The
Mesozoa are enigmatic, minuscule,
wormThe term worm is used to describe many different distantly-related animals which have a long cylindrical body and no legs.Most animals called "worms" are invertebrates, but the term is also used for the amphibian caecilians and the slow worm Anguis, a legless burrowing lizard...
-like parasites of marine invertebrates. It is still unclear as to whether they are degenerate platyhelminthes (
flatwormThe flatworms, known in scientific literature as Platyhelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s) or truly-primitive, basal metazoans. Generally, these tiny, elusive creatures consist of a somatoderm (outer layer) of ciliated cells surrounding one or more reproductive cells. Decades ago, Mesozoa was classified as a phylum.
The
Mesozoa are enigmatic, minuscule,
wormThe term worm is used to describe many different distantly-related animals which have a long cylindrical body and no legs.Most animals called "worms" are invertebrates, but the term is also used for the amphibian caecilians and the slow worm Anguis, a legless burrowing lizard...
-like parasites of marine invertebrates. It is still unclear as to whether they are degenerate platyhelminthes (
flatwormThe flatworms, known in scientific literature as Platyhelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s) or truly-primitive, basal metazoans. Generally, these tiny, elusive creatures consist of a somatoderm (outer layer) of ciliated cells surrounding one or more reproductive cells. Decades ago, Mesozoa was classified as a phylum. But
molecularA molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense...
phylogeny studies have shown that the mysterious
mesozoans are polyphyletic. That is, they consist of at least two unrelated groups.
As a result of these recent findings in
molecular biologyMolecular biology is the study of biology at a molecular level. The field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
, the label
mesozoan is now often applied informally, rather than as a formal
taxon|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement. Defining what belongs or does not belong to such a...
. Some workers previously classified Mesozoa as the sole
phylumIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
of the lonely subkingdom
AgnotozoaAgnotozoa is the name of a taxon of simple animals. The name first appeared in an invertebrate paleontology book as one of the "branches" of the subkingdom Metazoa. The branch contained only one group: Mesozoa....
.
Wikispecies places the mesozoans in kingdom protista.
In the 19th century, the Mesozoa were a wastebasket taxon for multicellular organisms which lacked the
invaginatingInvagination means to fold inward or to sheath. In biology, this can refer to a number of processes.*Invagination is the morphogenetic processes by which an embryo takes form, and is the initial step of gastrulation, the massive reorganization of the embryo from a simple spherical ball of cells,...
gastrula which was thought to define the Metazoa.
Evolution
Mesozoa were once thought to be
evolutionIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
ary intermediate forms between Protozoans and Metazoans, but now they are thought to be degenerate or simplified metazoa. Their ciliated larva are similar to the miracidium of trematodes, and their internal multiplication is similar to what happens in the sprocysts of trematodes. Mesozoan
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
has a low
GC-contentGC-content , in molecular biology, is the percentage of nitrogenous bases on a DNA molecule which are either guanine or cytosine . This may refer to a specific fragment of DNA or RNA, or that of the whole genome...
(40%). This amount is similar to ciliates, but ciliates tend to be binucleate. Others relate mesozoa to a group including
annelidThe annelids, collectively called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches. They are found in marine environments from tidal zones to hydrothermal vents, in freshwater, and in moist terrestrial environments...
s, planarians, and nemerteans.
Groupings
The two main mesozoan groups are the
RhombozoaRhombozoa, or Dicyemida, is a phylum of tiny parasites that live in the renal appendages of cephalopods. Although the name Dicyemida precedes Rhombozoa in usage, and is preferred by most contemporary authors, Rhombozoa still enjoys much popular support.Classification is controversial...
and the
OrthonectidaOrthonectida 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.-Characteristics:...
.
Other groups sometimes included in the Mesozoa are the Placozoa and the
MonoblastozoaMonoblastozoa was granted the title of phylum after the recognition that Mesozoa was too diverse to be a phylum unto itself.-Species:Only one species have been described, Salinella salvae. Discovered in 1892 by J. Frenzel in the saltpans of Argentina. Hasn't been found since .More organised than...
.
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.
As described, the animal had only a single layer of tissue.
Rhombozoan mesozoans
RhombozoaRhombozoa, or Dicyemida, is a phylum of tiny parasites that live in the renal appendages of cephalopods. Although the name Dicyemida precedes Rhombozoa in usage, and is preferred by most contemporary authors, Rhombozoa still enjoys much popular support.Classification is controversial...
, or dicyemid mesozoans, are found in the nephridia of cephalopods (squid and octopuses). 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.
Orthonectid mesozoans
OrthonectidaOrthonectida 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.-Characteristics:...
are found in the body spaces of various marine invertebrates including tissue spaces, gonads, genitorespiratory bursae. This pathogen causes host castration of different species.
The best known of Orthonectida is the parasite of brittle stars. The
multinucleateMultinucleate cells have more than one nucleus per cell, which is the result of nuclear division not being followed by cytokinesis. As a consequence, multiple nuclei share one common cytoplasm. This can be the consequence of a disturbed cell cycle control Multinucleate (also multinucleated,...
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 monoecious (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 way to another star.