Rhombozoa, or
Dicyemida, is a phylum of tiny parasites that live in the
renal appendagesThe kidneys are paired organs, which have the production of urine as their primary function. Kidneys are seen in many types of animals, including vertebrates and some invertebrates. They are part of the urinary system, but have several secondary functions concerned with homeostatic functions. ...
of
cephalopod{Taxobox| name = Cephalopods| fossil_range = | image = Tafel 054 300.jpg| image_caption = A variety of cephalopod forms from Ernst Haeckel's 1904 Kunstformen der Natur| regnum = Animalia| image_width = 220px| phylum = Mollusca| classis = Cephalopoda...
s. Although the name
Dicyemida precedes
Rhombozoa in usage, and is preferred by most contemporary authors,
Rhombozoa still enjoys much popular support.
Classification is controversial. Traditionally, dicyemids have been grouped with 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:...
in the
MesozoaThe Mesozoa are enigmatic, minuscule, worm-like parasites of marine invertebrates. It is still unclear as to whether they are degenerate platyhelminthes or truly-primitive, basal metazoans. Generally, these tiny, elusive creatures consist of a somatoderm of ciliated cells surrounding one or...
; however, molecular phylogenies indicate that dicyemids may be more closely related to the Platyhelminthes (flatworms).
Dicyemids are divided into two families, Conocyemidae and Dicyemidae.
Adult dicyemids range in length from 0.1-9.0mm, and they can be easily viewed through a light microscope.
Rhombozoa, or
Dicyemida, is a phylum of tiny parasites that live in the
renal appendagesThe kidneys are paired organs, which have the production of urine as their primary function. Kidneys are seen in many types of animals, including vertebrates and some invertebrates. They are part of the urinary system, but have several secondary functions concerned with homeostatic functions. ...
of
cephalopod{Taxobox| name = Cephalopods| fossil_range = | image = Tafel 054 300.jpg| image_caption = A variety of cephalopod forms from Ernst Haeckel's 1904 Kunstformen der Natur| regnum = Animalia| image_width = 220px| phylum = Mollusca| classis = Cephalopoda...
s. Although the name
Dicyemida precedes
Rhombozoa in usage, and is preferred by most contemporary authors,
Rhombozoa still enjoys much popular support.
Classification is controversial. Traditionally, dicyemids have been grouped with 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:...
in the
MesozoaThe Mesozoa are enigmatic, minuscule, worm-like parasites of marine invertebrates. It is still unclear as to whether they are degenerate platyhelminthes or truly-primitive, basal metazoans. Generally, these tiny, elusive creatures consist of a somatoderm of ciliated cells surrounding one or...
; however, molecular phylogenies indicate that dicyemids may be more closely related to the Platyhelminthes (flatworms).
Dicyemids are divided into two families, Conocyemidae and Dicyemidae.
Structure
Adult dicyemids range in length from 0.1-9.0mm, and they can be easily viewed through a light microscope. Dicyemids display
eutelyEutelic organisms have a fixed number of cells when they reach maturity, the exact number being constant for any one species. Development proceeds by cell division until maturity; further growth occurs via cell enlargement only....
, a condition in which each adult individual of a given species has the same number of cells, making cell number a useful identifying character. The organism's structure is simple: a single axial cell is surrounded by a jacket of ciliated cells. The anterior region of the organism is termed a "
calotteCalotte may refer to:*Calotte, in architecture, a round cavity or depression, in the form of a cup or cap, lathed and plastered; used to diminish the rise or elevation of a moderate chapel, cabinet, alcove, etc, which, without such an expedient, would be too high for other pieces of the apartment...
" and functions to attach the dicyemid to folds on the surface of its
hostIn biology, a host is an organism that harbors a parasite , or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...
's
renal appendagesThe kidneys are paired organs, which have the production of urine as their primary function. Kidneys are seen in many types of animals, including vertebrates and some invertebrates. They are part of the urinary system, but have several secondary functions concerned with homeostatic functions. ...
.
Life cycle
Dicyemids exist in both
asexualAsexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. Only one parent is involved in asexual reproduction. A more stringent definition is agamogenesis which refers to reproduction without the fusion of gametes...
and
sexualSexual reproduction is characterized by processes that pass a combination of genetic material to offspring, resulting in diversity. The main two processes are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration...
forms. The former predominate in juvenile and immature hosts, and the latter in mature hosts. The asexual stage is termed a
nematogen; it produces
vermiform larvaA larva is a young form of animal with indirect development, going through or undergoing metamorphosis ....
e which mature through direct development to form more nematogens. Nematogens proliferate in young cephalopods, filling the kidneys.
As the infection ages, perhaps as the nematogens reach a certain density, vermiform larvae mature to form
rhombogens, the sexual life stage, rather than more nematogens. This sort of density-responsive reproductive cycle is reminiscent of the asexual reproduction of sporocysts or rediae in larval trematode infections of
snailThe word snail is a common name for almost all members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word snail is used in a general sense, it includes sea snails, land snails and freshwater snails. Snails lacking a shell or having only a very small one are...
s. As with the trematode asexual stages, a few nematogens can usually be found in older hosts. Their function may be to increase the population of the parasite to keep up with the growth of the host.
Rhombogens contain hermaphroditic
infusorigens which produce
infusoriform larvae. These larvae possess a very distinctive morphology, swimming about with ciliated rings that resemble headlights. It has long been assumed that this sexually produced infusoriform, which is released when the host eliminates urine from the kidneys, is both the dispersal and the infectious stage. The mechanism of infection, however, remains unknown, as are the effects, if any, of dicyemids on their hosts.
Some part of the dicyemid life cycle may be tied to
temperateIn geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally mild, rather than extreme hot or cold. But in continental areas, such as central North America the variations between summer...
benthic environments, where they occur in greatest abundance. While dicyemids have occasionally been found in the tropics, the infection rates are typically quite low, and many potential host species are not infected. Dicyemids have never been reported from truly oceanic cephalopods, who instead host a parasitic
ciliateThe ciliates are a group of protists characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...
fauna. Most dicyemid species are recovered from only one or two host species. While not strictly host specific, most dicyemids are only found in a few closely related hosts.