Acoelomorpha is a
subphylum
In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum.
The taxonomic rank of "subdivision (rank), subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists ...
of very simple and small
soft-bodied animals with
planula
A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species and also in some species of Ctenophores. Some groups of Nemerteans also produce larvae that are very similar to the planula, which ...
-like features which live in
marine
Marine is an adjective meaning of or pertaining to the sea or ocean.
Marine or marines may refer to:
Ocean
* Maritime (disambiguation)
* Marine art
* Marine biology
* Marine debris
* Marine habitats
* Marine life
* Marine pollution
Military
* ...
or
brackish water
Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estua ...
s. They usually live between grains of
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
, swimming as
plankton
Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...
, or crawling on other organisms, such as
algae
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
and
coral
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...
s.
[Cannon, L. R. G. (1986) ''Turbellaria of the World. A guide to families and genera''. Brisbane, Queensland Museum, 136 p.] With the exception of two acoel freshwater species, all known Acoelomorphs are marine.
Systematics
Etymology
The term "acoelomorph" derives from the
Ancient Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...
words (), the ''
alpha privative An alpha privative or, rarely, privative a (from Latin ', from Ancient Greek ) is the prefix ''a-'' or ''an-'' (before vowels) that is used in Indo-European languages such as Sanskrit and Greek and in words borrowed therefrom to express negation or ...
'', expressing negation or absence, (), meaning "cavity", and (), meaning "form". This refers to the fact that acoelomorphs have a structure lacking a fluid-filled body cavity.
Classification
![Nemertodermatida species](https://upload.wikimedia.org/wikipedia/commons/9/9d/Nemertodermatida_species.png)
The subphylum Acoelomorpha is divided into two classes. There are at least 408 described species, with a majority of these falling within the
Crucimusculata infraorder in Acoela.
*
Acoela
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 ...
comprise small flattened worms, classified into a dozen of families.
*
Nemertodermatida
Nemertodermatida is a class of Acoelomorpha, comprising 18 species of millimetre-sized turbellariform, mostly interstitial worms.
Taxonomy
The order ''Nemertodermatida'' contains two families with 6 genera. The high level of cryptic diversity ...
comprise millimetre-sized, mostly interstitial worms, distributed into two families.
Phylogeny
The soft bodies of acoelomorphs and the lack of some of the key bilaterian traits make them difficult to classify. Traditionally, based on
phenotypic features, acoelomorphs were considered to belong to the phylum
Platyhelminthes
The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a Phylum (biology), phylum of relati ...
, which was long seen as the sister group to all other
bilateria
The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...
n phyla.
[Conway-Morris, S.; George, J. D.; Gibson R.; Platt, H. M. (1985) ''The Origins and relationships of lower invertebrates''. Oxford, Clarendon Press, 397 p.] However, a series of molecular phylogenetics studies at the hinge between the 20th and 21st centuries demonstrated that they are fast-evolving organisms not closely related to platyhelminthes, therefore involving the
polyphyly
A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of converg ...
of flatworms.
Actually, Acoelomorpha appeared to constitute a separate,
deep-branching phylum, kingpin of bilaterian evolution.
Yet their evolutionary affinities remain enigmatic as they might be the sister-group either to all other bilateral animals
or to all deuterostomes.
Resolving this debate would indicate whether acoelomorphs are simple or simplified. If they are the sister group to
Bilateria
The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...
, it would point to a simple
body plan
A body plan, ( ), or ground plan is a set of morphological features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many.
This term, usually applied to animals, envisages a "blueprin ...
for the first bilaterian. Alternatively, if acoelomorphs are related to
deuterostomes
Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some exampl ...
, this would imply that their organisation is the result of secondary simplification.
![Xenoturbella japonica](https://upload.wikimedia.org/wikipedia/commons/3/3b/Xenoturbella_japonica.jpg)
In addition, comparative analyses of morphological, developmental, and molecular characters raised two points.
*
Xenoturbellida
''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.
The first known species (''Xenoturbella bocki'') was discovered in 1915 by Sixten Bock, but it wa ...
is the sister group to acoelomorphs, constituting the so-called
Xenacoelomorpha
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 (physical app ...
clade.
