Patella are a herbivorous genus of sea snails limpets in the family Patellidae, further placed under the class Gastropoda. Primarily a marine based genus of prosobranch gastropods they most commonly live on coastal tidal shores in varying non tropical climates.^[2] As limpets under family Patellidae, they share sister taxa with 3 other accepted genus Cymbula, Helcion, and Scutellastra.^[2] Patella is characterized by their cup-shaped shell and gills, with their form of movement in waves.^[3][4]

The genus of Patella are geographically found around north eastern Atlantic and Mediterranean tidal beds. Surveys conducted over the 4 genus' habitat distribution show that Patella are generally the northern most genus with the other 3 distributing along south eastern Africa towards Asia.^[5][6] They tend to be subjected to high volumes of environmental stresses and therefore act as key indicator species for the tidal coast health in those regions. As such conservation efforts along coastal regions have marked many tidal ecosystems as Marine Protected Areas (MAPs), both in efforts of conserving Patella populations from overharvesting and monitoring coastal biodiversity.^[6]

Patella are important to keeping ecological balance of its habitats, through managing interactions with other organisms within its environment.^[7] The Patella genus is prone to marine pollution and is often used as a key genus upon research for marine biodiversity.^[7] Species such as Patella caerulea are used in bioindicator studies to understand ocean acidification and effects of climate change on oceans.^[7]

Patella is the genus name of the species Patella vulgata, Patella aspera, and Patella caerulea etc, and it belongs to family Patellidae^[8] (common name: common limpet).

Patella shells are single-valved shells with radial sculpture ^[9] and are made of calcium.^[10] Their shells are usually reduced conical and non-coiled and can protect them from floods with their stable structure.^[11] As a Limpet, Patella has a ventral muscular foot, providing adhesion force to substrates, like rocks, by secreting mucus and using a sucking force.^[4] Their foot is made of dorso-ventral and transverse muscles with no longitudinal muscle. Their movement uses retrograde locomotory waves to move forward that foot lifts from the substrate for 0.2 mm.^[4] Patella usually moves forward by a ditaxic pedal wave, but rarely moves backwards.^[4]

The radula of Patella consists of iron and silica. The formation of Patella is that, in bases and cusps, proteins tyrosine and tryptophane and chitin becomes amino groups and argentaffin, then they become iron and silica.^[12] The radula of common limpets has opaque teeth with median teeth surrounded by laterals and uncini/marginals, and it can be detected by transmitted lighting.^[13]

As a marine species genus group, Patella breath by using gills. The morphology of their gills are akin to a "double comb" structure, hence the name ctenidia, it functions as the gas exchange organ which uptakes oxygen and discards carbon dioxide.^[14] Patella grow 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 breath air through gills.^[14]

The eyes of Patella are a simple-cup shape lining with pigmented cells that can detect only dark and bright fields by receiving lateral rays of light connecting with nerve fibres to nerve system but cannot form images. This is the same as some clams due to their sedentary nature and preference of staying on the rock.^[14]

Patella also have balancing organs called: statocysts. Statocysts consists 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 ganglion.

Another chemo-sensory organ osphradium come in pairs as large structures in the pallial cavity.^[14] Osphradium function to detect water quality and mating opportunities. It also can prevent predators like sea stars and netted dog whelk from hunting them.^[14]

The nervous system of marine prosobranch gastropods Patella experience torsion similarly to pulmonate snails (land snails).^[14] After torsion, the pericarya of nerve cells concentrated at the anterior end forms 2 pairs of cerebral ganglia connected by circumoesophageal connectives without myelin sheath.^[14] Cerebral ganglia connects to structures such as : tentacles, eyes statocysts, lips, penis, pharynx, salivary glands and stomach.^[14] No brain presents in Patella due to the brains of molluscs being highly reduced except for in cephalopods.^[15]

Patella are mainly distributed on the rocky shores at costal regions exposed to waves to in varying degrees. Patella vulgata have been documented to living in salinities lower than 20 psu and with low seaweed abundance.^[2] 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 variant sunshine to cold Arctic like temperatures as the species expands.^[15]

The preferred niche of Patella are coasts,^[2] embayment or estuaries with salinity between 30-40 psu, and their preferred substratum habitats are Bedrock, Cobbles and Crevices.^[15] Their niche tend to bear changing levels in suspended sediment, water flow rates, temperature, turbidity, and wave exposure.^[15]

Patella have a larva stage after spawning between October and January through external fertilization. The larva are planktotrophic and green in coloration due to the pigment chromoprotein Y.^[15] These larvae can swim as the pelagic life forms for 2–10 days and travel greater than 10 km to find rocks to settle on and form shells.^[15]

Both hermaphroditism and gonochorism are common in gastropods. The common limpet, Patella vulgata, is a protandrous hermaphrodite,^[16] while Patella depressa is gonorchic.^[17] Being protandrous, the limpets with larger sizes in P. vulgata are dominated by females, changing from male to female as they grow.^[18]

Extant gastropods have lost one of their gonads, with the one that they do possess being situated next to the digestive gland.^[19] The gonoduct carries the gametes produced by their one 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.^[19] Thus, this duct functions to transport both gametes and urine.^[19] P. vulgata does not possess a penis or uterus,^[20] thus it engages in broadcast spawning.^[17]

P. vulgata undergoes spiral cleavage and during gastrulation yolk cells occupy the blastocoel.^[21] Planktonic trochophore larva that later on develops into a veliger larva is also present within this species of Patellid.^[21] 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.^[21]

