New Paper: Acanthocephalans from Australian Marine Fish

I've got a new paper out, this time on Acanthocephalans. This was one of those projects that started off simple and then snowballed into a serious undertaking.

D.C. Huston . T.H. Cribb and L.R. Smales. 2020. Molecular characterisation of acanthocephalans from Australian marine teleosts: proposal of a new family, synonymy of another and transfer of taxa between orders. Systematic Parasitology.
https://doi.org/10.1007/s11230-019-09896-2

Acanthocephalans are a strange group of obligate endoparasites. Generally, they have two-host life cycles, with an invertebrate intermediate and vertebrate definitive host. Despite being worm-like and traditionally lumped in with the parasitic 'helminths', molecular phylogenetic study has demonstrated that acanthocephalans are most closely related to the rotifers. A most curious evolutionary trajectory for an ancient rotifer-like beast.

A lot of the specimens I was after for my PhD research came from species of an unusual family of fishes, the Kyphosidae. Well, many of the kyphosids I found had heavy infections of acanthocephalans. This project began because I wanted to identify them. Next thing I knew I was taking all the acanthocephalans from the Marine Parasitology Lab at The University of Queensland to the South Australian Museum to work with Dr Lesley Smales identifying them. I noticed that, at that time, there weren't any molecular sequences available for any Australian acanthocephalan. Thus I started sequencing all the species in my collection. This took years - I was doing this work as a side-project to my PhD on digenetic trematodes, and more and more acanthocephalan specimens kept getting added as the years wore on. It was worth it in the end - we got some interesting phylogeny results which led us to do some much needed taxonomic revisions and we even got to propose a new family.

Publishing this paper also allowed me to experience the joys of making a mistake in something I've put out on public record. Well, I say mistake, but really it was mistakes... I requested a correction shortly after some things were brought to my attention - and its just come out (https://doi.org/10.1007/s11230-019-09896-2.) Somehow a couple typos made it all the way through drafting, revision and peer review. You would think that after reading through a draft you've been writing 10-20+ times you would notice these things, but I guess sometimes its easy to glaze over as you go, especially when reading over convoluted scientific names (note to self - always add scientific names to the word processor dictionary). 

As if that wasn't enough, I found myself greatly embarrassed when another researcher let me know that two of the cox1 sequences I uploaded to GenBank were coming up in odd places in the phylogeny they were building. Well, I double checked them and sure enough they were contaminated with DNA from other acanthocephalans from the same sequencing batch. I immediately contacted GenBank and had them flag those sequences - so they are no longer available for download. This problem seems to be a result of a labeling error in the spreadsheet I was using to track the sequences as I generated them over the many years of the project. I provided the cox1 sequences as part of the project, but didn't actually do anything with them for the paper, so I didn't notice their odd phylogentic positions myself. (second note to self - always double check each sequence via a quick tree and BLAST before uploading). All the analyses in the paper used only 18S and/or 28S sequences. Anyway, embarrassing, but at least the error didn't impact any of the results of the study...! So overall a bit of a lesson to self. It is easy to make silly mistakes even with a doctorate and huge amounts of experience writing and doing science. Accepting mistakes and making corrections is part of the scientific endeavor - transparency and honesty are key. I've heard it said somewhere, in discussions on corrections to scientific works: 'to err is human, to correct divine' !

New Paper on digenean cercariae (Pronocephaloidea)

It was studying snails infected with digeneans that sparked my interest in the group originally, so I'll always have a soft spot for projects working on digenean cercariae. A recent paper that has come out of the Marine Parasitology Laboratory at the University of Queensland (with me as a co-author) characterizes a number of cercariae which belong to a interesting digenean lineage, marine species of which parasitise the respiratory and alimentary tracts of birds, turtles and mammals.

T.H. Cribb, P.A. Chapman, S.C. Cutmore and D.C. Huston. 2020. Pronocephaloid cercariae (Platyhelminthes: Trematoda) from gastropods of the Queensland coast, Australia. The Journal of Helminthology. https://doi.org/10.1017/S0022149X19000981

We described five new pronocephaloid cercariae and provided molecular sequences for each species. We suspect that these species are parasites of marine turtles or perhaps mammals (dugongs), as none of them were found in a clade with the major family of bird parasites of this lineage (Notocotylidae). There are almost no molecular resources for adult pronocephaloids from turtles, so unsurprisingly we did not get a match for any of our cercariae with an adult. Marine turtles are, and will likely continue to be, under threat and are protected in Australia - so I don't foresee a large, modern, molecular-grade collection of pronocephaloids from marine turtles becoming available for study any time soon (and that's probably a good thing). Thus, collections from gastropods represent a great molecular resource for expanding the pronocephaloid phylogeny.

