Теңіздің дернәсілді жануарлары: Appendicularia класы

Class of marine animals in the subphylum Tunicata

Larvaceans or appendicularians, class Appendicularia, are solitary, free swimming tunicates found throughout the world's oceans. While larvaceans are filter feeders like most other tunicates, they keep their tadpole like shape as adults, with the notochord running through the tail. They can be found in the pelagic zone, specifically in the photic zone, or sometimes deeper. They are transparent planktonic animals, usually ranging from to in body length including the tail, although giant larvaceans can reach up to in length. More species were quickly discovered, with Oikopleura in 1830 providing the first evidence of the larvacean house, although its role in feeding wouldn't be understood until Eisen's discoveries in 1874.

Another jump in the study of larvaceans was the beginning of in situ observations, which allowed to study the creatures inside their fragile houses without damage. Researchers such as Kakani Katija Young from the Monterey Bay Aquarium Research Institute pioneered imaging techniques such as the particle image velocimetry instrument DeepPIV, revealing the complexity and inner structure of larvacean houses and leading to the first 3D simulations of their internal currents. the larvae of doliolids goes through their metamorphosis while still inside the egg, and salps and pyrosomes have both lost the larval stage, it makes the larvaceans the only tunicates that feed and have fully functional internal organs during their tailed "tadpole stage", which in Appendicularia is permanent. The full development of Oikopleura dioica and the fate of its cell lineages have been well documented, providing insight into larvacean anatomy. In some genera like Oikopleura, the tract is U shaped, with the anus located in a forwards position compared to the stomach and intestine. Others like Fritillaria present a more segmented appearance, with a straighter digestive tract and well separated pharyngeal and digestive sections. The species Appendicularia sicula doesn't have any anus at all, leading to accumulation of undigested material. Appendicularia retains the ancestral chordate characteristics of having the pharyngeal spiracles and the anus open directly to the outside, and by the lack of the atrium and the atrial siphon found in related classes. The gonads are located in the posterior section of the trunk, beyond the digestive tract. They are the only section of the body not to be well distinguished in the juvenile post tail shift, instead only growing in size in the days leading to spawning.

The tail of larvaceans contain a central notochord, a dorsal nerve cord, and a series of striated muscle bands enveloped either by epithelial tissue (oikopleurids) or by an acellular basement membrane (fritillarids). Unlike the ascidian larvae, the tail nerve cord in larvaceans contains some neurons. The tail twists during development, with its dorsal and ventral sides becoming left and right sides respectively. In this way, the dorsal nerve cord actually runs through the tail to the left of the notochord, connecting to the rest of the nervous system at the caudal ganglion at the base of the tail. The muscle bands surrounding the notochord and nerve cord consist of rows of paired muscle cells, or myocytes, running along the length of the tail.

To assist in their filter feeding, larvaceans produce a "house" made of mucopolysaccharides and cellulose, secreted from specialized cells termed oikoplasts. In most species, the house surrounds the animal like a bubble. Even for species in which the house does not completely surround the body, such as Fritillaria, the house is always present and attached to at least one surface. The house is secreted from oikoplasts, a specialized family of cells constituting the oikoplastic epithelium. Derived from the ectoderm, it covers part (in Fritillaria) or all (in Oikopleura) of the trunk. In larvae, surface fibrils are secreted by the epithelium prior to the differentiation of the oikoplasts, and have been suggested to play a part in the development of the first house, as well as the formation of the cuticular layer. The houses possesses several sets of filters, with external filters stopping food particles too big for the larvacean to eat, and internal filters redirecting edible particles to the larvacean's mouth. Including the external filters, the houses can reach over one meter in giant larvaceans, an order of magnitude larger than the larvacean itself. The house varies in shape: incomplete in Fritillaria, it is shaped like a pair of kidneys in Bathochordaeus, and toroidal in Kowalevskia. The arrangement of filters allows food in the surrounding water to be brought in and concentrated prior to feeding, with some species able to concentrate food up to 1000 times compared to the surrounding water. Larvaceans have been found to be able to select food particles based on factors such as nutrient availability and toxin presence, although both laboratory feeding experiments and in situ observations show no difference in feeding rate between their usual food sources and microplastics. In some species, houses are discarded and replaced regularly as the animal grows in size and its filters become clogged; in Oikopleura, a house is kept for no more than four hours before being replaced. In other genera such as Fritillaria, houses can be regularly deflated and inflated, cleaning off particles clogging the filters. Houses being reused in this manner leads to a smaller contribution in marine snow from these genera. This has been confirmed through genetic studies on Oikopleura dioica and the ascidian Ciona, pinpointing their common cellulose synthase genes as originating with a horizontal gene transfer from a prokaryote. However, houses and tunics share key differences — while houses are gelatinous and can be deflated or even discarded at will, tunics are rigid structures definitively incorporated into the animal's filter feeding apparatus.