The close evolutionary relationship between Acoelomorpha and ''Xenoturbella'' is supported by the morphology (structure of epidermal
cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
), the embryology (direct development without a feeding larval stage
), and the concatenation of hundreds of proteins.
* The phylogenetic placement of Xenacoelomorpha among bilaterian animals is not yet well defined, despite increased taxon and gene sampling, (re)-analyses of published data sets, and use of more sophisticated
models of sequence evolution in
phylogenomic
Phylogenomics is the intersection of the fields of evolution and genomics. The term has been used in multiple ways to refer to analysis that involves genome data and evolutionary reconstructions. It is a group of techniques within the larger fields ...
studies. There is a conflict between two evolutionary hypotheses, with Xenacoelomorpha being the sister group to
Ambulacraria
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 ...
within
deuterostomes
Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some exampl ...
(i.e., the
Xenambulacraria
Xenambulacraria is a proposed clade of animals with symmetry (biology)#Bilateral symmetry, bilateral symmetry as an embryo, consisting of the Xenacoelomorpha (i.e., ''Xenoturbella'' and acoelomorphs) and the Ambulacraria (i.e., echinoderms and he ...
hypothesis) on the one hand,
and Xenacoelomorpha as sister group to all other bilaterians (i.e., the
Nephrozoa
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 (conventionally deut ...
hypothesis) on the other.
However, the Nephrozoa hypothesis might reflect methodological errors resulting from model violations in the phylogenomic inference.
Anatomy
![Symsagittifera roscoffensis(Jersey)](https://upload.wikimedia.org/wikipedia/commons/d/de/Symsagittifera_roscoffensis%28Jersey%29.jpg)
Acoelomorphs resemble flatworms in many respects, but have a simpler anatomy, not even having a gut. Like flatworms, they have no circulatory or respiratory systems, but they also lack an excretory system. They lack body cavities (acoelomate structure), a hindgut or an anus.
The
epidermal cells
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water relea ...
of acoelomorphs are unable to proliferate, a feature that is only shared with
rhabditophora
Rhabditophora (from ''rhabdito''-, rhabdite + Greek -φορος ''phoros'' bearer, i.e., "rhabdite bearers") is a class of flatworms. It includes all parasitic flatworms (clade Neodermata) and most free-living species that were previously groupe ...
n flatworms and was for some time considered a strong evidence for the position of Acoelomorpha within Platyhelminthes. In both groups, the epidermis is renewed from mesodermal stem cells.
The
nervous system
In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...
of acoelomorphs is formed by a set of longitudinal nerve bundles beneath the ciliated epidermis. Close to the anterior end, these bundles are united by a ring commissure, but do not form a true brain, although it is hypothesized that such organization was the precursor of the
cephalization
Cephalization is an evolutionary trend in which, over many generations, the mouth, sense organs, and nerve ganglia become concentrated at the front end of an animal, producing a head region. This is associated with movement and bilateral symmetry, ...
of the nerve system in more derived bilaterians.
After decapitation, such a "brain" (rather, a ''cerebroid ganglion'') regenerates in a few weeks.
The sensory organs include a
statocyst
The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, ctenophorans, echinoderms, cephalopods, and crustaceans. A similar structure is also found in ''Xenoturbella''. The statocyst cons ...
– which presumably helps them orient to gravity –, and, in some cases,
ancestral
An ancestor, also known as a forefather, fore-elder or a forebear, is a parent or (recursively) the parent of an antecedent (i.e., a grandparent, great-grandparent, great-great-grandparent and so forth). ''Ancestor'' is "any person from whom ...
pigment-spot
ocelli
A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...
capable of detecting light.
Acoelomorphs are simultaneous
hermaphrodite
In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes.
Many Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...
s, but have no
gonad
A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...
s and no ducts associated with the female reproductive system. Instead,
gamete
A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...
s are produced from the
mesenchymal
Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every o ...
cells that fill the body between the epidermis and the digestive vacuole.
References
External links
Phylogeny of Lower Worms of the Meiofauna (Acoelomorpha)Acoelomorpha at the Turbellarian taxonomic databaseAcoelomorpha: A Phylogenetic Headache Earthling Nature'
{{Taxonbar, from=Q131486
Animal subphyla