"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

May 25, 2014

Loxothylacus panopei

Photo by Inken Kruse via the Hare Lab
Some parasites can manipulate their host's behaviour in very spectacular ways, but there are also other parasites that change their host's habits in more subtle manners. While such alteration to the host can seem fairly minor, they can still result in some very profound impact on the rest of the ecosystem.

There is a group of parasitic barnacles call Rhizocephala (the most well-known species is Sacculina carcini) that are capable of castrating their host, turning them into unwitting babysitters that nurture the parasites' brood. The infected crab display some very obvious changes to their behaviour, and in some cases, their appearance. But the study we are featuring today shows that apart from turning them into doting mothers for the parasite's babies, these barnacles can also alter the crab's behaviour in less obvious ways that have ramifications for other marine inhabitants.

The flatback mud crab (Eurypanopeus depressus) lives in estuaries on the coast of South Carolina and it is infected by a species of rhizocephalan call Loxothylacus panopei. In addition to doing the usual host castrating and commandeering trick, L. panopei also changes how this crab responds to potential prey. Usually, the mud crab has an omnivorous diet, dining on algae as well as worms, smaller crustaceans, and sponges. Sometimes they may also have a crack at more armoured prey like mussels. But crabs that are infected with L. panopei lose their appetite for such shell-covered fares.

When researchers offered uninfected crabs with piles of mussels, the crabs acted like they were at an all-you-can-eat seafood buffet and ate as much as they can - the more mussels the researchers presented them with, the more they ate. But no matter how many mussels they offered to crabs that were infected with L. panopei, they simply eat one and call it a day. The parasitised crabs also took longer to get their act together and this seems to be related to the size of the crab's parasite - the larger the parasite has grown, the longer the crab takes to start digging into a mussel.

Based on a field survey of the estuary where the study took place, the researcher concluded that about a fifth of the crab at that location were infected with L. panopei. Given the effects that L. panopei has on their crab's appetite for shellfish, it seems that the mussels might have an unlikely ally in the form a parasitic barnacle. The finding of this study share some parallel to another paper that we featured on this blog earlier this year, on the muscle-wasting parasite that infects a predatory shrimp and curb its otherwise ravenous appetite.

Ecosystems are made up of complicated networks of biological interactions and parasites can mediate predator-prey interactions in different, and sometimes conflicting ways. While some parasites can make prey animals more vulnerable or accessible to predators, there are other like L. panopei that may be reducing the appetite of the said predators. The subtle interplay of such parasite-mediated interactions are often overlooked or ignored, but their effects on the ecosystem are certainly there if you know what to look for.

Toscano, B. J., Newsome, B., & Griffen, B. D. (2014). Parasite modification of predator functional response. Oecologia 175: 345-352.


  1. Are there any theories concerning the reasons fot this odd behaviour modification? Or is it merely a side effect of the overall manipulation?

  2. This is most likely a side effect - it wouldn't necessarily the benefit the parasite much for the host to feed less.