Like the majority of limpets, Patella spp. are herbivores, meaning that they primarily eat thin films of algae on the substrates they attach to. Thus, their diet is mainly composed of algae and other cyanobacteria.^[22] They use their radula, an anatomical feature that is unique to Mollusks, to rasp and grind up food.^[19] Although this feature is a synapomorphy of the Phylum Mollusca, it is one trait absent in bivalves.^[19]

Patella spp. feeding behaviors is dependent on circatidal and circadian rhythms, often seeing higher feeding activities with the correct season and tidal cycle.^[23] P. caerulea shows significant activity during the night time and under submergence, following a 12 hour circatidal clock for its feeding behavior.^[24] 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.^[25]

Mollusks are traditionally used as marine bioindicator species, specifically bivalves such as Mytilus spp.^[5] Assessing the bioaccumulation within a chosen indicator species is the method that is most frequently utilized when assessing an environment's overall health, especially when Mytilus spp. are used as the bioindicator.^[5] However, there are other commonly used approaches to bioindication, such as observing the way in which an organism responds to changes in the environment ^[26] and how the environmental structure is modified as a result of the introduction of a toxic substance.^[26][27] Because of the many approaches to biomonitoring and the different trophic levels that can be observed, the demand for more species used in official monitoring programs has increased.^[5] There are certain characteristics that an organism must possess in order to qualify as an effective bioindicator species. These features include having a wide body of research and knowledge pertaining to the organism, being largely abundant, easy to collect, and mainly sedentary.^[26] Patella spp. possess many of these characteristics that make them a suitable alternative bioindicator species.

Patella spp. are herbivorous, which makes them particularly well suited for monitoring algal coverage.^[28] 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.^[29] Patella feeds on both fucoid and green algae, thus removing them from their substrate, allowing S. balanoides area for substrate attachment.^[29] Thus, Patella grazing regulates algal growth, while enhancing S. balanoides settlement, outlining Patella's role as a keystone species as well.^[22]

While grazing for algae and other cyanobacteria, their sharp radula scrapes away at limestone too.^[30] Therefore, Patella also contribute greatly to the erosion and breakdown of limestone coasts.^[31]

Human activity and distribution of Mediterranean marine species^[32] can be the cause of the problem of the conservation of Patella.^[33] The problem of Patella population is that the genetics variability detected by polymorphic microsatellite markers, which has only 0.41% variability between populations without isolation of the species.^[33] The difficulty of exchanging and communicating between species is due to both biotic and abiotic factors, in which the stochastic factor is messy.

The solution of conserving Patella ferruginea is that establishes marine protected area (MAPs) to protect the adult of Patella, which promote larva to travel further and increase the gene glow.^[33] Increasing gene flow of Patella would promote the conservation of the species.^[34]

Another solution is to use using artificial portable plates (APPs) on the rocks of Western Mediterranean Sea to retain more Patella^[35] because moving to new environment will cause death of limpets and prevent detachment and translocation by using APPs could reduce mortality and conserve the population.^[36]

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 (status in recent literature not researched by editor)
• Patella aspera Röding, 1798^[37]
• Patella caerulea Linnaeus, 1758^[38]
• Patella candei d'Orbigny, 1839^[39]
• 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.^[40]
• Patella depsta Reeve, 1855:^[41] (taxon inquirendum)
• † Patella estotiensis Lozouet, 1999
• Patella ferruginea Gmelin, 1791^[42]
• Patella gomesii Drouet, 1858^[43]
• † Patella hebertiana (d'Orbigny, 1850)
• Patella lugubris Gmelin, 1791^[44]
• † Patella mahamensis Martín-González, 2018
• † Patella matheyi De Loriol in De Loriol & Koby, 1890
• † Patella maxoratensis Martín-González & Vera-Peláez, 2018
• † Patella nelsonensis Trechmann, 1918
• Patella ordinaria Mabille, 1888
• Patella pellucida Linnaeus, 1758^[45]
• Patella piperata Gould, 1846^[46]
• † Patella protea Doderlein, 1862
• Patella rangiana Rochebrune, 1882^[47]
• Patella rustica Linnaeus, 1758^[48]
• Patella skelettensis Massier, 2009^[49]
• † Patella staceata Gründel, Hostettler & Menkveld-Gfeller, 2020
• Patella swakopmundensis Massier, 2009^[50]
• Patella tenuis Gmelin, 1791
• † Patella tintina Martín-González & Vera-Peláez, 2018
• Patella ulyssiponensis Gmelin, 1791 - synonyms: Patella aspera Lamarck, 1819; Patella athletica Bean, 1844^[51]
• Patella variabilisKrauss, 1848
• Patella vulgata Linnaeus, 1758^[52]

• Limpet

This article incorporates CC-BY-3.0 text from the reference.^[40]

• Linnaeus, C. (1758). Systema Naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Editio decima, reformata [10th revised edition], vol. 1: 824 pp. Laurentius Salvius: Holmiae
• Sowerby, G. B., II (1839). A Conchological Manual. G. B. Sowerby, London, v, 130 pp., 24 pls
• Pallary, P. (1912). Catalogue des mollusques du littoral méditerranéen de l'Egypte. Mémoires de l'Institut d'Egypte. 7(3): 69-207, pls 15-18.
• Monterosato, T. A. di. (1884). Nomenclatura generica e specifica di alcune conchiglie mediterranee. Virzi, printed for the Author, Palermo, 152 pp.
• Christiaens J. (1973). Révision du genre Patella. Bulletin du Muséum National d'Histoire Naturelle de Paris (3)Zoologie 121: 1305-1392
• Powell, A. W. B. (1973). The patellid limpets of the world (Patellidae). Indo-Pacific Mollusca. 3(15): 75-206