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).

References

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.

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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: http://jeb.biologists.org/content/current

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:

parasitet

A revolution in taxonomy: electronic-only publication OK’d by ICZN.

I’m excited.

The International Commission on Zoological Nomenclature (ICZN) has published an amendment to the International Codeof Zoological Nomenclature (which confusingly can also be abbreviated to ICZN, but is usually called the Code instead) allowing for publication of taxonomic works in online-only journals.

This is great news. Previously, all taxonomic works (i.e., species descriptions, revisions etc) had to be published in such a way that they had a permanent hard-copy. In the old days, ALL journals were published in hard-copy, so this wasn’t a problem. But in our new  digital age, there have been several journals spring up that do not have a hard-copy version – they’re only available online, for example the journals in the PLoS and BMC stables. Any taxonomic works published in those journals have been invalid until now.

The major change to the Code is that if you wish to publish a description in an online-only journal, you must register your new species with ZooBank, and include information on where the permanent (electronic) record of your work will be held, along with the ISSN or ISBN for the publication.

Go forth and describe!

Science round-up: what has interested me this week?

photo credit: belizar / Fotolia

Not a lot, actually. I’ve been distinctly unimpressed with the offerings of new science bits on the interwebs lately. However, I have found some things that have piqued my interest. Some highlights for this week have included:

  • deep-sea floor microbes are really, really old; and
  • naked mole rats live for years (and are completely awesome, but we already knew that).

Microbes!

From work newly published in Science, we have some interesting insight into just how long microbes can live for. Researchers took sediment cores from the mud under the seabed in the Pacific Ocean, and measured the amount of available oxygen in each of the mud strata. Because they could plot the amount of oxygen that diffused through the mud strata, they could determine how much of it had been used by respiring microorganisms (ie, the amount of oxygen that was absent from the available amount). It turned out that samples had oxygen available 28 metres below the seabed, because the microbe communities were too sparse to consume all the available oxygen. It’s been 86 million years since those microbes were actually on the seabed itself. There have been many, many layers of mud deposited on top of them, yet they’re still there. Slowly consuming oxygen. Cool.

Naked mole rats!

I like naked mole rats. Apart from their really bizarre looks and ecology, they are also very useful models for human medicine as they have some amazing biological properties (they don’t seem to get cancer, for a start). One new aspect is their ability to get old, which for small rodents, is not usually an option. Many rodents have short lifespans, and for an animal of their size, the mole rats should not really live for very long – maybe 3-5 years. But they tend to live for about 30 years. And, instead of getting increasingly decrepit as they age, they stay healthy and sprightly.

We now have a better understanding of how they do that, thanks for research published in the journal Aging Cell. Researchers have found that naked mole rats don’t seem to have the same levels of decline in a growth factor called NRG-1 as observed in mice and humans as they age. What this means is that the brains of the naked mole rats seem to benefit from the ongoing protective aspects of having appropriate amounts of the brain growth factor – suggesting that it might not be increasing oxidation or other detrimental effects that cause reduction in brain function and lifespan, but rather the decline of the protective factors that allow it to happen. Either way, this further reinforces my assertion that naked mole rats are awesome critters.

Octopus homeowners.

While looking around the interwebs for interesting science, I came across a really cute video of an octopus carrying a tin can and using it as a shelter. I know that there is a very cephalopod-heavy science-esque blog around the place, and I’m not trying to tread on their toes, but this is really cool.

This video from CNN iReport, was taken in the Philippines. It’s a very cute octopus. It carries the tin around (which looks like it requires a lot of effort! Poor octopus!), and at the end, as it folds itself into the can it seems that it has also been carrying a shell to use as a door. For some reason, I always visualise octopuses as being enormous, but with the tin providing context, it’s actually a rather small animal:

http://ireport.cnn.com/themes/custom/resources/cvplayer2.0/IReportEmbedPlayer.swf?site=ireport&profile=ireport-embed&context=embed&contentId=782459/0

(this link might work better: http://ireport.cnn.com/docs/DOC-782459)

This reminded me of something I saw a while ago – octopuses using coconut shells as shelter, in a rare demonstration of tool use by an invertebrate. The researchers observed the octopuses carrying around coconut shell halves in a rather ungainly fashion, and used them to hide under, or in some cases, carried two and ended up being sheltered inside the reassembled coconut. Cool. This research was published by some staff at the Museum of Victoria (in Current Biology), and they also uploaded some videos of the octopuses, as seen below. I love the way the octopus saunters past the camera at the start; and the excellent demo of how tube-feet work at the end.

The use of tools, and the carrying of the tools for the ‘just in case’ scenario indicate that these little cephalopods are much smarter than we give them credit for. Whether they are really psychic or not is still not resolved.

Dolphins can’t taste sweet….

… which is just as well, because cupcakes are hard to come by underwater.

"That was really lame. Hey, can you toss a pilchard my way?"

OK, it was pretty bad. But it’s not entirely ridiculous, as research published in PNAS and reported on the ABC News website shows that some carnivores have lost the ability to taste sweet, savoury and bitter flavours. Continue reading

Obvious science, or, Why didn’t anyone think of it sooner?

Occasionally, I read about research that surprises me because it’s based on everyday things, or is obvious it makes me wonder why no-one found it out before.

It's a tarsier. Is it just surprised, or is it shouting at you? (photo credit New Scientist website)

Today I read about how tarsiers (cute little primates) have been discovered to communicate using ultrasound. Previously, these little monkeys had been thought to yawn a lot because they are often seen with their mouths open and no sound coming out.

Continue reading