Sex is Weird: Rhombozoans

The Rhombozoan Dicyema macrocephalum . Image credit: Brockhaus and Efron Encyclopedic Dictionary, Wikimedia Commons.

The Rhombozoan Dicyema macrocephalum Image credit: Brockhaus and Efron Encyclopedic Dictionary, Wikimedia Commons.

Today our exploration of strange sex takes us into the vast wilderness of cephalopod kidneys. Within this renal wonderland there is a phylum (or maybe a class) of parasitic animals known as rhombozoans. Scientists know very little about these worm-like creatures (including how they get into the kidneys in the first place), but they are certain about one thing regarding their method of reproduction: it’s really weird.

As usual, I’ll start with a bit of basic biology. Rhombozoans are vermiform (worm-shaped) animals that are covered in cilia. Each individual of a particular species has the same number of body cells as the next individual (we call this eutely). One of those cells, the axial cell, is much larger than the rest, and this serves as the reproductive center of the animal. Both asexual and sexual reproduction happens inside this cell.

Top: Mature nematogen.  Bottom: Developing nematogen (outlined in yellow) inside the axial cell of a mature individual. Note: all specimens were stained pink to enhance detail. Image credit: Seth Goodnight

I’ll start with the asexual stages. The asexual individual is called a nematogen. Within its axial cell, the next generation of nematogens (up to 100 at a time in some species) develops from stem cells. These nematogens are clones of the original. Once they are released they go on to produce more nematogens in the same manner. The process allows them to quickly fill the entire kidney very quickly. The next stage is the development and release of rhombogens, probably triggered by the nematogens reaching a certain density. This is where the fun happens.

Top: Mature rhombogen.  Bottom: Mature infusorigens (outlined in yellow) inside the axial cell of a mature individual. Note: all specimens were stained pink to enhance detail. Image credit: Seth Goodnight

The rhombogen, which is only around for one “generation”, asexually produces a third type of individual called an infusorigen. Whereas all of the other stages are vermiform, the infusorigen is more like a ciliated ball. It also doesn’t leave the axial cell of its “parent”. Really it’s just a hermaphroditic gonad. It produces both eggs and sperm and self-fertilizes. The result of the fertilization is called an infusiform larva (because what these things need is another type of individual). The larvae are then able to exit the host and infect a new one. At least that’s the theory. No one has even seen one outside of the host, and (to my knowledge at least) no one has done the research to see how new infections occur.

At this point I want to draw your attention back to the really weird bits. Sexual reproduction happens inside the axial cell of the rhombogen. The adults (infusorigens) live and mate inside a juvenile (rhombogen). Oh yeah, and the infusorigens only self-fertilize. All of that trouble to create sexually reproducing individuals (four different growth forms), and they don’t even exchange DNA with one another (which is really the whole point of sex in the first place). What sort of evolutionary process caused them to lose their ability to mate with other individuals? Do they gain any benefit from maintaining the infusorigen life stage? Maybe one day a scientist will find the answers to these questions. Then again, maybe not.

References

 

Pechenik, J. A. 2005. Biology of the Invertebrates. McGraw-Hill, Higher Education.