Worms living within worms: a new endosymbiotic rhabdocoel flatworm (Umagillidae)

Describing new species of obscure invertebrates is how I get my kicks, thus I'm quite pleased with one of my most recent papers.

Huston, D.C. 2019. Collastoma esotericum (Neodalyellida: Umagillidae), a new species of sipunculan-inhabiting rhabdocoel from Queensland, Australia. Zootaxa, 4701 (6), 563-573.

http://dx.doi.org/10.11646/zootaxa.4701.6.5 

The most well-known parasitic flatworms are the highly-derived monogeneans, digeneans and cestodes, members of the platyhelminth subclade Neodermata. However, there are numerous ecto- and endosymbiotic ‘turbellarian’ flatworms scattered throughout the remaining lineages of the Platyhelminthes.

Invertebrates host a variety of endosymbiotic flatworms and these relationships are likely among the most ancient of metazoan symbioses. Echinoderms host two distinct groups of endosymbiotic flatworms, species of the families Umagillidae and Pterastericolidae. The pattern of host-association is also peculiar. The crinoid, echinoid and holothuroids host umagillids, while the asteroids host pterastericolids. There is apparently no overlap. Further, some umagillids have been reported from species of the Sipuncula (peanut-worms). The overall patterns suggests (to me at least!) multiple transitions from free-living to endoparasitism. However, none of these flatworm lineages have been studied in great detail, and there are few molecular resources for these species.

In my MS I describe a new species of the peanut-worm inhabiting umagillids. I was hoping that my molecular analyses would provide strong evidence for my above hypothesis (multiple transitions from free-living to endoparasitism), however I was only able to obtain sequences of the 18S rDNA gene as amplification of all other regions failed, despite the many different primers and cycling conditions I tried. Ultimately the results of my analyses didn't provide enough support to show this for sure just yet, but I suspect that future work will.

New paper on the Digenean family Enenteridae

A recent-ish paper of mine was the culmination of around 3 years of work (so most of my PhD)

2019. DC Huston, SC Cutmore, TH Cribb. An identity crisis in the Indo-Pacific: molecular exploration of the genus Koseiria (Digenea: Enenteridae). International Journal for Parasitology 49 (12), 945-961.
https://doi.org/10.1016/j.ijpara.2019.07.001

Much of my PhD focussed on the parasites of a unique group of herbivorous marine fishes, species of the family Kyphosidae (the drummers or sea-chubs). These fish are remarkable in that they eat almost entirely algae, and have lots of endosymbiotic organisms which help them with hindgut fermentation. Microbial fermentation has been poorly studied in fish, but I suspect it may have something to do with the unique parasite fauna of such fish species.

Anyway, the family Enenteridae needs a lot of work, and the genus Koseiria was a natural first choice to start the process. It turned out to be a quite complicated and challenging project, but resulted in a good paper, with a new genus, three new species and some new combinations. Next project for the family Enenteridae is the major lineage (the genus Enenterum)!

Gyliauchenidae work featured on The Lab Down Under

One of my papers, published with Terry Miller, Scott Cutmore and Tom Cribb has been featured on the Science Journalism Blog The Lab Down Under. The paper details the discovery and description of a new species of the trematode family Gyliauchenidae, Endochortophagus protoporus from the western buffalo bream Kyphsus cornellii. Members of the closely related Enenteridae exploit kyphosid fishes, but this is the first occurrence of a gyliauchenid in such a fish. A link to the article at The Lab Down Under.

Exploring the dark food web? Start with molluscs first.

Parasites with complex life cycles, such as the ubiquitous digenetic trematodes, exploit different hosts for different reasons (i.e. asexual reproduction in a molluscan intermediate host and sexual reproduction in a vertebrate definitive host). At the same time, for transmission between different hosts, trematodes exploit trophic interactions between these hosts. Parasites with complex life cycles are hidden hitchhikers in food webs, creating an unseen web of interactions – what we might call the ‘dark food web’ (well, that’s what I like to call it because it sounds rad and you know, up with the times).

