Patella is a genus of herbivorous marine sea snails, commonly known as limpets, in the family Patellidae within the class Gastropoda. Primarily a marine group of prosobranch gastropods, they typically live on tidal shores in non-tropical climates. As limpets in the family Patellidae, they are a sister group to the other three accepted genera: Cymbula, Helcion, and Scutellastra. Members of the genus Patella are characterized by a cup-shaped shell and gills, and move by means of muscular waves along the foot.

Patella species are found in north-eastern Atlantic and Mediterranean tidal zones. Surveys conducted over the habitat distributions of the four generas show that Patella is generally the northernmost genus, the other three distributed along southeastern Africa towards Asia.

Taxonomy and morphology The Patella genus includes the species Patella vulgata, Patella aspera, and Patella caerulea, among others, and is a member of the family Patellidae (common name: common limpet).

Shells

Patella shells are single-valved shells with radial sculpture and are made of calcium carbonate. Their shells are typically depressed, conical, and non-coiled, protecting them from heavy wave action through their stable structural shape. As a limpet, Patella has a ventral muscular foot, providing adhesive force to substrates such as rocks through suction and the secretion of mucus. The foot is composed of dorso-ventral and transverse muscles. Longitudinal muscles are absent. Locomotion relies forward-motion retrograde waves, during which sections of the foot lift approximately 0.2 mm from the substrate. Patella typically moves in a forward direction via a ditaxic pedal wave. Backward motion is rare.

Radula

The radula of Patella consists of iron and silica. During radular development, the protein matrix of the bases and cusps, which are rich in tyrosine and tryptophan complexes with chitin, undergoes mineralization to incorporate iron and silica. The radula of common limpets has opaque teeth and can be detected by transmitted light. The median teeth are surrounded by laterals and uncini.

Gills As marine species, Patella breathe using gills. The morphology of their gills is akin to a "double comb" structure, hence the name ctenidia. It functions as the gas exchange organ which takes up oxygen and discards carbon dioxide. Patella possess secondary gills with the gill filaments located at the pallial groove, unlike the terrestrial snails which have a reduced primary gill and are unable to breathe air through gills.

Eyes and other sensory organs

The eyes of Patella form a simple cup lined with pigmented cells. These structures receive lateral rays of light to detect shifts between dark and light fields but cannot construct true images. This minimal sensory architecture matches that of certain clams, corresponding with their sedentary lifestyle on rocky substrates.

Patella also have balancing organs called statocysts. Statocysts consist of statoliths floating against the wall of statocysts and function as sensory organs. Hairs and cilia detect and transfer the statolith signal to pedal ganglia or the pleuro-pedal connectives which are controlled by cerebral ganglia.

Another chemo-sensory organ, the osphradium comes in pairs as large structures in the pallial cavity. The osphradium functions to detect water quality and mating opportunities. It also can prevent predators like sea stars and netted dog whelk from hunting them.

Nerve system The nervous system of Patella experiences torsion similar to pulmonate snails (land snails). After torsion, the pericarya of nerve cells concentrated at the anterior end form two pairs of cerebral ganglia connected by circumoesophageal connectives without myelin sheaths. Cerebral ganglia connect to structures such as: tentacles, eyes, statocysts, lips, penis, pharynx, salivary glands and stomach. No brain is present in Patella, though this isn't abnormal as the brains of molluscs are highly reduced except in cephalopods.

Habitat, niche and life cycle The preferred niche of Patella is coasts, embayments or estuaries with salinity between 30-40 psu, and their preferred substratum habitats are bedrock, cobbles and crevices. Their niche tends to withstand changing levels in suspended sediment, water flow rates, temperature, turbidity, and wave exposure.

Patella vulgata has been documented to live in salinities lower than 20 psu (practical salinity units) and with low seaweed abundance. In their habitat, they can bear strong tides, changing water levels, with preference to different levels of eulittoral zones. The climates of their habitats vary from seasonally variable sunshine to cold Arctic-like temperatures as the species' range expands.

Patella have a larval stage after spawning between October and January through external fertilization. The larvae are planktotrophic and green in coloration due to the pigment chromoprotein Y. These pelagic larvae can swim for 2–10 days and travel distances greater than 10 kilometers to find rocks to settle on and form shells.

Reproduction and development

Reproduction Both hermaphroditism and gonochorism are common in gastropods. The common limpet, Patella vulgata, is a protandrous hermaphrodite, while Patella depressa is gonochoric. Being protandrous, the limpets with larger sizes in P. vulgata are predominantly females, changing from male to female as they grow.

Extant gastropods have lost one of their gonads, and the one that they possess is situated next to the digestive gland. The gonoduct carries the gametes produced by their gonad out of the body. In patellogastropods and vetigastropods, the gonoduct is called the urogenital duct, as the gonoduct develops alongside the right nephridium in both clades. Thus, this duct functions to transport both gametes and urine. P. vulgata does not possess a penis or uterus, and engages in broadcast spawning.

