"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

July 24, 2014

Special Report: #ASP2014 (Australia) Part II: Something Fishy This Way Come

This is Part 2 of my report on the annual meeting of the Australian Society for Parasitology (ASP 2014) I attended earlier this month. If you had missed Part 1 of my report, you can read it here

Barramundi photo by Nick Thorne
At the end of the previous post about ASP 2014, I alluded to the abundance of fish and their parasites. In this post I cover research on fish parasites presented at the conference - and there was quite a bit of it. There were a quite a few talks and posters that were focused on the parasite of Barramundi / Asian seabass (Lates calcarifer), which is a prominent aquaculture species in Australia. Like many other production animal, they have their fair share of parasites and there were a number of presentations focused on those said parasites from the Hutson lab including their identification, tracking, and means of control.

 One of the most persistent and common parasites of barramundi in Australian aquaculture is a tiny parasitic flatworm call Neobenedenia. Though they can be quite numerous on an afflicted fish, they are also are tiny and transparent, making them difficult to spot and even harder to study in situ. However, Alejandro Gonzalez presented a method for making these otherwise near-invisible parasites visible by labelling the parasite larvae with a fluorescent dye. Under the sight of an epifluorescence microscope, these treated parasites stands out like glow sticks at a rave club. Gonzalez was able to track how they distribute themselves over the fish's body

But Neobenedenia is just one of many different parasite species clinging to barramundi, a poster presented by Soranot Chotnipat found that there are at least eight different species of parasitic flatworms from the Diplectanidae family alone which are found on the skin of farmed barramundi of Asia-Pacific. But with all these parasites, what can be done about them? Kate Hutson presented a poster with a number of methods being trialled for treating farmed barramundi, including garlic and seaweed extracts, but of which the most novel is the use of cleaner shrimp. She found that fish housed with these shrimps have half as many external parasites as those without, and those shrimps consume all stages of the parasites - including their eggs which the shrimps happily grind up like crunchy treats.

Cleaner shrimp photo by Chris Moody
While there is still much to be learned about the parasites of farmed fish, that is nothing in comparison with the diversity of fish parasite outside of captivity, where there is a wild world of parasites full of murky unknowns. A parasite which has captured the imagination of the public is the tongue-biters which are related to a plethora of parasitic crustaceans in the Cymothoidae family. This family encompasses 361 described species and they range in life-style from skin-clingers to face-huggers to gill-tuggers and belly-burrowers. So how are face-huggers like Anilocra related to belly-burrowers like Ourozuektes? Melissa Martin presented a poster on some preliminary results on their interrelationship which seems to show that they might have independently evolved their respective attachment sites.

For most fish parasites, we do not even know what is out there let alone how they are related to each other, especially on a site of rich biodiversity like the Great Barrier Reef (GBR). Thomas Cribb from University of Queensland has been studying and describing flukes for over 20 years and he presented an overview of the current sum of knowledge about parasitic flukes on the GBR. Currently 326 species of flukes are known from 505 species of fish on the GBR, yet that represent only a small fraction of the 16000 or so species of fish found the the GBR, most of which are yet to be examined for parasites. The fluke fauna on the GBR are also very picky about their host, sticking to just two or so host species on average, and about 45% of them are found exclusively on the GBR. Cribb estimated that at this rate, it will take another 150 years to describe all the flukes (not even counting the other groups parasites) inhabiting the fishes of the GBR.

It is clear that underneath the surface of a tropical reef like the GBR is an extensive network of parasite life-cycles and transmission. To get a glimpse into this hidden world, Abigail Downie examined over 700 fish from 191 species, finding a trove of fluke larvae that utilise those fish as a mean of reaching their final host. She found that one species of goby - Amblygobius phalaena - seems to be a parasite hotspot with 16 species of flukes infecting it. Seeing as all those flukes require their temporary fish host to be eaten to complete their life-cycle, it is not surprising that they have all homed in on a small fish which would be a tasty dish for a range of predators, many of which may serve as potential hosts. Indeed, comparatively small fish species also tend to harbour proportionately more larval parasites than adult stages.

Epaulette shark photo by Strobilomyces
Aside from diversity, Downie also found that the ecology of the fish can influence what families of flukes infect them. For example, flukes in the Heterophyidae family produce free-living larvae that are energetic swimmers that hang out near the water's surface. Accordingly they were mostly found in surface or shallow water fishes such as mullets and halfbeaks. In contrast, flukes from the Opecoelidae family have nub-like tails and move by crawling along the seafloor like microscopic leeches. There they encounter fish that spend most of their time near or resting on the seafloor such as damselfishes and gobies.

One of the surprising finds by Downie was an epaulette shark which was heavily infected with opecoelid cysts. The flukes larvae were lodged in the fins which, when viewed under a microscope, looked like a bag of (gross) marbles. While epaulette sharks do spend a lot of time resting on the sea floor, fluke larvae are not usually known to infect elasmobranchs. At this point, it is unknown if shark serves as a viable transmission pathway for the opecoelids or if it is simply a dead-end parasite sink?

On that note, that is it for for my reports on the ASP 2014 (Australia) conference. It was fun to catch up with some colleagues and see some new research on parasites being presented. Start from next month, it is back to the usual parasite blog posts. Well kind of - as I did last year, next month I will be posting the best student blog posts from the Evolutionary Parasitology class of 2014 - so be sure to keep an eye out for that! Until then, you can check out some of the student blog posts from last year here.

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