Spotlight on invertebrate phylum: Gastrotricha

The phylum Gastrotricha is moderately small, but that’s OK because the animals themselves are small. There are around 450 species of gastrotrichs, and they are generally around 1mm long (Brusca and Brusca 2003). Their small size means that they live in interstitial environments of marine and freshwater benthos – i.e., they are meiofauna. Gastrotrichs are surprisingly abundant as constituents of marine meiofauna, being third most abundant invertebrate group behind nematodes and copepods (Todaro et al. 2006).

Gastrotrichs are characterised by a well-developed external cuticle, which can be armed with many spines and/or plates, along with cilia. This gives them a hairy appearance – hence their name is Greek for ‘hairy belly’. Despite their size, gastrotrichs are triploblasts, have a throughput gut, and they move by gliding on cilia on their ventral surfaces (Brusca and Brusca 2003). Most species of gastrotrichs are hermaphrodites. From an evolutionary point of view, gastrotrichs fit within the Lophotrochozoa, and are most closely related to other small meiofaunal groups like the Gnathostomula, but also to the Mollusca (Todaro et al. 2006).


Brusca & Brusca 2003. Invertebrates. 2nd ed. Sinauer Associates, Sunderland, USA.

Todaro et al. 2006. Interrelationships of the Gastrotricha and their place among the Metazoa inferred from 18S rRNA genes. Zoologica Scripta 35, 251-259.



Spotlight on invertebrate phylum: Dicyemida

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.

Phylum Dicyemida

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):

dicyemid reprod

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.

A new species of Acanthocephala (shameless self-promotion!)

My first paper for 2013 is out. If you’re interested in learning about a new species of Acanthocephala from fish, then this is the paper for you.

Weaver HJ and Smales LR (2013) Filisoma filiformis n. sp. (Echinorhynchida: Cavisomidae), a New Species of Acanthocephala from Kyphosus spp. (Perciformes: Kyphosidae) from the South Pacific, and a Key to the Genus Filisoma. Comparative Parasitology 80, 33 – 38.

Find it here: If you can’t access it, let me know by leaving a comment and I can email it to you.

Here’s the abstract: 

“We describe a new species of acanthocephalan from the reef fish Kyphosus bigibbus Lacepede, Kyphosus sydneyanus (Gunther) and Kyphosus vaigiensis (Quoy and Gaimardi)  from Heron Island, Queensland; Ningaloo Reef, Western Australia; and Moorea, French Polynesia, respectively. Filisoma filiformis n. sp. is differentiated primarily from other species by its long, slender proboscis, with 16–18 longitudinal rows of 42–48 hooks. The wide distribution and multiple host species of F. filiformis suggest that it could be found in other localities around the Indo-Pacific region where kyphosid fish occur. The differing patterns of host range and geographic distribution within the genus Filisoma are discussed.”


It’s been so long since I posted last. But my current project is nearly finished, so hopefully the blogging can recommence soon.

Dogs can drive better than many people.

Three dogs from a New Zealand SPCA shelter were taught how to drive a special modified car. See their success here:

Yes, it’s really driving. Apparently the car had been modified to only drive at walking pace, so the leadfoot dish-lickers couldn’t drive off into the sunset.

The point of this campaign was to get people thinking that rescue dogs are awesome and deserve a nice home as much as any other dog.

Perhaps worryingly, these dogs seem to be doing a better job of driving than many humans I see on the road each day…

Special issue on neuroparasitology in J. Exp.Biol.: I’m excited!

The latest issue of the Journal of Experimental Biology is out, and it’s all on how parasites manipulate the behaviour of hosts! Go see the contents here:

We all know about how Toxoplasma modifies the behaviour of infected rodents, making them more brazen and therefore less likely to run away from predators,and how fungus can take over ants and make them want to run to the tops of bades of grass in an effort to be eaten by another host. The papers in this special issue delve into the topic and provide some really great evidence andexamples of parasites being able to manipulate their hosts by altering their behviour.

I’d seen this special issue via a couple of different sources (@The_Episiarch and @Evolutionistrue on Twitter),but weirdly, on Jerry Coyne’s blog post linked to his tweet, he referred to parasites as  “so-called ‘simple’ organisms” (presumably his use of quotation marks means that he does not think they’re simple!). Perhaps it’s obvious only to parasitologists, but some parasites have incredibly complex and quite elegant biology and ecology, often with numerous different hosts and conditions required for different life stages, so I’d say that they are far from simple.

Maybe I should get some of these t-shirts made:


Monday Awesome: Total solar eclipse in Queensland

OK, so it happened last Wednesday, but it was amazing. I didn’t get to see the whole thing as it was rather cloudy, but the clouds parted during totality and I got a glimpse of the completely covered sun.

Here’s one of the best photos I have seen of it, taken by CairnsDiveAdventures (@cairnsdiver) from Twitter:

Solar eclipse, Cairns. Photo via CairnsDiveAdventures.

Monday’s music – Soundgarden

Today is usually an Awesome day, but today, I’m really just wanting to play music really loud. Given I share an office, my music will be confined to my headphones. Here’s a taste of what might be coming up on my all-purpose rage-against-everything playlist: Soundgarden, from the early 90s. The music video is very early 90s too, which is a bit of a laugh.