"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

July 15, 2013

Tetrabothrius bassani

It has been known for some time that intestinal parasites such as tapeworms can accumulate high concentrations of heavy metals, acting as a sink for such substances in the host's body. Back in 2010 a study on shark tapeworms accumulating heavy metals was featured on this blog, but most of such studies comparing the concentration of heavy metals in the host's organs with that of their parasites have been conducted on fish and fewer studies have looked at the heavy metal concentrations of intestinal parasites in birds and in particular seabirds, which form an important part of the marine ecosystem.

Photo of Tetrabothrius scolex (attachment organ)
from this paper
In the study we are featuring today, researchers tested the concentration of various heavy metals in the organs of twenty-three Northern Gannets from the central coast of Portugal. The birds had died when they were tangled up in fishing gear from commercial fishing boats, but they were otherwise in good health before they ended up on the wrong end of some fishing nets. They all had stomachs full of fish and the only parasite found in their intestines was the tapeworm we are featuring today - Tetrabothrius bassani. There are a number of different species in the Tetrabothrius genus, some species like T. bassani parasitise seabirds such as gannets and albatrosses, while other are found in whales - for example, I wrote a post a few years ago about some tapeworms I found in the gut of a beaked whale, which you can read about here.

For this study, the researchers collected at least one T. bassani from each gannet and took tissue samples from the bird's liver, kidney and pectoral muscle to measure the concentration of different heavy metals. They found that, on average, T. bassani accumulated twelve times as much cadmium as the gannet's pectoral muscles. Furthermore the tapeworms had seven to ten times more lead than the seabird's kidneys and liver. Since these worms seem to act like sponges that soak up and concentrate heavy metals, such substances would reach detectable level in the tapeworms well before they became noticeable in the host's own tissues. Because of that, these parasites can possibly serve as early warning indicators for the presence of pollutants in the environment.

Mendes, P., Eira, C., Vingada, J., Miquel, J., & Torres, J. (2013). The system Tetrabothrius bassani (Tetrabothriidae)/Morus bassanus (Sulidae) as a bioindicator of marine heavy metal pollution. Acta Parasitologica, 58: 21-25.


  1. The ability of certain intestinal parasites to accumulate heavy metals and, possibly, other toxins has been of interest to me since I read of the phenomenon several years ago. I have wondered, then, could an infection with such a toxin sequestering parasite ever be beneficial to a given host? Especially in areas where the bioavailability of said toxins is especially high, could selection pressure favor individuals parasitized by parasites with the ability to scavenge toxins? I feel this is worth investigating, at any rate. Sure, the host may have to share some of its resources with the parasite (depending on the parasite load and the species) but isn't that much better than, say, cadmium poisoning which is downright nasty.

  2. Sounds plausible, once the amount of toxins in the environment gets high enough. The worms might even become endosymbionts to their hosts.