

Physical characteristics

All living patellogastropods have cap-shaped shells. The apex of the shell is typically situated at the center of the shell or slightly towards the anterior. All shells are sculpted with concentric growth lines; in many species, additional radial ribs extend from the apex to the shell margin. These ribs may be very fine like growth lines or broad and strongly raised off the surface. The shell aperture is typically oval. The inner surface of the shell bears a horse-shaped muscle scar that opens anteriorly where the head is located. The head has one pair of tentacles and the mouth opens ventrally for feeding on the substrate. Inside the mouth is the radula. The patellogastropod radula has very few robust "teeth," which are brown in color because of the presence of iron compounds. Patellogastropods have two gill configurations: in the Patellina, the gill is located around the edge of the foot and extends around the aperture, while in the Acmaeina, the gill is located over the head, as it is in other gastropod species.

Patellogastropods range in size from about 0.19–7.8 in (5–200 mm) in length. The smallest species and the largest are typically found in the lowest intertidal zone or subtidally. Most species in the intertidal zone average between 0.78–1.5 in (20–40 mm) in length. Subtidal species are typically white or pink in color and intertidal species are typically drab brown or gray with white spots and radial rays. Coloration in patellogastropods is closely associated with their diet, and often the shell is similar in color to the substrate on which the limpet occurs because of the incorporation of plant compounds into the shell.

Distribution

The highest diversity of patellid species in the world occurs along the rocky shores of southern Africa. Patella faunas also occur in Western Europe and Australia. The highest diversity of Acmaeina species occurs around the Pacific Rim, while the Nacellidae reach their highest diversity in the tropical and warm temperate Pacific Ocean. Often, patellogastropod faunas represent an aggregation of lineages accumulated through geological time and are subject to the vagaries of colonization, origination, radiation, and extinction. Australia, like southern Africa, is the most diverse, with representatives of patellid, nacellid, and lottiid lineages. In Chile, only nacellid and lottiid lineages are present; in Europe, there are only lottiid and patellid taxa; and only lottiid species are found in the western United States.

Habitat

Patellogastropods typically occur on intertidal rock substrates. In near-shore subtidal habitats, they are commonly associated with calcareous substrates. Numerous species utilize, as adults, specific types of host plants as substrates, including the Laminariales and Fucales of the Phaeophyta (brown algae), the Cryptonemiales (corallines) of the Rhodophyta (red algae), and marine grasses (Zosteraceae, Angiospermae). The biogeographic distribution of recent marine plant limpets is cosmopolitan; they occur in all major oceans, except the Arctic Ocean. In the deep sea, patellogastropods are found at both cold seep and hydrothermal vent sites.

Behavior

Most patellogastropods are dioecious, although both simultaneous and protandric hermaphroditism are present in the taxon. Protandry is often correlated with territorial species. Because most patellogastropods discharge their gametes directly into the sea where fertilization and development take place, there is no courtship or mating between individual limpets.

Territoriality has independently evolved at least three times in the Patellogastropoda; the most famous example being the territorial Patellidae of South Africa. Territoriality is typically associated with specific food reserves, and limpets become highly aggressive by using their shells as battering rams to drive both conspecifics and other herbivorous species from their territory. Territorial species are often larger than related non-territorial species, and many territorial species are also protandric hermaphrodites—beginning life as males before becoming females—often upon the acquisition of a feeding territory.

Activity patterns of patellogastropods are often complicated. At high tide, moving limpets are susceptible to aquatic predators such as fish and crabs. At low tide, species are especially vulnerable to shore birds and foraging mammals. Moreover, low tide also places intertidal patellogastropods under physiological stress due to the effects of drying. Many species are most active at night during low tide when visual predation is less effective. Migratory movements of patellogastropods are limited to a general up-shore pattern, with recruitment ocurring in the lower intertidal and later movement leading to life in higher intertidal zones.

Feeding ecology and diet

Patellogastropods are grazers, and feed by removing diatoms, algal spores, and small bits of plant material from the substrate. Very few species are able to feed directly on large algae, although there are several species that live on algae or marine angiosperms and feed directly on the host plant. Often the shape of the radular teeth reflects the limpet's dietary preference—equal-sized blunt teeth are often seen in species that feed on coralline algae, while species that graze on rock substrates have unequal-sized, pointed teeth, and species that feed on marine angiosperms have broad, flat-topped teeth. Some tropical patellogastropods have small "gardens" of specific algal species that are maintained directly adjacent to the limpets' home site on the substrate.

Predators of patellogastropods include predatory gastropods, sea stars, nemertean worms, fishes, lizards, small mammals, and shore birds. Shore birds such as oystercatchers are especially voracious predators and can remove limpets in such numbers that entire expanses of the intertidal can become green from algal growth due to the lack of limpet feeding.

Reproductive biology

The eggs are typically small, about 0.0035 in (90 μm) in diameter, and contain sufficient yolk reserves to support the developing limpet through settling and metamorphosis. Larger species produce millions of eggs per reproductive season and typically have yearly cycles. Smaller species produce far fewer eggs, but can be gravid and capable of spawning gametes year-round. Patellogastropod larvae pass through a trophophore stage and a veliger stage before settling and undergoing metamorphosis. Parental brood protection has evolved at least twice in the patellogastropods. In some species, the eggs are retained in the mantle cavity over the head, where they are fertilized, and develop into crawl-away young. Other taxa have internal brood chambers and copulatory structures. In some Antarctic species, spawning occurs when individuals stack one on top of another, thereby reducing the distance between individuals and increasing the probability of fertilization.

Conservation status

Most patellogastropods are relatively common and broadly distributed. One exception is a group of Patella species endemic to the Azores that have been subject in recent years to over harvesting. There are also other species groups that are restricted to the Hawaiian Islands, and a unique brackish-water species that is known only from estuaries in India and Burma and has not been seen alive in more than 100 years.

One species is listed as Extinct by the IUCN: Lottia alveus. The extinction of this species does not appear to have been related to human activities. However, current local declines of patellogastropod species (such as in the Azores and the Hawaiian Islands) are almost always associated with over harvesting by humans for food.

Significance to humans

Because of their large size and intertidal habitat, patellogastropod species have been important components of aboriginal diets for more than 150,000 years, and there is evidence that human predation has (and continues to) reduced both maximum and mean limpet size at some localities. Patellid species are also finding use in biological-monitoring studies of the health of rocky-shore communities because of their ubiquitous presence and role in rocky-shore systems. There appears to be little economic interest in most patellogastropod species, although Cellana species are a significant fishery in Hawaii and patellids have been over harvested at the Azores in recent years.

Some of the flatter limpet species are often used as shell jewelry because of their medallion-like shape and nacreous shell surfaces. The most famous depiction of a limpet in art is the English painter J. M. W. Turner's War: The Exile and the Rock Limpet (1842), which figures a specimen of Patella being contemplated by Napoleon Bonaparte during his exile on St. Helena in the Atlantic Ocean.

Species accounts

Resources

Branch, G. M. "Limpets: Evolution and Adaptation." In The Mollusca. Volume 10, Evolution, edited by E. R. Trueman and M. R. Clarke. Orlando, FL: Academic Press, 1985.

Branch, G. M. "The Biology of Limpets: Physical Factors, Energy Flow, and Ecological Interactions." Oceanography and Marine Biology Annual Review 19 (1981): 235–380.

Lindberg, D. R. "The Patellogastropoda." Prosobranch Phylogeny. Malacological Review, Supplement 4 (1988): 35–63.

[Article by: David Lindberg, PhD]