37-400: Sea angels ( clade Gymnosomata ) are a large group of small free-swimming sea slugs , classified into six separate families. They are pelagic opisthobranchs in the clade Gymnosomata within the larger mollusc clade Heterobranchia . Sea angels were previously considered to be pteropods . Sea angels are also sometimes known as "sea butterflies" but this is potentially misleading because
74-479: A "ladder", with supposedly more "advanced" organisms at the top. Taxonomists have increasingly worked to make the taxonomic system reflect evolution. When it comes to naming , this principle is not always compatible with the traditional rank-based nomenclature (in which only taxa associated with a rank can be named) because not enough ranks exist to name a long series of nested clades. For these and other reasons, phylogenetic nomenclature has been developed; it
111-623: A clade can be described based on two different reference points, crown age and stem age. The crown age of a clade refers to the age of the most recent common ancestor of all of the species in the clade. The stem age of a clade refers to the time that the ancestral lineage of the clade diverged from its sister clade. A clade's stem age is either the same as or older than its crown age. Ages of clades cannot be directly observed. They are inferred, either from stratigraphy of fossils , or from molecular clock estimates. Viruses , and particularly RNA viruses form clades. These are useful in tracking
148-474: A constant depth. To keep itself upright during swimming, it uses two statocyst gravity-sensing organs that correct it to an upright posture using its tail. Clione limacina inhabits both the epipelagic and mesopelagic regions of the water column. Adults feed in a predator-prey relationship almost exclusively on the sea butterflies of the genus Limacina : on Limacina helicina and on Limacina retroversa . The feeding process of Clione limacina
185-460: A depth of 200 m in the Sea of Okhotsk. In winter, they migrate to the coast of north Hokkaido with drift ice. In this clade , the foot of the gastropod has developed into wing-like flapping appendages (parapodia) and larval sea angels discard their embryonic shells a few days after hatching. Both adaptations suit their free-swimming oceanic lives. The adaptations also explain the common name sea angel and
222-441: A gelatinous egg mass, and the eggs float freely until hatching. Their embryonic shells are lost within the first few days after hatching. The gymnosomes, like other shell-less opisthobranchs, discard their shells at metamorphosis, with the retractor muscles being severed and the shell lost. The group does not truly, therefore, lack a shell. Few larval shells have been described (and consequently an understanding of their fossil record
259-422: A revised taxonomy based on a concept strongly resembling clades, although the term clade itself would not be coined until 1957 by his grandson, Julian Huxley . German biologist Emil Hans Willi Hennig (1913–1976) is considered to be the founder of cladistics . He proposed a classification system that represented repeated branchings of the family tree, as opposed to the previous systems, which put organisms on
296-473: A rowing motion to propel their "perfectly streamlined" bodies through the upper 20 m of the water column. Although usually slow-moving, beating their wings once or twice per second, they are capable of bursts of speed when they need to pursue their prey, calling a separate suite of muscles into action to obtain the higher beat frequency. Like many gastropods, sea angels are simultaneous hermaphrodites with internal fertilization. A fertilized animal later releases
333-413: A size of 7–8.5 cm (2.8–3.3 in). This makes it by far the largest sea angel . In comparison, the size of the southern subspecies is 1.2 cm (0.5 in), C. elegantissima is up to 3 cm (1.2 in), C. okhotensis up to 0.8 cm (0.3 in), and C. antarctica up to 3 cm (1.2 in). C. limacina swims by beating its two wings to move upwards or maintain itself at
370-421: A subfamily of the family Clionidae. Clade In biological phylogenetics , a clade (from Ancient Greek κλάδος (kládos) 'branch'), also known as a monophyletic group or natural group , is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree . In the taxonomical literature, sometimes
407-429: A suffix added should be e.g. "dracohortian". A clade is by definition monophyletic , meaning that it contains one ancestor which can be an organism, a population, or a species and all its descendants. The ancestor can be known or unknown; any and all members of a clade can be extant or extinct. The science that tries to reconstruct phylogenetic trees and thus discover clades is called phylogenetics or cladistics ,
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#1733094444809444-701: Is about twice as fast as their prey, the sea butterfly. Tracing the wingtip showed a ‘figure-of-eight’ trajectory and stroke plane orthogonal to the caudal-rostral axis, this pattern was observed in some aerial insects, small birds, and sea butterflies such as hummingbirds and Limacina helicina . It is not yet clear whether the sea angel uses its swimming appendages as 'rowing paddles' or as 'wings'. However, cold water sea angel C. antarctica likely accommodates drag-based force production for several reasons, (1) Low aspect ratio for C.antarctica causes deleterious lift-drag ratios. (2) High angle of attack typically ranges from 60 to 80 degrees. (3) The orthogonality between
481-499: Is also used with a similar meaning in other fields besides biology, such as historical linguistics ; see Cladistics § In disciplines other than biology . The term "clade" was coined in 1957 by the biologist Julian Huxley to refer to the result of cladogenesis , the evolutionary splitting of a parent species into two distinct species, a concept Huxley borrowed from Bernhard Rensch . Many commonly named groups – rodents and insects , for example – are clades because, in each case,
