Anilocra nemipteri

Photo from Figure 1 of the paper

The parasite in the study being featured today makes a living riding around on the top of a fish's head and occasionally gnawing on its face. It is in the same family as the infamous tongue-biter, the Cymothoidae, though technically this one is more of a face-hugger.

Anilocra nemipteri is found on the Great Barrier Reef of Australia and it makes a living by hitching a ride (and feeds) from the bridled monocle bream, Scolopsis bilineata. It is a pretty common parasite - in some areas, up to 30 percent of monocle bream carry one of these crustaceans on their head like a nasty blood-sucking beret that stay attached for years.

As you can see from the photo, A. nemipteri is a fairly big parasites comparing with the size of the fish (in some case they can reach as almost one-third the length of the host fish!), and having a parasite of that size hanging off your face is going to be quite a drag - literally. That is bad news for a little fish like the monocle bream that needs to make a quick getaway from any hungry predators on the reef. So just how much of a drag is A. nemipteri? A related species - Anilocra apogonae - which clings to the cardinal fish (Cheilodipterus quinquelineatus) is known to cause their host to swim slower and have lower endurance. Does the same apply for A. nemipteri and the monocle bream?

To find out, scientists compared how quickly the fish can respond to an attack and their Flight Initiation Distance (FID) in both a laboratory setting and in the field. The FID is the distance from a predator at which an animal decides to flee - risk-takers have a shorter FID. They divided the monocle bream into three different groups: parasite-free fish, fish carrying an A. nemipteri, infected fish which just had their parasite removed.

Photo from Figure 1 of the paper

The research team simulated an attack by a bird (with a weighted PVC pipe) on fish in specially-designed experimental tanks and filmed the response to measure the fish's reaction time to the attack. Even though one would think all that face-gnawing from A. nemipteri would have weakened their host, and not to mention the body of the parasite itself causing significant drag, the escape performance of parasitised fish was not all that different from unparasitised - they reacted and got away from the attack just as quickly as their unburdened buddies. In the field experiment, the scientists donned snorkelling gear and tried to approach any monocle breams they spotted and measured how close they could get to the fish before they fled. There, they found parasitised fish have a slightly shorter FID than parasite-free fish, but not significantly so.

Fish that are infected by A. nemipteri are smaller than uninfected ones, and it just so happen that smaller fish tend to allow predators to get closer to them before fleeing. But whether this is due to the parasite is another matter. Are parasitised fish smaller because their growth have been stunt by A. nemipteri? Or does this face-hugger simply prefer smaller fish because larger and older fish might have built up an immunity to it?

Though it may seem less exciting when we find a parasite doesn't cause much behavioural changes in its host, it is vital to our understanding of host-parasite relationships. Perhaps it means the host is able to compensate for the presence of the parasite. Also it is not clear what the long term cost of having A. nemipteri might be over the life time of the fish. It is also important to treat such a case in its context. Unlike other parasite which have a complex life-cycle and depend upon its host getting eaten by a predator to reach maturity, A. nemipteri is an external parasite that simply sticks to a host and stay for life - if the parasitised fish is eaten by a predator, it'll go down with the host like a bit of garnish and be digested too.

So it is probably just as well that A. nemipteri is not too much of a drag to have around.

Reference:
Binning, S. A., Barnes, J. I., Davies, J. N., Backwell, P. R., Keogh, J. S., & Roche, D. G. (2014). Ectoparasites modify escape behaviour, but not performance, in a coral reef fish. Animal Behaviour 93: 1-7.