Development P. vulgata undergoes spiral cleavage and during gastrulation yolk cells occupy the blastocoel. Planktonic trochophore larva, that later on develop into veliger larva, are also present within this species of patellid. P. vulgata also undergoes torsion between its trochophore and veliger larval stages before it takes on a sedentary lifestyle, settling onto a substrate to metamorphose into an adult limpet.

Diet Like the majority of limpets, Patella spp. are herbivores, and primarily eat thin films of algae on the substrates they attach to. Thus, their diet is mainly composed of algae and other cyanobacteria. They use their radula, an anatomical feature that is unique to mollusks, to rasp and grind up food. Although this feature is a synapomorphy of the Phylum Mollusca, it is absent in bivalves.

Patella spp. feeding behaviors are dependent on circatidal and circadian rhythms, often seeing higher feeding activity with the correct season and tidal cycle. P. caerulea shows significant activity during the night time and under submergence, following a 12 hour circatidal clock for its feeding behavior. Patella spp. also exhibit feeding manoeuvres, such as P. vulgata presenting looped movements around their attachment site, often relying on decision making to efficiently use its energy.

Environmental impact Patella spp. are herbivorous, which makes them particularly well suited for monitoring algal coverage. The environmental effects associated with limpet grazing on Fucale and green algae are numerous. The barnacle species, Semibalanus balanoides, benefits from the grazing behavior of Patella. S. balanoides competes with algae for substrate. Patella feeds on both fucoid and green algae, thus removing them from their substrate, allowing S. balanoides area for substrate attachment. Thus, Patella grazing regulates algal growth, while enhancing S. balanoides settlement, outlining Patella's role as a keystone species as well.

While grazing for algae and other cyanobacteria, their sharp radula scrapes away at limestone too. Therefore, Patella also contribute greatly to the erosion and breakdown of limestone coasts.

Conservation Members of the genus Patella are important components of rocky intertidal ecosystems, where they help maintain ecological balance through their grazing activity and interactions with other organisms. They are also subject to high levels of environmental stress, making them useful indicator organisms for assessing the health of coastal habitats. As a result, many tidal ecosystems supporting Patella populations are included within Marine Protected Areas (MPAs), both to reduce impacts from overharvesting and to support monitoring of coastal biodiversity.

The genus is particularly sensitive to marine pollution and has been widely used in marine biodiversity and environmental monitoring research. Species such as Patella caerulea are employed in bioindicator studies to assess ocean acidification and to evaluate the impacts of climate change on marine ecosystems.

Human activity and distribution of Mediterranean marine species can be the cause of the problems with the conservation of Patella. Additionally, the genetic variability detected by polymorphic microsatellite markers in Patella populations has only 0.41% variability between populations without isolation of the species. The difficulty of gene flow between species is due to both biotic and abiotic factors.

Conservation of Patella ferruginea has involved the establishment of MPAs to safeguard adult populations. These protected areas help maintain viable breeding populations and can enhance larval dispersal, thereby promoting an increase in gene flow among populations. Increased gene flow is considered beneficial for the long-term conservation of the species by improving genetic diversity and population resilience.

Another solution is to use artificial portable plates (APPs) on the rocks of Western Mediterranean Sea to retain more Patella, because moving to new environment can cause death of limpets and using APPs could reduce mortality and conserve the population by preventing detachment and translocation.

Species

Species within the genus Patella include: † Patella alessiae Forli, Dell'Angelo, Montagna & Taviani, 2004 † Patella alternicostata Sandberger, 1859 † Patella ambroggii Lecointre, 1952 †Patella amuritica Wilckens, 1922 Patella argentaurum Lesson, 1831 Patella aspera Röding, 1798WoRMS (2010). Patella aspera Röding, 1798. In: Bouchet, P.; Gofas, S.; Rosenberg, G. (2010) World Marine Mollusca database. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=456570 on 2010-08-03 Patella caerulea Linnaeus, 1758Gofas, S. (2009). Patella caerulea Linnaeus, 1758. In: Bouchet, P.; Gofas, S.; Rosenberg, G. World Marine Mollusca database. Accessed throu+gh the World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=140677 on 2010-08-03 Patella candei d'Orbigny, 1839Gofas, S. (2010). Patella candei d'Orbigny, 1839. In: Bouchet, P.; Gofas, S.; Rosenberg, G. (2010) World Marine Mollusca database. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=140678 on 2010-08-03 Patella crocata R.P. Lesson, 1831 Patella cyanea R.P. Lesson, 1831 Patella depressa Pennant, 1777 - Patella depressa is a nomen dubium according to Christiaens (1973, 1983); Patella intermedia gets priority. Nevertheless the name Patella depressa is in usage among British authors (e.g. Graham, 1988) and is assumed as valid.WoRMS (2010). Patella depressa Pennant, 1777. In: Bouchet, P.; Gofas, S.; Rosenberg, G. (2010) World Marine Mollusca database. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=151374 on 2010-08-03 Patella depsta Reeve, 1855:"Patella depsta Reeve, 1855" . OBIS, accessed 3 August 2010. (taxon inquirendum) † Patella estotiensis Lozouet, 1999