Mesozoa

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Mesozoa
Dicyema macrocephalum.png
Dicyema macrocephalum
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
(unranked): Protostomia
(unranked): Spiralia
Clade: Platytrochozoa
(unranked): Mesozoa
van Beneden, 1876
Phyla

The Mesozoa are minuscule, worm-like parasites of marine invertebrates. Generally, these tiny, elusive creatures consist of a somatoderm (outer layer) of ciliated cells surrounding one or more reproductive cells.

Contents

A 2017 study recovered Mesozoa as a monophyletic group that emerged in the Lophotrochozoa as sister of the Rouphozoa. [1]

Some workers previously classified Mesozoa as the sole phylum of the lonely subkingdom Agnotozoa. Cavalier-Smith argued that at least some of the mesozoans are in fact protistans, not animals. [2]

In the 19th century, the Mesozoa were a wastebasket taxon for multicellular organisms which lacked the invaginating gastrula which was thought to define the Metazoa. [3]

Evolution

Mesozoa were once thought to be evolutionary intermediate forms between Protozoans and Metazoans, but now they are thought to be degenerate or simplified metazoa. Their ciliated larvae are similar to the miracidium of trematodes, and their internal multiplication is similar to what happens in the sporocysts of trematodes. Mesozoan DNA has a low GC-content (40%). This amount is similar to ciliates, but ciliates tend to be binucleate. Others relate mesozoa to a group including annelids, planarians and nemerteans.

Orthonectida have a very reduced muscular and nervous system, only consisting of a few cells, but so far no muscle cells or neurons have been found in Dicyemida. [4] [5]

Groupings

The two main mesozoan groups are the Dicyemida and the Orthonectida. Other groups sometimes included in the Mesozoa are the Placozoa and the Monoblastozoa.

Monoblastozoans consist of a single description written in the 19th century of a species that has not been seen since. As such, many workers doubt that they are a real group. [6] As described, the animal had only a single layer of tissue. [7]

Rhombozoan mesozoans

Rhombozoa, or dicyemid mesozoans, are found in the nephrid tracts of squid and octopuses. [8] They range from a few millimeters long with twenty to thirty cells that include anterior attachment cells and a long central reproductive cell called an axial cell. This axial cell may develop asexually into vermiform juveniles or it may produce eggs and sperm that self-fertilize to produce a ciliated infusiform larva.

There are three genera: Dicyema, Pseudicyema and Dicyemennea.

Molecular evidence suggests that this phylum are derived from the Lophotrochozoa. [9] [10]

Orthonectid mesozoans

Orthonectida are found in the body spaces of various marine invertebrates including tissue spaces, gonads, genitorespiratory bursae. This pathogen causes host castration of different species. [11]

The best known of Orthonectida is the parasite of brittle stars. The multinucleate syncytial stage lives within tissues and spaces of the gonad but can spread into arms. It causes the destruction of starfish ovary and eggs to cause castration (the male gonads are usually unaffected). The stages of the plasmodium develop into more plasmodia by simple fragmentation; at some point, they decide to go sexual. The syncytia are dioecious (either male or female), but young syncytia can fuse to produce both male and female. The males are ciliated and smaller than the females. The females and the males leave the starfish and mate in the sea. Tailed sperm enters the female and fertilizes the numerous oocytes. Each oocyst produces a small ciliated larva which makes its way to another star.

The genome of one of these species – Intoshia linei – has been sequenced. [12] The Orthonectids may be degenerate annelid worms. [6] [13]

Related Research Articles

<span class="mw-page-title-main">Invertebrate</span> Animals without a vertebral column

Invertebrates is an umbrella term describing animals that neither develop nor retain a vertebral column, which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordate subphylum Vertebrata, i.e. vertebrates. Well-known phyla of invertebrates include arthropods, mollusks, annelids, echinoderms, flatworms, cnidarians and sponges.

<span class="mw-page-title-main">Flatworm</span> Phylum of soft-bodied invertebrates

The flatworms, flat worms, Platyhelminthes, or platyhelminths are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Being acoelomates, and having no specialised circulatory and respiratory organs, they are restricted 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 can not be processed continuously.

