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