"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

September 26, 2011

Parvilucifera sinerae

Phytoplankton are microscopic single-celled "plants" which float in the upper surfaces of the ocean, and their photosynthetic action is responsible for generating most of the oxygen in our atmosphere. While you might think that something so tiny would not be host to anything, there are in fact a myriad array of viruses, bacteria, and flagellate organisms that infect and exploit phytoplankton, and the parasite for today is one of them. Parvilucifera sinerae is a single-celled, flagellated organism which infects dinoflagellate algae such as Alexandrium minutum. The photo shows an infected A. minutum cell. While earlier in this post we extolled the virtue of phytoplankton, dinoflagellate algae are also known to be responsible for harmful algal bloom events such as "Red Tides", so there is a lot of interest in their ecology and the factors that can influence their likelihood of blooming.

For P. sinerae, infecting its host is not an easy task - not only does it have to find a swarm of its tiny host in the vast ocean, it also needs to make contact and accomplish what amounts to a cellular heist - the parasite needs to break through the protective shell of the alga in order to steal its valuable content. As you can imagine, during such an intense operation, being jostled around will probably throw you off your game. And indeed that is what a group of scientists in Spain have found. It appears that even a slight turbulence is enough to reduce the infection success of P. sinerae and that it performs best under calm, still conditions. These researchers suggested that turbulence would erode the zone of chemical emission around the dinoflagellate, making them more difficult to detect. Turbulence would also shorten the period of time which P. sinerae are in constant contact with the host cell - which is a necessary precondition for the parasite to perform its little cellular heist.

While both P. sinerae and its host are tiny, their interactions can have far-reaching ecological consequences, and as explained earlier they are among the most important organisms on the planet. In addition, parasitic killers, such as today's parasite, have been suggested as a possible biological control for harmful algal blooms, but it is like that the effectiveness of any such control would be at the mercy of environmental factors such as small-scale marine turbulence.

Image from figure of the paper.

Llaveria, G., Garcés, E., Ross, O.N., Figueroa, R.I., Sampedro, N. and Berdalet, E. (2010) Small-scale turbulence can reduce parasite infectivity to dinoflagellates. Marine Ecology Progress Series 412: 45-56.

September 16, 2011

Sphaeromyxa cannolii

We've met other myxozoan parasites before, including the very well-known causative agent of whirling disease in salmonid fishes, Myxobolus cerebralis. Today, meet a newly described species of myxozoan that was found infecting seahorses collected from the Gulf of Mexico. Not only was this the first such species described from this seahorse, but this is also the first time that any pathology attributable to a species in this genus has been recorded. The abundance of the parasites in the liver was observed to obstruct the bile ducts of the fish, which caused noticeable accumulation of bile in the diseased hosts. The intermediate hosts are presumed to be some kind of annelid worm, but remain unknown for this species. And in case you were wondering, yes, the species name for this parasite comes from the fact that it looks like a cannoli.

Image from the paper.

Reference: Sears, B.F., P. Anderson, and E.C. Greiner. 2011. A new species of myxosporean (Sphaeromyxidae), a parasite of lined seahorses, Hippocampus erectus, from the Gulf of Mexico. Journal of Parasitology 97:713-716.

September 12, 2011

Cardicola forsteri

Today's parasite is a blood fluke that has been turning up in tuna ranches in South Australia. The blood fluke lives the tuna's circulatory system, and lays eggs that can become lodged in the fish's gills or other organs such the heart, and cause significant lesions in those tissues. This is obviously of great concern to the tuna ranchers, so they set out to find a way of alleviating their fish from infection.

Being a trematode, Cardicola fosteri must have an invertebrate host that is the source infection for the tuna. In their search for the first host of C. forsteri, researchers undertook a a truly heroic effort - sampling over 9000 (!) invertebrates, including all kinds of bivalves, snails, and polychaete worms from the pontoons on the tuna ranch and nearby areas, then meticulously dissected and examined every single one of them for parasitic infections. Those who have been following this blog would know that trematodes usually have a mollusc host in which they undergo asexual multiplication - usually a snail, but C. forsteri is very unusual - it turns out that it uses a polychaete worm, specifically tube-dwelling terebellids - also known as spaghetti worms - for asexual multiplication. Infected worms were packed with hundreds of sac-like sporocysts which continuously churn out the free-living cercarial stages that go on to infect the tuna.

The researchers then used specific sections of the DNA obtained from the parasites to match up the sac-like sporocyst stage in the worms with the adult stage in the tuna, and they were able to confirm that the blood flukes in the tuna were indeed originating from those infected tube-dwelling worms. As those sedentary worms usually live on the seafloor, researchers recommended that simply by moving them to deeper waters, the tuna would be infected by far fewer blood flukes. This study shows how understanding the ecology and life-cycle of a parasite can help us take straightforward measures that can mitigate their impact.

Photo by Robert Adlard

Cribb TH, Adlard RD, Hayward CJ, Bott NJ, Ellis D, Evans D, Nowak BF. (2011) The life cycle of Cardicola forsteri (Trematoda: Aporocotylidae), a pathogen of ranched southern bluefin tuna, Thunnus maccoyii. International Journal for Parasitology 41:861-70.