|Photo of infected (red) and uninfected (transparent) brine shrimps
From Fig 1 of the paper
All these parasites are using the shrimps as a temporary vehicle for getting into final host where they can mature into adult worms, and for that to happen, the shrimp needs to be eaten by a bird. However, in the environment that these shrimps dwell in, tapeworms like C. podicipina can convey some unexpected benefits. It seems that shrimps infected with tapeworms are more resistant towards arsenic.
Previously, we have featured a study on how tapeworms can act as a sink for heavy metal in seabirds soaking up the toxin before they get absorbed into the host's tissue. But that study was on adult tapeworms living in the gut of a bird host. Though they are also tapeworms, the physiological interaction between an adult tapeworm in the gut of a vertebrate host is very different to that of a larval tapeworm residing inside a small arthropod.
|Flamigolepis liguloides cysticerocoid (larger one on the left) and Confluaria podicipina cysticercoid (indicated by arrows)
From Fig 2 of the paper
Whereas F. liguloides seems to be present in high numbers all the time, C. podicipina only appear in April. This might be related to the seasonal movement of their final host - which are flamingos in the case of F. liguloides, but for C. podicipina, the final hosts are grebes, which only visit the lake during certain time of year. Indeed, that was the finding of a previous study which has been featured on this blog.
Additionally, it seems that the brine shrimps are better at handling arsenic in May when they are mostly only infected with F. liguloides. So why is that the case? Well, it could be that (1) C. podicipina is not as good at helping their host deal with arsenic, (2) it is harmful to the host in other ways that offset their detoxification effects, and (3) it only appears during the warmer months when the brine shrimp's overall resistance to arsenic is lower anyway, so it simply coincided with their appearance.
Of course, neither F. liguloides and C. podicipina are doing this as some kind of favour to the host - C. podicipina and its fellow tapeworm larvae are doing this for their own benefit. They are manipulating host physiology to make the host a more suitable shelter and vehicle for reaching the final host - increasing the fat content of the host makes it a cosier site for development, and increasing the carotenoid level makes the shrimp bright red and stand out more to the bird host. But it just so happens that all these changes also have a side effect of benefiting the shrimp, even if temporarily, before they end up between the beaks of a bird
Sánchez, M. I., Pons, I., Martínez-Haro, M., Taggart, M. A., Lenormand, T., & Green, A. J. (2016). When Parasites are Good for Health: Cestode Parasitism Increases Resistance to Arsenic in Brine Shrimps. PLOS Pathogen 12(3): e1005459.