A major group of marine snails, commonly called the true limpets. Patellogastropoda has long been recognized as one of the most primitive groups of living gastropods (class Gastropoda) and was once considered part of the Archaeogastropoda. Patellogastropoda was designated as an order by D. R. Lindberg in 1986; later, it was included in the subclass Eogastropoda. Although other investigators, including P. Bouchet and J.-P. Rocroi, treated it as a clade, Patellogastropoda was recognized in 2015 as a subclass in the classification by S. Gofas. In addition, Docoglossa is an older name that was often used in lieu of Patellogastropoda. See also: Gastropoda; Mollusca; Prosobranchia
Phylogeny and families
The monophyly (descent from a common ancestral form) of the Patellogastropoda is supported by many morphological characters (for example, secondarily uncoiled shells, a divided shell muscle, the presence of pallial gills, a ventrally positioned gonad, and labial ganglia) and is readily distinguishable from the other members of the Gastropoda. Although Patellogastropoda has been recognized as the most basal group of living gastropods based on morphological analyses, the phylogenetic position of the group has been variable as the result of molecular phylogenetic analyses. The higher classification and higher relationships of the Patellogastropoda among Gastropoda are still debated. See also: Invertebrate phylogeny; Phylogeny
By observations of simple shell morphology, early investigators recognized only a few families (Acmaeidae, Patellidae, and Lepetidae) within the Patellogastropoda. More recently, familial classifications within the Patellogastropoda have been extensively revised based on three mitochondrial genes [12S rRNA, 16S rRNA, and COI (cytochrome c oxidase I)]. As a result, eight families (Eoacmaeidae, Acmaeidae, Patellidae, Nacellidae, Lottiidae, Lepetidae, Pectinodontidae, and Neolepetopsidae) are currently assigned to the subclass Patellogastropoda. Approximately 40 genera and 300 species are recognized in this group (examples are shown in Figs. 1 and 2).
Patellogastropod limpets are familiar inhabitants of the intertidal rocky shores of all oceans. They also have adapted to live on algae, coralline algae, sunken wood, whale bones, the shells of other mollusks, vents, and seeps. They can be found in all marine environments. The highest diversity can be found in the temperate regions of the northwestern Pacific, northeastern Pacific, South Africa, and Australasia. See also: Marine ecology
Characteristics and behavior
Morphologically, patellogastropods have a simple conical shell with no coiling or operculum (an aperture-closing bony plate). However, shell morphology is variable, corresponding to the substrate. Recent molecular work has revealed that intraspecific variation in shell morphology and color are common. A most striking example can be found in the New Zealand lottiid species, Notoacmea scapha. Although N. scapha living on the leaves of seagrass (Zostera capricorni) has a small, narrow, straight-edged shell, it has a larger, typical conical ovate form on other hard substrates. Genetic analysis confirms that these forms are conspecific (belonging to a single species). Similar work also determined that N. turbatrix exhibits a high variety of shell morphology and color pattern. Limpets attaching to rock surfaces or shells of other gastropods have small and narrow shells with light color, whereas those inhabiting tide pools have large depressed shells with white or black color. Such intraspecific color variation has been reported in several species of true limpets, and it is thought to be caused by differences in food resources, responses to visual selection by predators, and desiccation stress. See also: Protective coloration
Limpets normally graze microalgae from rock surfaces or other hard substrata using their radula (Fig. 3). The radula teeth consist of chitin and are reinforced by goethite. Recent studies have suggested that the limpet radula is the strongest biological material. Limpets inhabiting sunken wood have sawlike teeth. See also: Feeding mechanisms (invertebrate)
When limpets are splashed or covered with water, they start to move around to graze the microalgae. When the tide is out, some patellogastropods exhibit a form of homing behavior, returning to the same place on the shore known as the home scar (an indentation in the rock). The scar tightly fits their shell shape, and the limpets are not active during this time in order to avoid predation and desiccation.
Patellogastropod limpets are preyed on by fish, starfish (sea stars), birds, and other gastropods. If the limpets are touched by a fish or bird predator, they clamp tightly against their substrate, making them very difficult to remove. By contrast, limpets exhibit a run, twisting, or mantle response (covering the shell using their mantle) when a snail predator attempts to use biochemicals to bore a hole through their shell. Some limpets also exhibit mushrooming behavior in low tide. They lift the shell to expose the foot for cooling to protect themselves from desiccation stress. See also: Predator–prey interactions
Patellogastopod limpets normally have a reproduction period of a few months each year. They release their gametes directly into the sea. Such broadcast spawning is a common reproductive mode among the gastropods. One exception is the genus Erginus, whose members are ovoviviparous (producing eggs that develop internally and hatch before or soon after extrusion). After the eggs hatch, trochophores are formed, which can swim actively. The later period of their larval stage, the veliger, settles on suitable substrates. Although limpets have the potential to extend their distribution during the larval phase, most dispersal covers short distances because the larval period is usually just 7–10 days. Recent molecular studies suggest that patellogastropod limpets disperse no more than 100–200 km (62–124 mi) during their larval period. Nevertheless, rafting of settled juveniles and adults, especially those living on buoyant macroalgae, is a plausible means for long-distance dispersal. See also: Invertebrate embryology; Population dispersal