518-424: Is as yet unknown). Clione antarctica Pneumodermopsis spe Spongiobranchaea australis Pneumoderma violaceum (19 species in 13 genera) The other suborder of pteropods, Thecosomata , is superficially similar to sea angels, but are not closely related; some authorities include both Thecosomata and Gymnosomata as separate branches of
555-415: Is closely linked to that of their prey species. Even the size of the gymnosomes is correlated to the size of their prey, which they recognize by means of touch and grab using their sometimes-suckered buccal cones. A combination of hooks and a toothed radula are employed to scour the flesh from the thecosomes' shells. C. limacina stimulates the neuromuscular system to chase the prey, this notably increases
592-471: Is in turn included in the mammal, vertebrate and animal clades. The idea of a clade did not exist in pre- Darwinian Linnaean taxonomy , which was based by necessity only on internal or external morphological similarities between organisms. Many of the better known animal groups in Linnaeus's original Systema Naturae (mostly vertebrate groups) do represent clades. The phenomenon of convergent evolution
629-515: Is responsible for many cases of misleading similarities in the morphology of groups that evolved from different lineages. With the increasing realization in the first half of the 19th century that species had changed and split through the ages, classification increasingly came to be seen as branches on the evolutionary tree of life . The publication of Darwin's theory of evolution in 1859 gave this view increasing weight. In 1876 Thomas Henry Huxley , an early advocate of evolutionary theory, proposed
666-455: Is somewhat extraordinary. The buccal ("mouth") apparatus consists of three pairs of buccal cones. These tentacles grab the shell of Limacina helicina . When the prey is in the right position, with its shell opening facing the radula of Clione limacina , it then grasps the prey with its chitinous hooks, everted from hook sacs. Then it extracts the body completely out of its shell and swallows it whole. Adult Limacina are absent for much of
703-489: Is still controversial. As an example, see the full current classification of Anas platyrhynchos (the mallard duck) with 40 clades from Eukaryota down by following this Wikispecies link and clicking on "Expand". The name of a clade is conventionally a plural, where the singular refers to each member individually. A unique exception is the reptile clade Dracohors , which was made by haplology from Latin "draco" and "cohors", i.e. "the dragon cohort "; its form with
740-641: The Latin form cladus (plural cladi ) is used rather than the English form. Clades are the fundamental unit of cladistics , a modern approach to taxonomy adopted by most biological fields. The common ancestor may be an individual, a population , or a species ( extinct or extant ). Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflect evolutionary history as populations diverged and evolved independently. Clades are termed monophyletic (Greek: "one clan") groups. Over
777-424: The life cycle of C. limacina appears to be at least 2 years. It is a hermaphrodite and observations suggest this is simultaneous . It breeds during the spring and summer, and the eggs are about 0.12 mm (4.7 thou). Clione limacina is a prey of planktonic feeders, such as the baleen whales , which historically led to sailors naming it "whale-food". Some fishes are also its predators. For example,
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#1733094444809814-424: The order Pteropoda , whereas others list them as distinct orders within the subclass Heterobranchia . They have larger, broader parapodia, and most of that species retain a shell; they are commonly known as sea butterflies . In the new taxonomy of Bouchet & Rocroi (2005), the clade Gymnosomata is arranged as follows: The group that used to be the family Thliptodontidae is treated as Thliptodontinae ,
851-472: The buccal apparatus from C. limacina’s head. Gymnosomata are carnivorous, feeding only on their fellow pteropods, the Thecosomata . Their lifestyles have coevolved with those of their prey, with their feeding strategy adapting to the morphology and consistency of the thecosome shell. By rowing their "wings" back and forth at 1–3 Hz, the sea angels swim at speeds up to 100 mm/s (0.22 mph). This
888-621: The family Clionidae is just one of the families within this clade, and the term "sea butterfly" is also applied to the shelled Thecosomata . Recent molecular data suggest the Gymnosomata form a sister group to the Thecosomata (other planktonic, weakly or nonmineralized gastropods), but this long-standing hypothesis has also had some recent detractors. These organisms have a wide geographic range, from polar regions, under sea ice, to equatorial (tropic) seas. From spring to autumn, sea angels live at
925-699: The genus, which formerly were included in C. limacina (either as subspecies, variants or subpopulations). These are C. elegantissima of the cold North Pacific (at least north to the Gulf of Alaska ; the Beaufort Sea is inhabited by C. limacina ), C. okhotensis of the Okhotsk Sea (where it overlaps with C. elegantissima ), and C. antarctica of Antarctic waters. There are two subspecies that differentiate in body length. The northern subspecies lives in colder water, matures at 3 cm (1.2 in) and can reach
