I’ve had a new gig for a couple of months now, lecturing in invertebrate biology and ecology at the University of the Sunshine Coast. As such, I’m rediscovering all kinds of information that I first learned many years ago, including lots of stuff about the ‘small’ invertebrate phyla. These are groups that are small in number not in body size, represented by relatively few taxa, and often overlooked because of the sheer magnitude of taxa in the other, ‘bigger’ phyla.
Everyone knows insects and their relatives – spiders, crabs, millipedes etc. Everyone knows snails, octopuses, sea stars, jellyfish, and, thanks to Finding Nemo, anemones. But what about the little groups? Who knows about the gastrotrichs, the bryophytes and the chaetognaths? Therefore, over the coming weeks I’m going to celebrate some of the smaller groups. I thought I’d start with one of the weirdest.
Dicyemids are small, worm-like metazoans. Approximately 112 species of dicyemids have been described, and they live as parasites in renal sacs of cephalopods (Catalano 2012). The body plan of the Dicyemida is quite simple, with a total number of cells ranging from 8-40 (depending on the species), with the ciliated peripheral cells arranged in a kind of spiral formation around a central elongated axial cell (Catalano 2012, Suzuki et al. 2010). In essence, they wear their peripheral cells like a jacket around their axial cell. Their ‘head’ is a calotte, where they use the slightly differently-shaped cells at the anterior end to attach to the renal lining of their host (Suzuki et al. 2010). Dicyemids have no differentiated organs, nor any body cavities.
While the body plan of dicyemids is extremely simple, their reproductive biology is, by contrast, fairly mind-boggling. Dicyemids have a complicated sexual/asexual reproductive cycle, involving two kinds of morphotypes as represented in the following diagram (from Furuya and Tsuneki 2003):
Life cycle of dicyemids. Abbreviations: A apical cell, AG agamete, An axial cell nucleus, AX axial cell, C calotte, DI developing infusiform embryo, DP dipolar cell, DV developing vermiform embryo, IN infusorigen, MP metapolar cell, PA parapolar cell, PP propolar cell, UP uropolar cell.
All of the reproductive activity of dicyemids occurs in the ctyoplasm of the axial cell. Essentially, what happens is this:
- The nematogen is produced asexually, from vermiform embryos. Nematogens can continue to produce vermiform embryos and cycle along like this.
- However, vermiform embryos can also form hermaphrodite rhombogens (instead of nematogens), which produce haploid gametes that are fertilised internally.
- Sexually-produced embryos arising from this are called ‘infusiform’ and they leave their parent and are free-living in the sea until such time as they find an octopus kidney to settle down in. Whereupon they produce gametes asexually and vermiform embryos are formed. Precisely how they do that, however, is not known.
The reason for the asexual/sexual switch is not known. It is speculated, however, since host opportunities are relatively rare, that asexual reproduction in the same host site will enable the dicyemid population to survive. The trigger for sexual reproduction to take over as the dominant mechanism is not known, but may be linked to density of dicyemids in the host tissue (Furuya and Tsuneki 2003).
Dicyemids are frequently host-specific, and most likely evolved from free-living ancestors. Hence, their asexual reproductive cycle is probably an adaptation to living as a parasite (Furuya and Tsuneki 2003). In terms of phylogeny, dicyemids have been assumed previously to related to platyhelminths, presumably because of their worm-like morphology and parasitic ecology. However,more recently, dicyemids have been found to be more closely related to annelids (Suzuki et al. 2010).
Catalano (2012) A review of the families, genera and species of Dicyemida Van Bereden 1876. Zootaxa 3479, 1-32.
Furuya and Tsuneki (2003) Biology of dicyemid mesozoans. Zoological Science 20, 591-532.
Suzuki et al. (2010) Phylogenetic analysis of dicyemid mesozoans (Phylum Dicyemida) from innexin amino acid sequences: Dicyemids are not related to Platyhelminthes. Journal of Parasitology 96, 614-625.