Dicyemida: Leading a double life

It always amazes me that some of the simplest animals have such complex life cycles; they make the reproductive strategies of humans pretty yawn-worthy. In focus today: the ultra-fascinating Dicyemida.

This is a very small group of invertebrates, part of what is known as the Mesozoa. They live as parasites within the kidneys of cephalopods. What is most interesting to me is that they have two different forms of larvae, one that reproduces sexually, the other asexually, and environmental/social cues appear to determine which larvae is produced at any given time.

It seems that the "default" form for dicyemids is the nematogen, which is a group of ciliated exterior cells surrounding the crucial center axial cell, which contains structures known as axoblasts. These axoblasts develop into asexual larvae, also known as vermiform larvae. These larvae will squeeze out of the nematogen, and go on to infect a new host and then become nematogens themselves, and continue the asexual life cycle.

If, however, the population density of the nematogens within the host reaches a certain critical density, the axoblasts begin to develop into a different type of individual. In this case, they become an infusorigen, which is a sexual cell that is retained within the axial cell of the nematogen. To review, because these are a lot of words and it's hard to picture, the axoblasts were in the center axial cell of the nematogen, and that is where the infusorigen remains, like a stacking doll of individuals, with the infusorigen inside the nematogen.

So, if the sexual infusorigen is all dressed up with nowhere to go, what good does it do? The cell is hermaphroditic (it is essentially just a little ciliated structure consisting of eggs cells surrounding a mass of sperm), and it self-fertilizes within its home in the axial cell. This produces a second form of larvae, known as the infusiform larvae. This form exits the cell, just like the vermiform, and then goes on to infect a new host, where it will attach and transform into a nematogen, restarting the entire cycle.

I wish I could illustrate this, but haven't been able to find good pictures/diagrams for the cycle. To summarize, dicyemids produce different forms of larvae depending on population density: asexual vermiform larvae when populations are low, and sexual infusiform larvae once density reaches a threshold level.

This raises a couple of thoughts worth kicking around. Why produce sexual forms when population density is high, if those sexual infusorigens are self-fertilizing anyway? It's not as if they were waiting to be assured mates were plentiful, because they don't mate with each other. It appears that it could be related to dispersal: when populations of a parasite are getting high within a host, it could be to the parasite's advantage to disperse to a new host to avoid crowding. But why must the dispersing larvae (infusiform) be produced from sex, what advantage does that confer to it that the vermiform larvae are lacking?

There isn't much information out there on this subject, but it is definitely interesting to think about. Parasitic animals have all kinds of amazingly creative lifestyles, it's a fascinating area of study.

(Picture credit to BIODIDAC)