Leptorhynchoides thecatus

Photo by Scott Bauer

Life is dangerous for a little crustacean like a freshwater amphipod. There are all kinds of things out there that would like to make a meal out of you, so you would sure want to get out of the way at the first sign of any would-be predator. While our sense of smell is relatively poor, other animals live in a far more aromatic and pungent world, filled all kinds of chemical signals. When it comes to chemoreception (what we would consider smell and taste), amphipods can tell the presence of a predator in main two ways, either smell their presence directly through the kairomones (basically BO) they release, or indirectly from the alarm chemicals of dead compatriots (so essentially, the scent of death).

However, this can be big problem for some parasites of these little crustaceans, as they need to be eaten by a predatory animal in order to complete their life cycles. In that case, some of these parasites have ways of making sure that their host never see (or in other ways sense) it coming when a predator comes knocking.

Proboscis of adult L. thecatus
modified from here

Hyalella azteca is a common species of amphipod that is found in many freshwater habitats in North America. It is also host to the larval stage of a thorny-head worm call Leptorhynchoides thecatus. For this parasite to complete its life-cycle the amphipod host needs to be eaten by a fish - such as a green sunfish - something that the amphipod is certainly not okay with. However, regardless of what the amphipod wants, the parasite needs to reach a fish's gut, and it does so by overriding the crustacean's usual response to alarm chemicals in the water. A pair of scientists conducted an experiment to see this in action.

First they made some scent solutions that correspond to the ones that the amphipods would usually respond to in the wild. Alarm chemical from dead or injured H. azteca was relatively straight forward to make as it simply involved mushing up some amphipods in a bit of water to get this "scent of death". But to get some liquid fish BO, they collected water from a tank housing green sunfish which had been circulating for a day without a carbon filter, so the water has been saturated with the "essence of fish" as it were (I'd imagine neither scent would sell all that well if you release it as a line of perfume or cologne).

To see how the amphipods reacted to the scents they've prepared, the scientists placed each H. azteca individually in an observation chamber which has a small shelter at the bottom. After it has settle down, they either drip a bit of that "scent of death", or some of the "essence of fish", or just plain water into the chamber, and watched the amphipod's response.

When uninfected H. azteca catch a whiff of fish BO or the scent of their dead companions, they hid in the shelter and try to keep still (especially at the scent of dead amphipods). But not the amphipods infected with L. thecatus - regardless of what's in the water, they just stayed completely oblivious and carried on with whatever they were doing as usual, as if the scientists had just added plain water to the chamber. If it had been in the wild, those infected amphipods would have been quickly snapped up by a hungry sunfish (and made L. thecatus really happy, if worms are capable of being happy...).

Being visual animals, we humans tend to take more notice when parasites manipulate their hosts in a flashy way that catches our eyes. But there are other ways that parasites can manipulate the sensory world of their hosts in order to complete their life cycle. We have not paid as much attention to those other senses - perhaps it is time that we do so.

Reference:
Stone, C. F., & Moore, J. (2014). Parasite-induced alteration of odour responses in an amphipod–acanthocephalan system. International Journal for Parasitology 44: 969-975.