You know how people can be going along, minding their own business, and then they see some cute big-eyed puppy and they go “Awwwww,” and their hearts melt, and then it’s all a big sloppy mushfest? I felt that way the other day, as I was meandering down some obscure byways of the developmental biology literature, and discovered the dicyemid mesozoa … an obscure phylum which I vaguely recall hearing about before, but had never seriously examined. After reading a few papers, I have to say that these creatures are much more lovable then mere puppy dogs. Look at this and say “Awwwww!”
O dicyemid mesozoan, how do I love thee? Let me count the ways.
The dicyemids are all parasites: specifically, parasites that live in the renal sacs of benthic (primarily) cephalopods. They take up residence inside the kidneys of cephalopods, attaching themselves to the renal tubules or the crypts (folds) of the sac with a specially shaped set of anterior cells called the calotte, and then they live there, bathed in octopus urine, from which they extract all the nutrients they need.
It’s a beautifully specific lifestyle in a very narrow environment; they occupy a niche that is populated only by other dicyemid mesozoans.
That niche is a kind of natural laboratory for evolutionary experimentation. The different species (there are over 100 total known so far) each vary in the shape and size of the calotte. There is of competition between the different species for different sites of attachment to the renal sac, and species with similar calottes are not found coexisting in a single sac, except when certain other species are also present. It’s like there’s a whole simple ecosystem flourishing in octopus urine, jostling about in some fascinating interactions.
They have a wonderfully intricate life cycle, and the name comes from the fact that the adults come in two forms, the nematogen and the rhombogen. This is not analogous to two sexes: the nematogen reproduces asexually by creating vermiform embryos, while the rhombogen makes a different kind of embryo, the infusiform.
The vermiform embryo (which is small, perhaps a few hundred micrometers long) grows up to be a larger nematogen or rhombogen, which are as much as a few millimeters long. The nematogen incubates more vermiform embryos in the cytoplasm of the axial cell, a characteristic long cell around which peripheral cells are arrayed. It releases these embryos into the renal sac; this is the tactic to rapidly populate a host with progeny. Under certain conditions, such as crowding, some of the vermiform embryos develop into rhombogens, which are functionally just like the nematogens, but differ in their reproductive apparatus. Rhombogens build infusorigens inside their axial cells, which produce haploid gametes that fuse to create the infusoriform embryos. The infusoriform embryos are the autonomous form that are not dependent on parasitizing cephalopods, and which are freed as the octopus urinates to swim about and seek out new hosts. Once they settle in to a new renal sac, they restart the whole process.
Dicyemids have a very simple body plan. They are eutelic, which means that the adult form has a species-specific number of cells, which varies between 10 and 40. You can see the body plan in the diagram above: a central axial cell which forms the elongated, wormlike shape of the animal and contains the reproductive cells; a cap of cells called the calotte which is responsible for attachment; and a surrounding set of ciliated cells that have the job of absorbing nutrients from the urine. That’s it; that’s all it takes.
These animals have gorgeous embryos with species-specific patterns of cell division; individual cells can be named and have specific developmental fates. This is a pattern we also see in another classic model system, the nematode.
Did I mention that they have species-specific patterns of development? Yes, I did, and it’s worth mentioning again. The lineages of the different forms can be followed, and what’s seen are divergences generated by varying the number of cell divisions — that is, these animals have an evolutionary history of generating novel forms by a relatively simple process of changing the regulation of embryonic mitoses.
Now here’s the part that really tickles the imagination: you might be wondering where these strange creatures came from. The cells are linked together by classic gap junctions; they produce the extracellular matrix molecules fibronectin, laminin, and collagen; it even has an Antp-like Hox-like gene, a member of a family not found in cnidarians; the evidence suggests that these beasties are descended from fairly elaborate ancestors. Comparison of 18S rDNA shows the dicyemids nested within the triploblastic metazoa, with some fuzzy results (we’re talking looooong branches here) suggesting affinities with the lophotrochozoans, like the flatworms.
What all that means is that the ancient ancestor of the dicyemids was probably a full triploblast, an animal with three embryonic tissue layers (endoderm, ectoderm, and mesoderm) and some kind of coelom, or body cavity. This is a lineage that threw all that away, and stripped itself down to a bare minimum, a single axial cell surrounded by an absorptive layer of cells. That’s all it needed to thrive in a very confined environment.
After saying how much I like these wonderful little creatures, though, I have to make a confession: there isn’t a single organism you can’t come to love if you get to know it well enough. That’s one of the joys of biology, that there is this never ending stream of fascinating characters, every one of them a star.
Furuya H, Hochberg FG, Tsuneki K (2001) Developmental patterns and cell lineages of vermiform embryos in dicyemid mesozoans. Biol Bull 201:405-416.
Furuya H, Hochberg FG, Tsuneki K (2003) Reproductive traits in dicyemids. Marine Biol 142:693-706.
Furuya H, Tsuneki K (2003) Biology of dicyemid mesozoans. Zoolog Sci. 20(5):519-32.
Katayama T, Wada H, Furuya H, Satoh N, Yamamoto M (1995) Phylogenetic position of the dicyemid mesozoa inferred from 18S rDNA sequences. Biol Bull 189:81-90.