Mobilida is a
group of
parasitic or
symbiotic
Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...
peritrich ciliate
The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...
s, comprising more than 280 species.
Mobilids live on or within a wide variety of aquatic organisms, including
fish
A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
,
amphibian
Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...
s,
molluscs
Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...
,
cnidaria
Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...
ns,
flatworm
Platyhelminthes (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a Phylum (biology), phylum of relatively simple bilaterian, Segmentation (biology), ...
s and other ciliates, attaching to their
host organism by means of an
aboral adhesive disk.
Some mobilid species are
pathogen
In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...
s of wild or farmed fish, causing severe and economically damaging diseases such as
trichodinosis.
Morphology and Feeding
As the name suggests, mobilida cells are mobile, capable of moving about on the body of a host organism, and of swimming between hosts. This sets them apart from the predominantly
sessile peritrichs of the order Sessilida, such as
Vorticella and
Epistylis, which, during the feeding, or vegetative, phase of the life cycle remain attached to submerged surfaces, often by means of a stalk.
Like all peritrichs, the mobilids possess a spiral wreathe of
cilia
The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
running counterclockwise around the oral region (peristome), at the anterior of the cell. Ciliature on the body is restricted to a posterior wreathe called the "trochal band," made up of three rings of cilia girdling the aboral region of the cell. This trochal band is also found in sessilid peritrichs, where it is ciliated only during the swarmer (telotroch) phase of the cell's life, during which the organism can swim freely. In mobilid ciliates, the trochal band is a permanent feature of the cell.
Mobilids possess a conspicuous "adhesive disk" at the aboral (posterior) pole, enabling the organism to attach itself temporarily to its host organism. This disk is radially symmetrical and composed of interlocking curved denticles and associated fibres.
The perimeter of the disk can be contracted, allowing it to act as a sucker to hold the ciliate against the surface of its host while it feeds. Because the adhesive disk is a complex and variable structure, and clearly visible in the light microscope, it has been used by
taxonomists to differentiate between species and genera within Mobilida.
Mobilids typically feed
commensally on bacteria and organic debris surrounding their host, but can also consume
epithelial
Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...
cells and other cellular matter shed by the host itself.
Classification
The
order Mobilida was created in 1933 by
Alfred Kahl, and is usually placed, along with its
sister group
In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree.
Definition
The expression is most easily illustrated by a cladogram:
Taxon A and ...
Sessilida, within the
subclass Peritrichia.
Some molecular
phylogenetic
In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
studies, based mostly on
small subunit rRNA, have raised doubts that Mobilida and Sessilida are sister
taxa
In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...
, indicating that the groups belong to separate lineages within the class
Oligohymenophorea.
In 2009, on the basis of such findings, Zifeng Zhan and his collaborators removed the mobilids from Peritrichia and elevated them to the subclass Mobilia. More recently, in a revised classification of
Ciliophora drawing on both molecular and morphological data, researchers have reaffirmed the traditional grouping of Mobilida with Sessilida. A
phylogenomic study released in September, 2016 robustly supports the classical view that Mobilida and Sessilida are sister
clade
In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
s within a
monophyletic
In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria:
# the grouping contains its own most recent co ...
Peritrichia.
Five families are recognized within the order:
Leiotrochidae,
Polycyclidae,
Trichodinidae
Trichodinidae is a family of ciliates of the order Mobilida, class Oligohymenophorea. Members of the family are ectoparasites (or, alternatively, ectocommensals) of a wide variety of aquatic organisms, including fish, amphibians, hydrozoans, ...
,
Trichodinopsidae and
Urceolariidae.
References
{{Taxonbar, from=Q10609751
Oligohymenophorea
Ciliate orders