Larvaceans are widespread, motile planktonic creatures, living through the water column. As their habitats are mostly defined by ocean currents, Larvaceans have been reported as far as the Southern Ocean, where they are estimated to comprise 10.5 million tonnes of wet biomass. Most species live in the photic zone at less than 100 meters in depth, and undescribed oikopleurid and fritillariid species have been reported through the bathypelagic zone, down to the 3,500 meters deep seafloor in Monterey Bay where they constitute the dominant particle feeders in most of the water column.

Larvaceans reproduce sexually, with all but one species being protandric hermaphrodites. Unlike all other known larvaceans, Oikopleura dioica shows separate sexes, which are distinguished on the last day of their life cycle through differing gonad shapes. Fertilisation is external. The body wall ruptures during egg release, killing the animal.

Through their discarded, nutrient rich houses — termed sinkers — and fecal pellets falling towards the deep seafloor, larvaceans transport large amounts of organic matter towards that region, constituting a significant component of marine snow. Still in Monterey Bay, giant larvaceans have been found to have the highest filtration rate of any invertebrate, and discarded larvacean houses have been observed as a consistent food source for both pelagic and benthic organisms in that same region.

Appendicularia is most often recovered as the sister group of the other tunicate groups (Ascidiacea and Thaliacea). Already in the late 19th to early 20th century, it was hypothesized by Seeliger and later by Lohmann that Appendicularia diverged first from a free swimming ancestral tunicate, with sessile forms evolving later in the sister lineage (often termed Acopa). The following cladogram is based on the 2018 phylogenomic study of Delsuc and colleagues.

Being delicate and soft bodied, Appendicularia has no definitive fossil record, although the Cambrian form Oesia disjuncta has historically been suggested to belong to the class. Vetulicolians have also been argued to represent stem group larvaceans by Dominguez and Jefferies, on the basis of synapomorphies comprising the reduction of the atria and of the gill slits, the position of the anus, and a 90° counter clockwise torsion of the tail (as seen from behind) around the anterior posterior axis.

The extant species of the class are divided into three families based on both morphological and genomic criteria: Kowalevskiidae, Fritillariidae and Oikopleuridae. The first two are believed to be closer to each other, sharing more derived characteristics compared to the primitive Oikopleuridae. Fritillariidae itself is subdivided into Fritillariinae and the monotypic Appendiculariinae, while Oikopleuridae is split into Bathochordaeinae and Oikopleurinae. Deeper phylogeny is unclear, with genera such as Oikopleura possibly being paraphyletic. Several key morphological differences distinguish the families. Fritillariidae presents a more tapered, compressed trunk, as compared to the rounder one of the other two families. Meanwhile, Kowalevskiidae is notable for lacking the heart and endostyle present in other families, the latter replaced by a ciliated groove without glandular cells. The shape of the spiracles also differs: they appear as simple holes in Fritillariidae, long narrow slits in Kowalevskiidae, and tubular passages in Oikopleuridae. Not all species are equally well studied. The popularity of Oikopleura dioica as a model organism and its ease of cultivation have led to studies disproportionately focusing on this species' anatomy, and in situ observations on Bathochordaeus charon have been performed by the Monterey Bay Aquarium Research Institute. Meanwhile, studies of Kowalevskiidae and Fritillariidae are comparatively rarer and more limited.

The dioecious Oikopleura dioica is the only larvacean species that has successfully been cultured in laboratory. The ease of cultivation, combined with extremely small genome size and recent development of techniques for expressing foreign genes in O. dioica, has led to the advancement of this species as a model organism for the study of gene regulation, chordate evolution, developmental biology, and ecology.