Xenoturbella bocki | |
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X. bocki. Black arrow indicates side furrow. a is the anterior tip. p is the posterior tip. Black triangle indicates mouth. White triangle indicates circumferential furrow. The scale bar in the bottom right is 1 cm. | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Xenacoelomorpha |
Family: | Xenoturbellidae |
Genus: | Xenoturbella |
Species: | X. bocki |
Binomial name | |
Xenoturbella bocki Westblad, 1949 | |
Xenoturbella bocki is a marine benthic worm-like species from the genus Xenoturbella . It is found in saltwater sea floor habitats off the coast of Europe, predominantly Sweden. It was the first species in the genus discovered. Initially it was collected by Swedish zoologist Sixten Bock in 1915, and described in 1949 by Swedish zoologist Einar Westblad. [1] The unusual digestive structure of this species, in which a single opening is used to eat food and excrete waste, has led to considerable study and controversy as to its classification. It is a bottom-dwelling, burrowing carnivore that eats mollusks (likely larval forms, as opposed to hard-shelled adults).
For the genus name Xenoturbella, Ancient Greek xénos, means foreign or strange, and Latin turbela, means a bustle or turbulence in water. Genus Xenoturbella is a member of sub-phylum Xenoturbellida, which are known as paradoxmaskar [2] – Swedish for "paradox worms" (a term that some popular media have applied to the species), because if it is classified as a deuterostome, it would be more closely related to humans than other, more complex, invertebrates such as lobsters. [3] Deuterostomes are a superphylum of animals whose anus forms before their mouth does during embryonic development. It includes humans, other chordates, echinoderms and hemichordates.
The species signifier bocki refers to Sixten Bock, who first collected the organism in 1915. [4] It was assigned by Swedish zoologist Einar Westblad, who described the species in 1949.
In 1999, examination of X. bocki specimens held at the Swedish Museum of Natural History showed that a small subset of them must belong to another species. [5] This population differed from specimens identified as X. bocki in internal fertilization, its small size of 12 mm (0.47 in) at most, and its pink coloration – in contrast to yellow-white coloration identified for X. bocki. The new taxon was named after Westblad, who collected the specimens from coarser and shallower habitats in the same range as X. bocki. However, mitochondrial DNA sequencing from the specimens identified with both species suggested that the two populations belonged to the same species, involving that X. westbladi is a junior synonym to X. bocki. [6]
Comparison of mitochondrial DNA and protein sequences showed that the species Xenoturbella bocki – often found off the coast of Sweden – is the sister group to X. hollandorum, a species discovered in 2016 in eastern Pacific Ocean. [7] In turn, these two species share evolutionary affinities with X. japonica into a clade of 'shallow-water' taxa. [8]
Species-level cladogram of the genus Xenoturbella. | |||||||||||||||||||||||||||||||||||||||
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The cladogram has been reconstructed from mitochondrial DNA and protein sequences. [7] [8] |
When it was discovered, X. bocki was placed in a new genus Xenoturbella . Above the genus level, the classification of this animal is controversial. Westbald placed it in the phylum Platyhelminthes in the class Turbellaria (free-living flatworms). [9] In 1999, based on genetic analysis, it was placed in Protostomia by Israelsson, grouped with the bivalves. [10] [11] Protostomia is a large clade including worms, mollusks and arthropods. In embryonic development, their mouth develops prior to the development of the anus for most protostomes, though some have evolved other developmental pathways. If placed in this clade, Xenoturbella would also be among these exceptions. [12]
However, today, this is understood as a misclassification due to contaminating DNA from its shellfish food. Swedish scientist Sarah J. Bourlat and her coauthors in 2006 placed it in its own phylum, Xenoturbellida. More recent studies suggest on the basis of genetic and developmental evidence (e.g. Hox genes) that it should be grouped with Acoela and Nemertodermatida into Acoelomorpha. These three taxa are sometimes placed within the deuterostomes (a large clade that includes humans and other chordates, sea stars and others), [12] while others classify these organisms as a basal offshoot that resembles a common ancestor of deuterostomes and protostomes. [11] A 2016 analysis of many genetic data sets supports the latter, and suggests that, like Xenoturbella bocki, the common ancestor of protostomes and deuterostomes likely had one opening, ciliated locomotion and a wormlike body. [13] However, if the deuterostome hypothesis is correct, then Xenoturbella must have lost many ancestral traits, such as an anus. [14]
