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Helkesida (formerly known as Sainouroidea) is a group of microorganism protists belonging to the supergroup Rhizaria, both discovered through phylogeny. It contains amoeboid flagellates with two flagella. They are either free-living, mostly on feces, or live inside the gut of animals. Among these amoebae, one lineage has independently evolved aggregative multicellularity similarly to slime moulds.
These organisms have an amorphous apical centrosome attached to the nucleus by a rhizoplast. The arises from 2–4 very short with dense fibrous roots that attach them to each other and to the nucleus. Their anterior flagellum is reduced to a stub without its 9+2 axoneme. The centrosome also generates numerous in larger cells. The dictyosome is seen attached to the nuclear membrane and the anterior rhizoplast. They have a microbody attached to the posterior end of the nucleus.
One helkesid genus, Guttulinopsis, represents an independent lineage in which aggregative multicellularity has evolution to generate "fungi-like" fruiting bodies called , similarly to slime moulds such as Dictyostelium.
Some host species can harbor different helkesid genera and species. One animal can be infected by multiple species simultaneously, and one species can also infect different animal hosts. More sampling of hosts, amoebae and molecular data is needed to better understand the life history and ecology of these protists.
The initial name for this group, Sainouroidea, had the -oidea suffix for superfamilies, but it was not assigned to any existing classes or orders due to the uncertainty of its phylogenetic position. In a 2018 revision, the class Helkesea and order Helkesida were created as a substitute for this name. Sainouroidea was then modified to only include one of the three helkesid families, Sainouridae. A second superfamily, Helkesimastigoidea, was created to host the remaining two families, Helkesimastigidae and Guttulinopsidae.
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