In my last post I included a photo of a freshwater gammarid amphipod (Gammarus lacustris) that had a big red spot on its side. I noted that I didn’t know if the spot was on the critter’s exoskeleton or was something inside the body. Well, that mystery has been solved thanks to information provided by Tony Irwin on the Diptera.info forum and “Deeky” on the PhotoForum. It turns out that the red spot was actually an acanthocephalan parasite called Polymorphus.
You may well ask: what the heck is an acanthocephalan? The phylum Acanthocephala is a group of animals commonly known as thorny-headed worms, or spiny-headed worms. According to Wikipedia there are about 1150 species in the phylum (but that text isn’t referenced, so I don’t know how accurate it is). All of the acanthocephalans are parasitic and have a spiny proboscis that can be everted (turned inside-out) to stab into and attach to the gut of its host (the animal in which it is living and feeding from). Below is a simple diagram of an acanthocephalan. They are basically a sac containing reproductive organs and musculature with a spiny proboscis that can stick out of one end. Acanthocephalans don’t have a digestive tract. They ingest nutrients (that have been digested by their host) directly through their body surface. Most of these animals are very small, but one species (Macracanthorhynchus hirudinaceus) which is a parasite of pigs (and sometimes humans) can reach 65 cm (25 inches) in length. [Yikes!]
Acanthocephalans have interesting life cycles which usually include at least two hosts: one vertebrate (fish, amphibian, bird or mammal) and one arthropod. In the case of Gammarus lacustris; these critters are preyed upon by ducks (amongst other predators). Needless to say they generally try yo avoid beint eaten and tend to avoid light and stay away from the surface (where the ducks are). However, a Gammarus that has been infected by Polymorphus eventually becomes attracted by light and swims towards the surface. That’s right; the acanthocephalan alters the behaviour of the Gammarus such as to increase the likelihood of being eaten by a duck. The bird then becomes infected by the parasite…
Now I am especially keen to get back to that stream this fall and see if I can find more infected Gammarus. Hopefully I’ll be able to dissect out a specimen of Polymorphus to have a look at and maybe photograph. For the time being the only photo I have is at the top of the page: a tightly cropped picture of the infamous red spot. I did manage to find the video below on YouTube of an (apparently) male acanthocephalan. It’s a rather surreal bit of video played to the tune of the song California Girls by the Beach Boys. Enjoy!
So…one mystery has been solved. But now I have another to share: take a look at the upper right corner of the red spot photo (at the top of this post). I wonder what that little clump is growing on the back of the amphipod…?