This animal usually grows to 1 cm (0.39 in) in length, [11] though individuals as long as 4 cm (1.6 in) have been reported. [11] Its nervous system consists of a nerve net with no defined brain or ganglia. The nerve net is found on the basal (away from the animal's surface) side of the skin. [15] This animal lacks a coelom. It also lacks an anus, excreting waste through the same opening as it intakes food. [9] Thus, the digestive organ is sac-like. The opening is on the belly of the animal, near the front. The animal is simultaneously hermaphroditic. [4]
A furrow runs along the circumference of the body in the middle of the animal. There are also side furrows. On its sides there are numerous tiny cilia that aid in locomotion. Small cells contain vesicles which may act as glands. An organ of unknown function, preliminarily called a statocyst, has been observed on the front end of the animal. Two leading hypotheses are that it aids in balance, as statocysts do in other invertebrates, or that it has endocrine functions. Experiments in which the animal was observed to cleave into two after a wound show that the statocyst is essential for normal behavior and long-term survival. [9]
The animal moves through the water via rhythmic muscle contraction, aided by its side cilia, and a tuft of longer cilia on its back. The organism can also use its musculature to roll up into a ball, and maintain that form for several months. [9] Adults are known to have a symbiotic relationship with Chlamydiae and Gammaproteobacteria, two bacterial endosymbionts found in their gastrodermis. [16] Genetic data confirms that its diet includes bivalve mollusks. [11] However, it has never been observed feeding, so it is unknown if it eats bivalve carcasses, eggs, sperm, mucus, feces, or live larval or adult bivalves. [9] It lacks any visible means to get through the shells of adult bivalves. Captive specimens survived for several months without food, and showed no interest in any of the proposed food items afterwards. This has led some to suggest that it feeds by absorbing dissolved organic matter through its skin. At least one specimen that has been proposed to show a consumed bivalve larvae is preserved in the Swedish Natural History Museum. This species burrows, and has been observed to make tunnels as deep as 15 cm (5.9 in) into substrate in a laboratory aquarium. [9]
This species has been found in ocean habitats off the coast of Europe, most often off the coast of Sweden. It is often collected using a Warén’s dredge from mud on the sea floor, at depths of 50–200 m (160–660 ft). [9] [17]
X. bocki has only been observed to reproduce sexually. [9] In the wild, this species spawns in the winter. It lays small, mucus-coated eggs, which sink in the water column. [11] The eggs have a pale-orange color, and are opaque. Young, upon hatching, are yellowish, nearly spherical, and move to the surface of the water. Larvae lack a blastopore and do not feed until they are fully developed. They may derive nourishment from the yolk which would make them lecithotrophic. Within five days muscular contractions are observed in a laboratory setting, which may aid locomotion. X. bocki is a direct developer. As of 2013, this animal is extremely challenging to grow in captivity. [11]
A chordate is a deuterostomic animal belonging to the phylum Chordata. All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa. These five synapomorphies are a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail. The name "chordate" comes from the first of these synapomorphies, the notochord, which plays a significant role in chordate body plan structuring and movements. Chordates are also bilaterally symmetric, have a coelom, possess an closed circulatory system, and exhibit metameric segmentation.
The flatworms, flat worms, Platyhelminthes, or platyhelminths are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Unlike other bilaterians, they are acoelomates, and have no specialised circulatory and respiratory organs, which restricts them to having flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. The digestive cavity has only one opening for both ingestion and egestion ; as a result, the food cannot be processed continuously.
Vetulicolia is a phylum of animals encompassing several extinct species belonging to the Cambrian Period. The phylum was created by Degan Shu and his research team in 2001, and named after Vetulicola cuneata, the first species of the phylum described in 1987. The vetulicolian body comprises two parts: a voluminous anterior forebody, tipped with an anteriorly positioned mouth and lined with a row of five round to oval-shaped features on each lateral side, which have been interpreted as gills ; and a posterior section that primitively comprises seven segments and functions as a tail. All vetulicolians lack preserved appendages of any kind, having no legs, feelers or even eyes. The area where the anterior and posterior parts join is constricted.
Bilateria is a large clade/infrakingdom of animals called bilaterians, characterized by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front and a rear end, as well as a left–right–symmetrical belly (ventral) and back (dorsal) surface. Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception is the echinoderms, which achieve secondary pentaradial symmetry as adults, but are bilaterally symmetrical as an embryo. Cephalization is also a characteristic feature among most bilaterians, where the special sense organs and central nerve ganglia become concentrated at the front/rostral end.
