Dicyemida, also known as Rhombozoa, is a phylum of tiny parasites that live in the renal appendages of cephalopods.
Taxonomy
thumb|200px|[[Dicyema macrocephalum]]
Classification is controversial. Traditionally, dicyemids have been grouped with the Orthonectida in the phylum Mesozoa and, from 2017, molecular evidence appears to confirm this.
However, other molecular phylogenies have placed the dicyemids more closely related to the roundworms. Additional molecular evidence suggests that this phylum is derived from the Lophotrochozoa.
The phylum (or class if retained within Mesozoa) contains three families, Conocyemidae, Dicyemidae and Kantharellidae, 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 and unambiguous name to cover all three families.
Anatomy
Adult dicyemids range in length from , and they can be easily viewed through a light microscope. 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. 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 dicyemid species, their body will likely be found to contain organisms with a variety of calotte shapes, which means they are infected with multiple different species. 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.
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. and many potential host species are not infected. Dicyemids have never been reported from truly oceanic cephalopods, who instead host a parasitic ciliate 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.