The system I use most often is the guppy (Poecilia reticulata) and its gyrodactylid parasites (mainly Gyrodactylus turnbulli). The fish is a super important model for evolutionary ecology, and we know a huge amount about its natural history. These worms are also really cool, though! Here are some moving around on a guppy tail (video credit: Mike Reynolds).
Gyrodactylus spp. parasites are great to work with, particularly the ones (like G. turnbulli) that only grow on the fish skin, and not on the gills or other hard to see places. They have a crazy reproductive strategy: daughter worms burst out of their mothers and latch on to the same fish, and are already pregnant with the 3rd generation! Once they’ve given birth, they grow penises and can inseminate each other. Yes, you heard right. Given that the species I work on have a generation time of about 24 hours (at 25◦C), this clearly means that the number of worms on a fish can change quite rapidly through time, with potential implications for the fish health, the parasites’ health, and the probability of transmission to a new host.
Perhaps the best thing about this system (apart from the weird parasite sex), is that you can count the exact number of worms on a fish without killing the fish. This means you can count the number of worms on the same fish multiple times through the course of its infection. In other disease systems, we often have to use proxies, such as the number of parasite eggs in the hosts’ poo, or the number of parasites in a blood sample. Gyrodactylus is therefore a great parasite (perhaps the best?) to investigate questions like ‘Are heavily infected hosts the most likely to transmit their infection?’ and ‘How does infected host behaviour depend on the number of parasites it has?’.
You can check out the other research questions I’m addressing using this system by going to the other research pages, although I can totally understand it’s hard to navigate away from the wriggling worms…
I used this system to look at reasons parasites move between host fish. Sea lice have an advantage over Gyrodactylus spp. for asking this question in that the must reproduce sexually (gyros can reproduce asexually, and in fact the first born daughter is always produced mitotically), so you can ask whether sex (presence or absence of potential mates on the same host) or food (level of resource competition) is more important to sea louse in their decision to transmit to a new host or stay put. I enjoyed this experiment a lot – sea lice are pretty charismatic – except for when the salmon I was using lost all their skin to a bacterial infection and I walked in to a load of salmon sashimi swimming around in the tank. Poor fish! Salmon are tricky to keep happy and healthy in captivity (see all the problems fish farmers have), so I moved on to the perpetually happy and super easy guppies.