<span class="mw-page-title-main">Nemertea</span> Phylum of invertebrates, ribbon worms

Nemertea is a phylum of animals also known as ribbon worms or proboscis worms, consisting of 1300 known species. Most ribbon worms are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many have patterns of yellow, orange, red and green coloration. The foregut, stomach and intestine run a little below the midline of the body, the anus is at the tip of the tail, and the mouth is under the front. A little above the gut is the rhynchocoel, a cavity which mostly runs above the midline and ends a little short of the rear of the body. All species have a proboscis which lies in the rhynchocoel when inactive but everts to emerge just above the mouth to capture the animal's prey with venom. A highly extensible muscle in the back of the rhynchocoel pulls the proboscis in when an attack ends. A few species with stubby bodies filter feed and have suckers at the front and back ends, with which they attach to a host.

<span class="mw-page-title-main">Alveolate</span> Superphylum of protists

The alveolates are a group of protists, considered a major clade and superphylum within Eukarya. They are currently grouped with the stramenopiles and Rhizaria among the protists with tubulocristate mitochondria into the SAR supergroup.

<span class="mw-page-title-main">Trochozoa</span> Taxonomic clade

The Trochozoa are a proposed Lophotrochozoa clade that is a sister clade of Bryozoa. The clade would include animals in five phyla: the Nemertea, the Annelida, the Mollusca, and the two Brachiozoan phyla, Brachiopoda and Phoronida. Both annelids and molluscs have been suggested as the sister group of Brachiozoa. It has also been proposed that nemerteans are actually a clade of annelids.

<span class="mw-page-title-main">Orthonectida</span> Phylum of marine invertebrate parasites

Orthonectida is a small phylum of poorly known parasites of marine invertebrates that are among the simplest of multi-cellular organisms. Members of this phylum are known as orthonectids.

<span class="mw-page-title-main">Myxosporea</span> Class of cnidarians comprising microscopic parasites

Myxosporea is a class of microscopic parasites, belonging to the Myxozoa clade within Cnidaria. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate and an ectothermic vertebrate, usually a fish. Each host releases a different type of spore. The two forms of spore are so different that until relatively recently they were treated as belonging to different classes within the Myxozoa.

<span class="mw-page-title-main">Lophotrochozoa</span> Superphylum of animals

Lophotrochozoa is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids, molluscs, bryozoans, and brachiopods.

<span class="mw-page-title-main">Dicyemida</span> Phylum of tiny parasites of cephalopods

Dicyemida, also known as Rhombozoa, is a phylum of tiny parasites that live in the renal appendages of cephalopods.

<span class="mw-page-title-main">Phoronid</span> Phylum of marine animals

Phoronids are a small phylum of marine animals that filter-feed with a lophophore, and build upright tubes of chitin to support and protect their soft bodies. They live in most of the oceans and seas, including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about 400 meters down. Most adult phoronids are 2 cm long and about 1.5 mm wide, although the largest are 50 cm long.

<span class="mw-page-title-main">Spiralia</span> Clade of protosomes with spiral cleavage during early development

The Spiralia are a morphologically diverse clade of protostome animals, including within their number the molluscs, annelids, platyhelminths and other taxa. The term Spiralia is applied to those phyla that exhibit canonical spiral cleavage, a pattern of early development found in most members of the Lophotrochozoa.

Dicyema shimantoense is a parasitic worm of the phylum Dicyemida. It is a vermiform mesozoan parasite that infects the renal appendages of the cephalopod Octopus sasakii. The name is derived from the Shimanto River, which is the longest river in Shikoku, and flows into Tosa Bay. A study from 2000-06 used 59 specimens obtained from fishermen at Tosa Bay and Kii Strait in Japan. O. sasakii is a cephalopod found mainly in the shallow-water of Southern Japan. Research found that only those of certain sizes and geographical locations can be infected by D.shimantoense.

<span class="mw-page-title-main">Dicyemidae</span> Family of rhombozoa animals

The Dicyemidae is a family of tiny parasites that live in the renal appendages of cephalopods. It contains the following genera and species:

<span class="mw-page-title-main">Annelid</span> Phylum of segmented worms

The annelids, also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecologies – some in marine environments as distinct as tidal zones and hydrothermal vents, others in fresh water, and yet others in moist terrestrial environments.

The Platytrochozoa are a proposed basal clade of spiralian animals as the sister group of the Gnathifera. The Platytrochozoa were divided into the Rouphozoa and the Lophotrochozoa. A more recent study suggests that the mesozoans also belong to this group of animals, as sister of the Rouphozoa.

