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32-412: Ascoglossa Bergh, 1876 Sacoglossa are a superorder of small sea slugs and sea snails , marine gastropod mollusks that belong to the clade Heterobranchia known as sacoglossans . There are 284 valid species recognized within this superorder. Sacoglossans live by ingesting the cellular contents of algae , hence they are sometimes called "sap-sucking sea slugs". Some sacoglossans simply digest

64-456: A cohors (plural cohortes ). Some of the plant families still retain the names of Linnaean "natural orders" or even the names of pre-Linnaean natural groups recognized by Linnaeus as orders in his natural classification (e.g. Palmae or Labiatae ). Such names are known as descriptive family names. In the field of zoology , the Linnaean orders were used more consistently. That is,

96-566: A distinct rank of biological classification having its own distinctive name (and not just called a higher genus ( genus summum )) was first introduced by the German botanist Augustus Quirinus Rivinus in his classification of plants that appeared in a series of treatises in the 1690s. Carl Linnaeus was the first to apply it consistently to the division of all three kingdoms of nature (then minerals , plants , and animals ) in his Systema Naturae (1735, 1st. Ed.). For plants, Linnaeus' orders in

128-644: A more diverse feeding range than the Oxynoacea, feeding on a wider range of green (and sometimes red) algae, and even, in three cases, being carnivorous. The ancestor of the Sacoglossa is presumed to have fed on a now-extinct calcifying green alga in the Udoteaceae . The first fossil evidence of the group comes from bivalved shells dating to the Eocene , and further bivalved shells are known from later geological periods, although

160-490: A process termed nutritional homochromy. Around 20% of sacoglossan species bear a shell. The Oxynoacea contains three shelled families, and all feed solely on algae of the genus Caulerpa . None of these organisms benefits from the photosynthesis of the ingested chloroplasts, but the chloroplasts may have been retained to perform a camouflaging function. The shells of the Volvatellidae and Oxynoidae somewhat resemble those of

192-469: Is that the slugs autotomized to escape predators, but when the researchers tried to mimic an enemy's attack by pinching and cutting the creatures, none cast off their bodies. The process itself takes several hours, which the scientists say would make it ineffective as means of escape. How the slugs survive without a heart and other vital organs for nearly a month remains a mystery. Mitoh and her colleagues suspect it may be tied to their ability to survive using

224-403: Is triggered by the shortage of food resources, and typically not preferred. If food is readily available, the animal will actively consume it. Starvation periods (with photosynthesis and no active feeding) vary between species of sacoglossans from less than a week to over four months, and photosynthesis is used as a last-resort mechanism to avoid mortality. Another unclear step in the process is how

256-529: Is usually written with a capital letter. For some groups of organisms, their orders may follow consistent naming schemes . Orders of plants , fungi , and algae use the suffix -ales (e.g. Dictyotales ). Orders of birds and fishes use the Latin suffix -iformes meaning 'having the form of' (e.g. Passeriformes ), but orders of mammals and invertebrates are not so consistent (e.g. Artiodactyla , Actiniaria , Primates ). For some clades covered by

288-520: The International Code of Zoological Nomenclature , several additional classifications are sometimes used, although not all of these are officially recognized. In their 1997 classification of mammals , McKenna and Bell used two extra levels between superorder and order: grandorder and mirorder . Michael Novacek (1986) inserted them at the same position. Michael Benton (2005) inserted them between superorder and magnorder instead. This position

320-769: The Systema Naturae and the Species Plantarum were strictly artificial, introduced to subdivide the artificial classes into more comprehensible smaller groups. When the word ordo was first consistently used for natural units of plants, in 19th-century works such as the Prodromus Systematis Naturalis Regni Vegetabilis of Augustin Pyramus de Candolle and the Genera Plantarum of Bentham & Hooker, it indicated taxa that are now given

352-407: The cephalaspid bubble snails. The Juliidae are extraordinary in that they are shelled, bivalved gastropods. They have a shell in two pieces, which resemble the valves of a minute clam. Living members of this family have been known since 1959, and had previously only been known to science as fossils (which had been interpreted as bivalves). The majority of sacoglossans are shell-less, consequently,