Parasites are consumers and constitute a large amount of biomass in communities (Kuris et al., 2008). There are a number of really great papers on the importance of incorporating parasites into food webs. For a start see Thompson et al. (2005), Lafferty et al. (2006, 2008) and the references within those papers. Furthermore, parasites are not only consumers, but are also predated upon (think gnathiid isopods and cleaner wrasse). Digenetic trematodes have a free-living larval stage, called a cercaria, which seeks to infect the next host in the digenean’s complex life-cycle. Cercariae are produced from asexual colonies residing in infected molluscs, and thousands of cercariae can emerge from an infected mollusc each day. In many communities a large proportion of the molluscs are infected with trematodes, so you can imagine that there are a lot of cercariae being pumped into the world’s ecosystems daily. This represents a huge, and mostly unstudied, path of energy flow in food webs (Thieltges et al. 2008; Morley, 2012).

Clearly incorporating parasites into food webs has great promise for understanding how complex ecological communities function. However, progress is generally hindered by a lack of knowledge of parasite life cycles. Most larval parasites are difficult to identify to species on the basis of morphology alone and in many cases these larval stages may only be identifiable to family (common for trematode cercaria). Thus traditional methods for elucidating life-cycles are slow and difficult. This process can be sped up significantly however, by using molecular barcodes to connect various life-cycle stages. I have done this twice previously (see this post and this post) – and in my recent(ish) paper I focused on molecularly characterising a whole community of trematodes parasitising a single species of gastropod on the Great Barrier Reef : 

Huston, D.C., Cutmore, S.C., and Cribb, T.H. 2018. Molecular systematics of the digenean community parasitising the cerithiid gastropod Clypeomorus batillariaeformis Habe & Kusage on the Great Barrier Reef. Parasitology international 67 (2018): 722–735.

It took me over 3 years to collect all the data (mostly working on the side while I was on Heron Island for other reasons). Fortunately, I had a head start on this particular project because of the work of Cannon (1978), who had previously morphologically characterised most of the cercariae which I sequenced in my study. Although I didn’t find all the cercariae that Cannon (1978) described, I found two which he had not, showing a shift in the community structure over time. The new tally of digeneans which utilise Clypeomorus batillariaeformis on the Great Barrier Reef stands at 14! That is a large diversity and volume of cercariae being pumped into the waters of the Great Barrier Reef.

Although the morphology of the cercariae typically tell us what family they belong to, molecular data takes us a step further. Phylogenetic placements for cercariae can tell us what sorts of definitive hosts we ought to expect each cercariae ultimately aims to end up in, and from that we might infer what the transfer mechanisms might be. For example, in my study we found three species of the heterophyid genus Galactosomum. With that knowledge we know that the definitive hosts ought to be birds, and because all three of these species of Galactosomum had large, visually conspicuous cercariae we can infer that the second intermediate hosts are likely surface feeding fishes.

Characterising whole communities of digeneans from molluscs with molecules seems a great way to advance our understanding of food-web dynamics and build on our understanding of trematode-mollusc evolutionary interactions. Thus, when setting out to explore the dark food web, start with the snails first.

References

Cannon, L.R.G. (1978). Marine cercariae from the gastropod Cerithium moniliferum Kiener at Heron Island, Great Barrier Reef. Proceedings of the Royal Society of Queensland 89, 45–57.

Kuris, A.M., Hechinger, R.F., Shaw, J.C., Whitney, K.L., Aguirre-Macedo, L., Boch, C.A., Dobson, A.P., Dunham, E.J., Fredensborg, B.L., & Huspeni, T.C. (2008). Ecosystem energetic implications of parasite and free-living biomass in three estuaries. Nature 454, 515.

Lafferty, K.D., Allesina, S., Arim, M., Briggs, C.J., De Leo, G., Dobson, A.P., Dunne, J.A., Johnson, P.T.J., Kuris, A.M., & Marcogliese, D.J. (2008). Parasites in food webs: the ultimate missing links. Ecology letters 11, 533–546.

Lafferty, K.D., Dobson, A.P., & Kuris, A.M. (2006). Parasites dominate food web links. Proceedings of the National Academy of Sciences 103, 11211–11216.