<i>Sepioloidea lineolata</i> Species of cuttlefish

Sepioloidea lineolata or more commonly known as the striped pyjama squid or the striped dumpling squid is a type of bottletail squid that inhabits the Indo-Pacific Oceans of Australia. Although traditionally falling within Sepiida, the cuttlefish order, it lacks a cuttlebone. More recent phylogenomic evidence suggests bottletail and bobtail squid may form their own order, Sepiolida. The striped pyjama squid lives on the seafloor and is both venomous and poisonous. When fully mature, a striped pyjama squid will only be about 7 to 8 centimetres in length. Baby striped pyjama squid can be smaller than 10 millimetres (0.39 in).

<i>Dicyema</i> Genus of rhombozoa animals

Dicyema is a genus of worms belonging to the family Dicyemidae.

<i>Dicyemennea</i> Genus of rhombozoa animals

Dicyemennea is a genus of worms belonging to the family Dicyemidae.

Intoshia linei is a species of parasitic worms belonging to the family Rhopaluridae.

<span class="mw-page-title-main">Pleistoannelida</span> Clade of annelid worms

Pleistoannelida is a group of annelid worms that comprises the vast majority of the diversity in phylum Annelida. Discovered through phylogenetic analyses, it is the largest clade of annelids, comprised by the last common ancestor of the highly diverse sister groups Errantia and Sedentaria and all the descendants of that ancestor. Most groups in the Clade find their ancestors within the Cambrian explosion when Annelid diversity expanded dramatically. The Pleistoannelida clade covers a variety of traits. However, the evolution of simple to complex eyes, developed papillae for burrowing, and for some specialized radioles for feeding can be seen universally across every species. New findings have discovered the range of Annelid diversity have led to uncertainty if groups with developed ancestral traits should remain within the clade. Furthermore There's been a lack of recently discovered Annelid traits being used in the categorization of groups within the clade, leading to many hypothesis on how to do so and which should remain within the clade. Currently three smaller clades that were originally a part of the groups Errantia and Sedentaria have been proven to fall outside while still being connected to the basal groups.

References

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  2. Hanelt, B.; Van Schyndel, D.; Adema, C.M.; Lewis, L.A.; Loker, E.S. (November 1996). "The Phylogenetic Position of Rhopaluva ophiocomae (Orthonectida) Based on 18s Ribosomal DNA Sequence Analysis" (PDF). Molecular Biology and Evolution . 13 (9): 1187–1191. doi: 10.1093/oxfordjournals.molbev.a025683 . PMID   8896370. Archived (PDF) from the original on 2022-10-09. Retrieved 2013-03-14.
  3. Syed, Tareq; Schierwater, Bernd (December 2002). "Trichoplax adhaerens: discovered as a missing link, forgotten as a hydrozoan, re-discovered as a key to metazoan evolution". Vie et Milieu. 52 (4): 177–187.
  4. Slyusarev, George S.; Nesterenko, Maksim A.; Starunov, Viktor V. (2019). "The structure of the muscular and nervous systems of the male Intoshialinei (Orthonectida)". Acta Zoologica. 100 (4): 451–458. doi:10.1111/azo.12279.
  5. Zverkov, O. A.; Mikhailov, K. V.; Isaev, S. V.; Rusin, L. Y.; Popova, O. V.; Logacheva, M. D.; Penin, A. A.; Moroz, L. L.; Panchin, Y. V.; Lyubetsky, V. A.; Aleoshin, V. V. (2019). "Dicyemida and Orthonectida: Two Stories of Body Plan Simplification". Frontiers in Genetics. 10: 443. doi: 10.3389/fgene.2019.00443 . PMC   6543705 . PMID   31178892.
  6. 1 2 Schiffer, Philipp; Robertson, Helen; Telford, Maximilian (2018-04-03). Orthonectids are highly degenerate annelid worms (Data set). doi: 10.5281/zenodo.1206208 .[ better source needed ]
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  9. Kobayash, M.; Furuya, H.; Wada, H. (September–October 2009). "Molecular markers comparing the extremely simple body plan of dicyemids to that of lophotrochozoans: insight from the expression patterns of Hox, Otx, and brachyury". Evolution & Development. 11 (5): 582–589. doi:10.1111/j.1525-142X.2009.00364.x. PMID   19754714. S2CID   6070504.
  10. Suzuki, T. G.; Ogino, K.; Tsuneki, K.; Furuya, H. (June 2010). "Phylogenetic Analysis of Dicyemid Mesozoans (Phylum Dicyemida) From Innexin Amino Acid Sequences: Dicyemids Are Not Related to Platyhelminthes". Journal of Parasitology. 96 (3): 614–625. doi: 10.1645/GE-2305.1 . PMID   20557208. S2CID   25877334.
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