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384-468: The Caribbean and Indo-Pacific . These three areas have distinct ranges of species, indicating a high degree of biogeographic separation. Where sacoglossans are present further from the equator, in places such as Australia or Japan, diversity is lower, and the species present are typically tropical species that have a higher tolerance for temperature variation. Their temperate distribution closely corresponds to

416-452: The Greek words σάκος sákos "shield" and γλώσσα glóssa "tongue" because the species have single toothed radulas . This taxonomy follows Marin 2004. In the taxonomy of Bouchet & Rocroi (2005), the clade Sacoglossa is arranged as follows: In this taxonomy, the family Elysiidae Forbes & Hanley, 1851 is considered a synonym of the family Placobranchidae Gray, 1840 , and

448-584: The Plakobranchoidea are commonly described using the vernacular term "sea slugs", which can lead to their confusion with the only very distantly related nudibranchs . However, the plakobranchoid Elysia (and undoubtedly others) do develop a shell before hatching from their egg. Indeed, at least the Elysiidae, Limapontiidae, and Hermaeidae all bear larval shells, which are spiral, and possess between three-quarters and one complete whorl. The plakobranchoids have

480-519: The Sacoglossa. Bouchet et al. (2017) moved Sacoglossa from Panpulmonata to the subterclass Tectipleura . Extreme autotomy has been observed on two species, Elysia marginata and E. atroviridis , studied in vitro . Over the course of the study, some individuals decapitated themselves, a behavior known as autotomy. The neck wound usually closed within one day, and the heads, especially in younger specimens, began to feed on algae within hours. Twenty days later, an entirely new body had regrown, while

512-488: The animal can only feed for part of the year, relying on the ingested chloroplasts to survive whilst the foodstuff is calcified, until later in the season when the calcification is lost and the grazing can continue. Sacoglossans can also use antiherbivory compounds produced by their algal foodstuffs to deter their own would-be predators, in a process termed kleptochemistry. This may be achieved by converting algal metabolites to toxins, or by using algal pigments for camouflage in

544-401: The chloroplasts are protected from digestion, and how they adapt to their new position in animal cells without the membranes that would control their environment in the algae. However it is achieved, kleptoplasty is an important strategy for many genera of Placobranchacea . One species of Elysia feeds on a seasonally calcifying alga. Because it is unable to penetrate the calcified cell walls,

576-429: The discarded bodies never regrew heads. In E. atroviridis , three of 82 studied individuals autotomized, and two of the three eventually grew new bodies. All of these animals were infected with small crustaceans known as copepods . In another group of 64 without parasites, none self-decapitated, leading the researchers to hypothesize that animals cast off their bodies as a means to get rid of parasites. Another possibility

608-419: The distribution of their important food source, Caulerpa spp. They typically live at very low population densities, making scientific study of the group difficult. The sacoglossans can use the chloroplasts of the algae on which they feed, which they keep alive for hours to months after their ingestion. They maintain the cells and metabolise the photosynthetic products; this process is termed kleptoplasty, and

640-408: The dorsal surface. The majority of sacoglossans are 1–3 cm in length; they are typically uniform in color because the chloroplasts they ingest end up installed in their own cells. Sacoglossa species are found worldwide in tropical and temperate oceans, but most live in the central Pacific Ocean , where they frequent the shorelines of tropical islands; diverse tracts of species are also known in

672-494: The families Oleidae O'Donoghue, 1926 and Stiligeridae Iredale & O'Donoghue, 1923 are synonyms of the family Limapontiidae Gray, 1847 . The family Cylindrobullidae belongs to the superfamily Cylindrobulloidea in the sister "group" Cylindrobullida. Jörger et al. (2010) moved Sacoglossa into the Panpulmonata . A molecular phylogeny analysis by Maeda et al. (2010) confirmed the placement of Cylindrobulla within

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704-503: The fluid which they suck from the algae, but in some other species, the slugs sequester and use within their own tissues living chloroplasts from the algae they eat, a very unusual phenomenon known as kleptoplasty , for the "stolen" plastids . This earns them the title of the "solar-powered sea slugs", and makes them unique among metazoan organisms, for otherwise kleptoplasty is known only among other euthyneurans and single-celled protists . The Sacoglossa are divided into two clades -

