| Dicyemida | |
|---|---|
 | |
| Photomicrograph of Dicyema japonicum | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Subkingdom: | Eumetazoa |
| Clade: | ParaHoxozoa |
| Clade: | Bilateria |
| Clade: | Nephrozoa |
| (unranked): | Protostomia |
| (unranked): | Spiralia |
| Clade: | Platytrochozoa |
| (unranked): | Mesozoa |
| Phylum: | Dicyemida |
| Class: | Rhombozoa |
Dicyemida, also known as Rhombozoa, is a phylum of tiny parasites that live in the renal appendages of cephalopods.
Classification is controversial. [1] Traditionally, dicyemids have been grouped with the Orthonectida in the phylum Mesozoa and, from 2017, molecular evidence [2] [3] appears to confirm this.
However, other molecular phylogenies have placed the dicyemids more closely related to the roundworms. [4] Additional molecular evidence suggests that this phylum is derived from the Lophotrochozoa. [5] [6]
The phylum (or class if retained within Mesozoa) contains three families, Conocyemidae, Dicyemidae and Kantharellidae, [7] which have sometimes been further grouped into orders. Authors who treat Dicyemida as an order and separate the family Conocyemidae into a different order (Heterocyemida) prefer 'Rhombozoa' as a more inclusive name for the phylum or class. [3] [4] [8] [9]
Adult dicyemids range in length from 0.5 to 7 millimetres (0.020 to 0.276 in), and they can be easily viewed through a light microscope. [10] They display eutely, a condition in which each adult individual of a given species has the same number of cells, making cell number a useful identifying character. Dicyemida lack respiratory, circulatory, excretory, digestive, and nervous systems.
The organism's structure is simple: a single axial cell is surrounded by a jacket of twenty to thirty ciliated cells. The anterior region of the organism is termed a calotte and functions to attach the parasite to folds on the surface of its host's renal appendages. [10] When more than one species of dicyemida exist within the same host, they have distinctly shaped calottes, which range in shape from conical to disk shaped, or cap shaped.
To this day, there has never been a recorded case of two separate species of dicyemida existing in the same host and having exactly the same calotte. [3] Species that share similar or even identical calottes have been found on occasion, but have never been found within the same host. Because of the constant variation in calotte size between species (even within one given host) there is very rarely observable competition between the multiple Dicyemida species for habitat or other resources. [5] Calotte shape determines where a dicyemid can comfortably live. In general, dicyemida with conical shaped calottes fit best within the folds of the kidneys, while those with rounded calottes (disk or cap shaped) are more easily able to attach to the smooth surfaces of the kidneys. [5] This extreme segregation of habitats allows multiple species of dicyemids to comfortably exist within the same host while not still competing for space or resources (by occupying different ecological niches).
While most dicyemid species have been found to prefer to live within specific cephalopods, no one species is unique in their preferences. [11] In fact, It is also almost unheard of that a host infected with a dicyemid is only infected with one species. This means that if a select cephalopod is found to be infected with one species Dicyemid, their body will likely be found to contain organisms with a variety of calotte shapes, which means they are infected with multiple different species. [12] On the occasion that similar (but not identical) calotte shapes happen to be present within one host’s body, one species usually ends up dominating the other, indicating that it has adapted more readily to the environment within the host. [12] However, this occurrence is very rare and has only been observed a handful of times. In a study done on octopi, it was found that Dicyemida that had similarly shaped calottes rarely coexisted in the same individual host, which suggested a strong level of competition for habitat. [13]
In Japan, two types of dicyemid parasites, D. misakiense and D. japonicum, have often been discovered living in the same host. In 1938, when the two species were initially discovered, scientists did not classify them as separate species due to their large amount of morphological similarities. In fact, the only difference between the two species that scientists were able to observe was between the shape of their calottes.The idea that D. misakiense and D. japonicum are two different species is still very controversial among scientific groups. Some scientists have speculated that when closely related species of dicyemids coexist in the same region, such as in the case of D. misakiense and D. japonicum, competition for habitat causes them to evolve to develop two distinct calotte shapes. [12]
Dicyemids exist in both asexual and sexual 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 larvae within the axial cell. These mature through direct development to form more nematogens. [10] 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 snails. 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 gonads developed within the axial cell. These gonads, more correctly termed infusorigens, self-fertilise to 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. [10]
Some part of the dicyemid life cycle may be tied to temperate benthic environments, where they occur in greatest abundance[ citation needed ]. While dicyemids have occasionally been found in the tropics, the infection rates are typically quite low, [14] [15] and many potential host species are not infected. Dicyemids have never been reported from truly oceanic cephalopods, who instead host a parasitic ciliate fauna[ citation needed ]. 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[ citation needed ].
The Apicomplexa are organisms of a large phylum of mainly parasitic alveolates. Most possess a unique form of organelle structure that comprises a type of non-photosynthetic plastid called an apicoplast—with an apical complex membrane. The organelle's apical shape is an adaptation that the apicomplexan applies in penetrating a host cell.
The Mesozoa are minuscule, worm-like parasites of marine invertebrates. Generally, these tiny, elusive creatures consist of a somatoderm of ciliated cells surrounding one or more reproductive cells.
Plasmodium is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of Plasmodium species involve development in a blood-feeding insect host which then injects parasites into a vertebrate host during a blood meal. Parasites grow within a vertebrate body tissue before entering the bloodstream to infect red blood cells. The ensuing destruction of host red blood cells can result in malaria. During this infection, some parasites are picked up by a blood-feeding insect, continuing the life cycle.
Trematoda is a class of flatworms known as flukes or trematodes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is usually a snail. The definitive host, where the flukes sexually reproduce, is a vertebrate. Infection by trematodes can cause disease in all five traditional vertebrate classes: mammals, birds, amphibians, reptiles, and fish.
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.
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.
Amphioctopus fangsiao, called webfoot octopus, is a species of octopus, a cephalopod belonging to the genus Amphioctopus. It is found in the Pacific Ocean, including off the coasts of New Zealand as well as in the Yellow Sea and surrounding Chinese shores. It is also commercially fished.
Aggregata is a genus of parasitic alveolates belonging to the phylum Apicomplexa.
The Dicyemidae is a family of tiny parasites that live in the renal appendages of cephalopods. It contains the following genera and species:
Chromidina is a genus of apostome ciliates of the family Opalinopsidae. Species of Chromidina are parasitic in the renal and pancreatic appendages of cephalopods.
Chromidina elegans is a species of parasitic ciliates. It is a parasite of the cuttlefish Sepia elegans.
Chromidina elegans is a species of ciliates, described in 2016. It is parasitic in the kidney appendages of the cuttlefish Loligo vulgaris. The type-locality is off Tunisia in the Mediterranean Sea.
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.
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).
Opisthoteuthis depressa, also known as the Japanese flapjack octopus, is an octopus found in waters near Japan.
Kantharellidae is a family of worms belonging to the class Rhombozoa, order unassigned. The family consists of only one genus: KantharellaCzaker, 1994. The only species in the genus is Kantharella antarctica. This species is most closely related to other species of the order Dicyemida, which only has one other family, Dicyemidae.
Dicyema is a genus of worms belonging to the family Dicyemidae.
Dicyemennea is a genus of worms belonging to the family Dicyemidae.
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.
{{cite journal}}: Cite journal requires |journal= (help) in Kinne, Otto. Diseases of Marine animals. Vol. 3. Hamburg: Biologische Anstalt Helgoland. pp. 47–202. 
Data related to Rhombozoa at Wikispecies