Larvacean

Huxley was the first to suggest the identity of larvaceans as tunicates in 1851. Their relationship with other tunicates remained unclear, with larvaceans being argued to be ascidian larvae or a free-swimming generation of ascidians.

The trunk can roughly be divided into three regions — pharyngeo-brachial, digestive and genital — which are more or less distinct depending on the genus.[11] Like in vertebrates, the digestive system comprises in order a mouth, pharynx, oesophagus, stomach, intestine and rectum.

The pharynx is equipped with an endostyle on its lower side, a specialized organ helping direct food particles inside. It also possesses two spiracles, each surrounded by a ring of cilia.[1]

In some genera like Oikopleura, the tract is U-shaped, with the anus located in a forwards position compared to the stomach and intestine.[12] Others like Fritillaria present a more segmented appearance, with a straighter digestive tract and well-separated pharyngeal and digestive sections.

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 contain some neurons.[13]

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.[14]

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,[15] secreted from specialized cells termed oikoplasts.[16][17] 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.[10] 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.[4] By regularly beating the tail, the larvacean can generate water currents within its house that allow the concentration of food. For this purpose, the tail fits into a specialized tail sheath, a funnel of the house connected to the exhalent aperture.[14] The high efficiency of this method allows larvaceans to feed on much smaller nanoplankton than most other filter feeders.

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. No other tunicate is able to abandon its test in this fashion. Discarded larvacean houses account for a significant fraction of organic material descending to the ocean depths.[18] 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.[10]

Larvaceans are widespread, motile planktonic creatures, living through the water column. As their habitats are mostly defined by ocean currents,[1] many species have a cosmopolitan distribution, with some like Oikopleura dioica being found in all of the world's oceans.[19] Larvaceans have been reported as far as the Southern Ocean, where they are estimated to comprise 10.5 million tonnes of wet biomass.[5]

Most species live in the photic zone at less than 100 meters in depth,[19] although giant larvaceans such as Bathochordaeus mcnutti can be found up to 1,400 meters deep.[20]

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.[9]

The immature animals resemble the tadpole larvae of ascidians, albeit with the addition of developing viscera. Once the trunk is fully developed, the larva undergoes "tail shift", in which the tail moves from a rearward position to a ventral orientation and twists 90° relative to the trunk. Following tail shift, the larvacean begins secretion of the first house.

The life cycle is short. The tadpole-shaped larva usually performs the tail shift less than one day after fecundation, becoming fully functional juveniles. Adults usually reproduce after 5 to 7 days depending on the species.[9]

Fertilisation is external. The body wall ruptures during egg release, killing the animal.[21]

Through their discarded 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.[5] In that way, they massively contribute to the oceanic carbon cycle, with giant larvaceans having the highest filtration rate of any invertebrate in Monterey Bay.[4]

Both larvacean houses and fecal pellets were also found to trap microplastics, before sinking towards the seafloor. In this way, larvaceans are believed to play a part in the missing plastic paradox, transporting microplastics through the water column and to the seafloor. Experiments performed on the giant larvacean Bathochordaeus stygius confirm their ability to filter and discard microplastics.[22]

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.[23] More recently, microfossils covered in an organic coat found in vanadium-rich Cambrian black shales in South China have been suggested to be traces of early larvaceans in their houses, putatively termed "paleoappendicularians".[25][26]

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.[27]

The extant species of the class are divided into three families based on both morphological and genomic criteria: Kowalevskiidae, Fritillariidae and Oikopleuridae.[10][11] The first two are believed to be closer to each other, sharing more derived characteristics compared to the primitive Oikopleuridae.[28] 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: simple holes in Fritillariidae, they appear as long narrow slits in Kowalevskiidae, and tubular passages in Oikopleuridae.[1]

While the number of described species is comparatively low, the class is believed to harbour massive diversity in the form of cryptic species. For instance, Oikopleura dioica comprises at least three distinct, reproductively incompatible clades despite a similar morphological appearance.[29]

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.[30] Meanwhile, studies of Kowalevskiidae and Fritillariidae are comparatively rarer and more limited.[10]