Trophomera marionensis

This planet is full of parasites, and no matter what you are or where you live, there seems to be no escape from getting parasitised. A few years ago, I wrote a post about some microsporidian parasites which live in deep sea nematodes (roundworms) - well this time it is a deep sea nematode which is the parasite. Trophomera marionensis is a nematode which is found in one of the deepest part of the ocean, in the inky depths of the Kermadec Trench about 7000 to 10000 metres below sea level. This is a part of the ocean known as the Hadal Zone - a realm of perpetual darkness and immense water pressure, named after the underworld of Greek mythology.

Sample of the deep sea amphipods (top left), parasitised H. dubia (bottom right), an immature female T. marionensis (right)
Image from Fig. 1 and Fig. 4 of the paper

Trophomera marionensis belongs to a family of roundworms call Benthimermithidae which are mostly found in the deep sea. They share a similar lifecycle to the Mermithidae and Marimermithidae families which are found in the sunlit realm - some of which have previously been featured on this blog here, here, and here. Much like those families of nematodes, the benthimermithids are also body-snatchers that infects their host, take over the insides, and make a xenomorph-style exit at the end of their stay. But whereas those shallow water roundworms infect mostly insects and crustaceans, these deep sea nematodes are found in a more diverse range of hosts.

While T. marionensis infects the deep sea amphipod Hirondellea dubia which makes it comparable to some of its shallow water marine mermithid cousins, the hosts of the other 40 or so known species of Trophomera covers a wide variety of deep sea invertebrate animals. Given how sparsely distributed potential hosts are in the deep sea, you tend to take what you can get. The ecology of the hadal zone had placed enormous evolutionary selection pressure on the benthimermithids to diversify and infect invertebrates other than just arthropods. In addition to infecting deep sea crustaceans, species from that genus have been recorded from priapuplid worms (also known as the penis worm), mussels, and even other nematodes.

Much like other deep sea creatures, the population of T. marionensis is very sparsely distributed. Out of the several thousand amphipods that the researchers examined, they only came across 32 infected ones, containing a total of 40 worms. Most amphipods were infected with a single worm, though there was one rather unfortunate individual that was host to four worms. Furthermore, all the worms they found were female worms - so at this point we don't know how the male worms look like!

The most likely way that those deep sea amphipods become infected by T. marionensis is through accidentally ingesting the larval parasite during early stages of their development, while feeding on scraps of "marine snow" which had settled on sea floor. Currently, it is unclear what effects T. marionensis has on its crustacean host, but given the size of this nematode in comparison with the amphipod, they must have at least some effects on their growth and reproduction.

Amphipods are common in deep sea habitats, and benthimermithid nematodes have also been recorded in deep sea environments from all over the world. So there is no doubt there are many more parasite-host combinations lurking in the dark abyss of deep sea habitats which are yet to be discovered.

Reference:
Leduc, D., & Wilson, J. (2016). Benthimermithid nematode parasites of the amphipod Hirondellea dubia in the Kermadec Trench. Parasitology Research 115: 1675-1682

Pseudolynchia canariensis (revisited)

Ever since birds and mammals have evolved to have feathers and fur respectively, many different orders of insects have also evolved to take advantage of the opportunities that they provide. Fleas, lice and some families of flies have become ectoparasites that dwell in the cosy environments offered by animals covered in feathers or fur.

Top: P. canarienesis with hitch-hiking lice
Bottom: Pigeon lice; (A) Columbicola columbae,
(B) Campanulotes compar, (C) Hohorstiella lata,
and (D) Menacanthus stramineus. Image from the paper

While there are many biting flies that feed on the blood of feather- and fur-covered animals, few are as specialised as the hippoboscid flies - also known as Louse Flies. Louse flies are flies that have evolved to be obligate ectoparasite - some of them can fly, but they prefer spending most of their time crawling around the feathers of birds or the fur of mammals. The species featured in the study we are covering today is Pseudolynchia canariensis, which parasitises the common rock pigeon (Columba livia). The flattened body and long legs of the louse fly allows it to go scrambling amidst the feathers of its host, as it finds a sweet spot to chow down on some pigeon blood. The prime spot to do so is on the pigeon's belly amongst all the soft downy feathers

But P. canariensis is not the only ectoparasite on pigeons - as would be expected, pigeons are also home to many other parasites include a variety of actual lice. There are four species of lice that regularly hang out around the pigeon's belly, Columbicola columbae (which has been featured on this blog before), Campanulotes compar, Hohorstiella lata, and Menacanthus stramineus. So it can get pretty crowded on a pigeon's belly and there are plenty of opportunities for these parasites to mingle

Unlike the louse flies, lice don't have wings - so to travel from one host to another, they have to either crawl the whole way themselves, or borrow someone else's wings. By that I mean some lice hitch a ride on their fly-based namesake. In this study, scientists measured the mobility of the above mentioned four lice species commonly found on the rock pigeon, and their ability to hitch a ride on those P. canariensis.

They first tested how well each of those lice move about on their own by placing them on a piece of filter paper, and watch how far they managed to move in two minutes. Because lice tend to dislike light, the scientist shone a small light on them to coax them to move. Next, they tested the lice's ability to attach to louse flies by placing louse individually in a clear tube with a louse fly, and see how quickly they climb onboard - if at all. Finally, they test how well these lice managed to stay on the louse fly by repeating the attachment experiment, but this time they let P. canariensis does its thing and fly to the other side of a small room with a closed window, then recapture it to see whether the hitch-hiking louse had manage to hang on.

They found that not all lice are equally adept when it comes either moving on their own, or the finer art of leaving on a louse fly - and it seems aptitude in those two skills are inversely related. The most athletic lice like M. stramineus are also the worst at attaching themselves to P. canariensis, whereas those that can't move all that well off-host, such as C. columbae, are louse fly riders par excellence. Rock pigeons are pretty gregarious, so for the more mobile lice, they can easily cover the distance under their own steam. At the speed which the scientists recorded, M. stramineus is capable of covering one metre in the period of six minutes, which makes it quite the marathon runner in the louse world. In contrast, Co. columbae and Ca. compar are downright helpless anywhere away from a bed of pigeon feathers, but they are very skillful when it comes to piggybacking on a louse fly.

For some lice, leaving on a louse fly is not such a lousy way to travel.

Reference:
Bartlow, A. W., Villa, S. M., Thompson, M. W., & Bush, S. E. (2016). Walk or ride? Phoretic behaviour of amblyceran and ischnoceran lice. International Journal for Parasitology 46: 221-227.