962-546: The group consists of a common ancestor with all its descendant branches. Rodents, for example, are a branch of mammals that split off after the end of the period when the clade Dinosauria stopped being the dominant terrestrial vertebrates 66 million years ago. The original population and all its descendants are a clade. The rodent clade corresponds to the order Rodentia, and insects to the class Insecta. These clades include smaller clades, such as chipmunk or ant , each of which consists of even smaller clades. The clade "rodent"
999-436: The laboratory the length of slugs have decreased on average from 22.4 to 12 mm (0.88 to 0.47 in). The earliest larvae stages of C. limacina feed on phytoplankton , but from the later laval stage this changes to Limacina . The development of these two species is parallel and small C. limacina feed on Limacina of a size, while large C. limacina avoid small Limacina (including its larvae). In Svalbard ,
1036-525: The largest species ( Clione limacina ) reaching 5 cm. C. limacina is a polar species; those found in warmer waters are far smaller. Some species of sea angels feed exclusively on sea butterflies ; the angels have terminal mouths with the radula common to mollusks, and tentacles to grasp their prey, sometimes with suckers similar to cephalopods . The wing comprises seven groups of muscles, anterior oblique muscles for dorsal and ventral sides, posterior oblique muscles for dorsal and ventral sides,
1073-590: The last few decades, the cladistic approach has revolutionized biological classification and revealed surprising evolutionary relationships among organisms. Increasingly, taxonomists try to avoid naming taxa that are not clades; that is, taxa that are not monophyletic . Some of the relationships between organisms that the molecular biology arm of cladistics has revealed include that fungi are closer relatives to animals than they are to plants, archaea are now considered different from bacteria , and multicellular organisms may have evolved from archaea. The term "clade"
1110-518: The latter term coined by Ernst Mayr (1965), derived from "clade". The results of phylogenetic/cladistic analyses are tree-shaped diagrams called cladograms ; they, and all their branches, are phylogenetic hypotheses. Three methods of defining clades are featured in phylogenetic nomenclature : node-, stem-, and apomorphy-based (see Phylogenetic nomenclature§Phylogenetic definitions of clade names for detailed definitions). The relationship between clades can be described in several ways: The age of
1147-421: The longitudinal and transverse wing retractors muscles, and dorso-ventral muscles. The first four groups form a continuous sheet where the dorsal muscles are perpendicular to the ventral muscles. Transverse and longitudinal retractors muscles retract the wing along the span and the chord. The dorso-ventral muscles control the thickness of the wing by changing the haemocoelic pressure inside the wing, this also ejects
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1184-434: The motion of the wings, minimizes the gap between wingtips at the end of each stroke, lessening the aspect ratio, and increases the deflection of the wings. Therefore, reducing the drag forces and overcoming stall. Also, the increment in the angle of attack along with low Reynold number for sea angels resembles the fluid dynamic forces acting on insects such as Drosophila . Gymnosomes slowly beat their wing-like parapodia in
1221-438: The scientific name of the order: From Greek gymnos meaning "naked" and soma meaning "body". The lack of a heavy shell for gymnosomes allows them to hover on the upper water surfaces for longer periods, along with a streamlined body that reduces the drag coefficient. Also, gymnosomes have a low aspect ratio which aids them to maneuver and accelerate quickly. Sea angels are gelatinous, mostly transparent, and very small, with
1258-522: The spread of viral infections . HIV , for example, has clades called subtypes, which vary in geographical prevalence. HIV subtype (clade) B, for example is predominant in Europe, the Americas and Japan, whereas subtype A is more common in east Africa. Clione limacina Clione limacina , known as the naked sea butterfly , sea angel , and common clione , is a sea angel (pelagic sea slug ) found from
1295-759: The surface of the wing and ascending trajectory of the sea angel. Furthermore, Clione antarctica , defends itself from predators by synthesizing a previously unknown noxious molecule, pteroenone . Because of this secretion, predators will not eat the sea angel. A species of amphipod takes advantage of this trait: The amphipod will seize an individual of C. antarctica out of the water column, and carry it around for protection. Local population density of C. antarctica may reach extraordinary levels; up to 300 animals per cubic metre have been recorded. Their hunting strategies are variable; some forms are ambush predators, sitting and waiting for their prey; whilst others actively pursue their prey; their metabolic rate
1332-676: The surface to greater than 500 m (1,600 ft) depth. It lives in the Arctic Ocean and cold regions of the North Atlantic Ocean . It was first described by Friderich Martens in 1676 and became the first gymnosomatous (without a shell) " pteropod " to be described. Clione limacina is found in cold waters of the Arctic Ocean and North Atlantic Ocean, ranging south at least to the Sargasso Sea . There are three other species in
1369-427: The year, leaving C. limacina without access to their main food source. A study of 138 C. limacina during a period without adult Limacina found that the stomachs of 24 contained remains of amphipods and 3 contained remains of calanoids . This temporary prey change may allow them to survive in periods of starvation , although the species can survive for one year without food. Under such exceptional starvation in
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