Rhombozoa

Rhombozoa are a phylum of small, multicellular animals that are parasitic. They were formerly classified under the phylum Mesozoa, but are now recognized as distinct due to significant morphological and life history differences.1

Rhombozoans, such as the genus Pseudicyema, are found parasitizing the kidneys of squids and octopuses. Their bodies are simple, consisting of two cell layers with a limited number of cells, and they lack complex organ systems like digestive, respiratory, nervous, and excretory systems.1

Their life cycle is complex, involving two reproductive phases within the host. This includes asexual reproduction to increase the parasite population and sexual reproduction that produces ciliated larvae, though the method of infecting new hosts remains unclear. Rhombozoans are widely distributed in shallow marine environments where their hosts are found.1