Acoelomorpha is a subphylum of very simple and small soft-bodied animals with planula-like features which live in marine or brackish waters. They usually live between grains of sediment, swimming as plankton, or crawling on other organisms, such as algae and corals. With the exception of two acoel freshwater species, all known Acoelomorphs are marine.
Xenoturbella is a genus of very simple bilaterians up to a few centimeters long. It contains a small number of marine benthic worm-like species.
Acoela, or the acoels, is an order of small and simple invertebrates in the subphylum Acoelomorpha of phylum Xenacoelomorpha, a deep branching bilaterian group of animals, which resemble flatworms. Historically they were treated as an order of turbellarian flatworms.
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. As of 2022, 2.16 million living animal species have been described—of which around 1.05 million are insects, over 85,000 are molluscs, and around 65,000 are vertebrates. It has been estimated there are around 7.77 million animal species. Animals range in length from 8.5 micrometres (0.00033 in) to 33.6 metres (110 ft). They have complex interactions with each other and their environments, forming intricate food webs. The scientific study of animals is known as zoology.
Ambulacraria, or Coelomopora, is a clade of invertebrate phyla that includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggests the echinoderms and hemichordates separated around 533 million years ago. The Ambulacraria are part of the deuterostomes, a clade that also includes the many Chordata, and the few extinct species belonging to the Vetulicolia.
Deuterostomes are bilaterian animals of the superphylum Deuterostomia, typically characterized by their anus forming before the mouth during embryonic development. The three major clades of extant deuterostomes include chordates, echinoderms and hemichordates.
Protostomia is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's members, although the reverse is typically true of its sister clade, Deuterostomia. Well known examples of protostomes are arthropods, molluscs, annelids, flatworms and nematodes. They are also called schizocoelomates since schizocoely typically occurs in them.
The embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating bilaterian animals into two natural groupings: the protostomes and deuterostomes.
Nephrozoa is a major clade of bilaterians, divided into the protostomes and the deuterostomes, containing almost all animal phyla and over a million extant species. Its sister clade is the Xenacoelomorpha. The Ambulacraria are occasionally thought to be sister to the Xenacoelomorpha, forming the Xenambulacraria as basal Deuterostomia, or basal Bilateria invalidating Nephrozoa and Deuterostomia in multiple studies. The coelom, the digestive tract and excretory organs (nephridia), and nerve cords developed in the Nephrozoa. It has been argued that, because protonephridia are only found in protostomes, they cannot be considered a synapomorphy of this group. This would make Nephrozoa an improper name, leaving Eubilateria as this clade's name.
Xenacoelomorpha is a small phylum of bilaterian invertebrate animals, consisting of two sister groups: xenoturbellids and acoelomorphs. This new phylum was named in February 2011 and suggested based on morphological synapomorphies, which was then confirmed by phylogenomic analyses of molecular data.
Xenoturbella japonica is a marine benthic worm-like species that belongs to the genus Xenoturbella. It has been discovered in western Pacific Ocean by a group of Japanese scientists from the University of Tsukuba. The species was described in 2017 in a study published in the journal BMC Evolutionary Biology, and amended in 2018.
Xenambulacraria is a proposed clade of animals with bilateral symmetry as an embryo, consisting of the Xenacoelomorpha and the Ambulacraria.
Xenoturbella churro is a marine, benthic, deep-water worm-like species that belongs to the genus Xenoturbella. It was discovered in eastern Pacific Ocean by a group of Californian and Australian scientists. The species was described in 2016 from a single specimen.
Xenoturbella profunda, the purple sock or sock worm, is a marine, benthic, deep-water worm-like species that belongs to the genus Xenoturbella. It was discovered in eastern Pacific Ocean by a group of Californian and Australian scientists. The species was described in 2016 from seven specimens.
Xenoturbella monstrosa, a deep-sea giant purple sock worm, is a marine, benthic, deep-water worm-like species that belongs to the genus Xenoturbella. It was discovered in eastern Pacific Ocean by a group of Californian and Australian scientists. The species was described in 2016 from several specimens.
Xenoturbella hollandorum is a marine, benthic worm-like species that belongs to the genus Xenoturbella. It was discovered in eastern Pacific Ocean by a group of Californian and Australian scientists. The species was described in 2016.