736-597: The genes have been laterally transferred from algae to the animals. DNA amplification experiments on Elysia chlorotica adults and eggs using Vaucheria litorea derived primers revealed the presence of psbO, an algal nuclear gene. These results were likely an artefact, as most recent results based on transcriptomic analysis and sequencing of genomic DNA from the slug's eggs reject the hypothesis that lateral gene transfer supports kleptoplast longevity. Sacoglossans are able to choose which method of feeding they use. The switch from active feeding to photosynthesis in sacoglossans

768-698: The orders in the zoology part of the Systema Naturae refer to natural groups. Some of his ordinal names are still in use, e.g. Lepidoptera (moths and butterflies) and Diptera (flies, mosquitoes, midges, and gnats). In virology , the International Committee on Taxonomy of Viruses 's virus classification includes fifteen taxomomic ranks to be applied for viruses , viroids and satellite nucleic acids : realm , subrealm , kingdom , subkingdom, phylum , subphylum , class, subclass, order, suborder, family, subfamily , genus, subgenus , and species. There are currently fourteen viral orders, each ending in

800-545: The photosynthetic algae in their diet while other energy sources are unavailable. Superorder What does and does not belong to each order is determined by a taxonomist , as is whether a particular order should be recognized at all. Often there is no exact agreement, with different taxonomists each taking a different position. There are no hard rules that a taxonomist needs to follow in describing or recognizing an order. Some taxa are accepted almost universally, while others are recognized only rarely. The name of an order

832-547: The precursor of the currently used International Code of Nomenclature for algae, fungi, and plants . In the first international Rules of botanical nomenclature from the International Botanical Congress of 1905, the word family ( familia ) was assigned to the rank indicated by the French famille , while order ( ordo ) was reserved for a higher rank, for what in the 19th century had often been named

864-444: The presence of a single row of teeth on the radula . The teeth are adapted for the suctorial feeding habits of the group. Many of these gastropods (e.g. Elysia spp.) resemble winged slugs with a pair of cephalic tentacles. In photosynthetic members of the group, the wings, or parapodia , can be unfurled to maximise the area of the organism that is struck by sunlight. In others (e.g. Placida spp.), cylindrical cerata extend from

896-494: The rank of family (see ordo naturalis , ' natural order '). In French botanical publications, from Michel Adanson 's Familles naturelles des plantes (1763) and until the end of the 19th century, the word famille (plural: familles ) was used as a French equivalent for this Latin ordo . This equivalence was explicitly stated in the Alphonse Pyramus de Candolle 's Lois de la nomenclature botanique (1868),

928-416: The sacoglossans are the only animals to employ it; some ciliates and foraminifera (protists) also employ the strategy. Sacoglossans have been known to survive for months living solely on the photosynthetic products of their acquired plastids. This process is somewhat mystifying, as the upkeep of chloroplasts usually requires interaction with genes encoded in the plant cell nucleus. This seems to suggest that

960-541: The shelled families (Oxynoacea) and the shell-less families (Plakobranchacea). The four families of shelled species are Cylindrobullidae , Volvatellidae , Oxynoidae , and Juliidae , the bivalved gastropods. The shell-less Plakobranchacea are grouped in six families, divided between two clades ("superfamilies"), the Plakobranchoidea and the Limapontioidea . All sacoglossans are distinguished from related groups by

992-606: The thin nature of the shells and their high-erosion habitat usually make for poor preservation. The corresponding fossil record of algae points to an origin of the group deeper in time, perhaps as early as the Jurassic or Cretaceous. The loss of the shell, which was apparently a single evolutionary event, opened up a new ecological avenue for the clade, as the chloroplasts of the green algae on which they fed could now be retained and used as functioning chloroplasts, which could generate energy by photosynthesis . The suborder name comes from

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1024-418: Was adopted by Systema Naturae 2000 and others. In botany , the ranks of subclass and suborder are secondary ranks pre-defined as respectively above and below the rank of order. Any number of further ranks can be used as long as they are clearly defined. The superorder rank is commonly used, with the ending -anae that was initiated by Armen Takhtajan 's publications from 1966 onwards. The order as

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