An animal reduced to essentials
Inside the renal sacs of benthic octopuses and cuttlefish lives one of the simplest animals known, the Dicyemida. These organisms, also called Rhombozoa, have a body plan reduced to an absolute minimum. Adults consist of only 8 to 40 cells, a fixed number for each species in a condition known as eutely. This simple structure lacks respiratory, circulatory, digestive, and nervous systems. A single, long axial cell runs down the center, containing the reproductive potential of the animal. This is surrounded by a jacket of 20 to 30 ciliated outer cells which absorb nutrients directly from the host's urine.
The anterior end, or head region, is a specialized structure called a calotte. It functions as an anchor, attaching the parasite to the folded inner surfaces of the cephalopod's kidney. Despite their parasitic nature, dicyemids appear to cause little to no harm to their hosts. Over 140 species have been described, many of them specific to a single host species, and they are found in oceans around the globe, including the Mediterranean Sea.
A life of two forms
The life cycle of a dicyemid is complex, alternating between two distinct reproductive modes and body forms. The specific path depends on the age of its cephalopod host. In young, immature hosts, the parasite exists in an asexual stage called a nematogen. Nematogens produce worm-like, or vermiform, larvae within their axial cell. These larvae mature directly into more nematogens, rapidly populating the host's kidneys. The fully formed vermiform embryo consists of just 23 cells.
As the host matures, a change occurs. The nematogens begin to produce a sexual stage called a rhombogen. Rhombogens are hermaphroditic and contain a self-fertilizing gonad called an infusorigen. This process produces a completely different kind of larva: the infusoriform. These larvae are small, ciliated, and look somewhat like swimming headlights. An infusoriform embryo is more complex than its vermiform counterpart, consisting of 37 to 39 cells. These are the dispersal stage, released from the host into the sea to infect a new cephalopod, although the exact mechanism of infection remains unknown.
A phylogenetic puzzle
When first described in the 19th century, the dicyemids' simple structure led scientists to classify them as "Mesozoa," a proposed intermediate link between single-celled protozoans and multicellular metazoans (animals). For decades, their place in the tree of life was a subject of intense debate. Were they truly primitive animals, or something else entirely?
The advent of molecular phylogenetics has provided a clearer, though still debated, picture. Analysis of their genes suggests that dicyemids are not a primitive lineage. they are members of the Lophotrochozoa, a major group of animals that includes mollusks, annelids, and flatworms. This indicates that their simplicity is not an ancestral trait but a derived one. They likely evolved from a more complex, free-living ancestor and underwent a process of extreme simplification after adapting to a parasitic lifestyle. This "secondary simplification" involved losing the complex organs and systems they no longer needed while living inside a nutrient-rich host environment.