Morley, N. (2012). Cercariae (Platyhelminthes: Trematoda) as neglected components of zooplankton communities in freshwater habitats. Hydrobiologia 691, 7-19.

Thieltges, D.W., de Montaudouin, X., Fredensborg, B., Jensen, K.T., Koprivnikar, J., & Poulin, R. (2008). Production of marine trematode cercariae: a potentially overlooked path of energy flow in benthic systems. Marine Ecology Progress Series 372, 147-155.

Thompson, R.M., Mouritsen, K.N., & Poulin, R. (2005). Importance of parasites and their life cycle characteristics in determining the structure of a large marine food web. Journal of Animal Ecology 74, 77-85.

Isorchis cannoni n. sp. (Digenea: Atractotrematidae) from Great Barrier Reef rabbitfishes

This paper came out as an 'online-first-view' last year, and is still languishing in 'in press' purgatory, so I thought I would go ahead and do a quick write up. Here's the citation and link:

2018. Huston, D.C., Cutmore, S.C. and Cribb, T.H. Isorchis cannoni n. sp. (Digenea: Atractotrematidae) from Great Barrier Reef rabbitfishes and the molecular elucidation of its life cycle. The Journal of Helminthology (in press). PDF

This particular story begins with Dr. Lester Cannon’s study of the cerithiid gastropod Clypeomorus batillariaeformis (then called Cerithium moniliferum, a junior synonym) from Heron Island on Australia’s Great Barrier Reef. In his 1978 publication ‘Marine Cercariae from the Gastropod Cerithium moniliferum Keiner at Heron Island, Great Barrier Reef’ Cannon described 10 new trematode cercariae from C. batillariaeformis, and gave each cercariae a placeholder name and a numeral, e.g. Cercariae queenslandae I-X. The morphology of one of these cercariae (Cercaria queenslandae II) aligned closely with those known from the trematode family Haploporidae. In fact, Cannon noted that this cercariae might be that of Atractotrema sigani which was, at that time, considered a member of the Haploporidae, although today it is placed in the haploporid’s sister family Atractotrematidae. Atractotrema sigani parasitises rabbitfishes (family Siganidae), an important group of medium-sized, herbivorous marine fishes, several species of which feed on algae growing on the beachrock along Heron Island where C. batillariaeformis is found.

Cannon was about as close to the mark as one could get in those days, those days being those without molecular sequencing. In the past, elucidating life-cycles was rather difficult, and could only really be accomplished with captive populations and somewhat time-consuming experimental infections. Nowadays it is easy enough to generate molecular sequences from adult and larval trematodes and match them up!

Had molecular tools been available to Dr. Cannon, he would have quickly realised that the cercariae he had found were not those of A. sigani, and actually a species of atractotrematid yet to be discovered! This is what I found when I started a project sequencing the cercariae of C. batillariaeformis from Heron Island. When I began the project in 2015, the adult of Cercariae queenslandae II had not yet been discovered, but in 2017 we found the adults. Cannon was pretty close to the mark again: the definitive hosts turned out to be the same rabbitfishes that host A. sigani, Siganus lineatus and Siganus fuscescens. The twist was that rather than being the cercaria of a species of the genus Atractotrema, the adult match was an undescribed species of the genus Isorchis. We named this species Isorchis cannoni in honour of Dr. Lester Cannon and his work on trematodes of the Great Barrier Reef (and that he pretty much figured out the life-cycle for us already!).

Isorchis cannoni new species. A) Holotype, B) Ovarian complex

Species of Isorchis were previously known only from the milkfish Chanos chanos (family Chanidae) and the scat Selenotoca multifasciata (family Scatophagidae). This was all pretty rad as our study added a new host family to those known to be exploited by species of Isorchis and we revealed the first life-cycle known for the genus (actually the first life-cycle for a species of the Atractotrematidae). This life-cycle information may provide new insights into the evolutionary split between the Atractotrematidae and the Haploporidae, though we need to discover a few more life-cycles before we can truly delve.


References
Cannon, L. (1978) Marine cercariae from the gastropod Cerithium moniliferum Kiener at Heron Island, Great Barrier Reef. Proceedings of the Royal Society of Queensland, 89, 45-57.