I have in a couple of earlier blogs hinted at my interest of bizarre symbiotic relationships.
For many reasons close symbiotic relationships are very interesting from an evolutionary point of view, as animals in most cases can be expected to maximize their own progress at the cost of anything or everyone else. In many symbiotic interactions such a total self interest is not obvious. Mutualistic interactions where both participants convey a certain amount of success to their mutualistic partner are of course prime examples,
but even interactions between parasites and their hosts are often in that realm, as the parasite or disease in some sense has an evolutionary pressure on it not to be to good and efficient in translocating energy from the host. A parasite that is to efficient will in many cases to quickly use up the hosts resources and cause the host to die. Diseases share many of the same evolutionary pressures and can often be used as examples for understanding such relationships. Parasites or diseases that manage to “hold back” can be very common and widespread. Think for example on common colds, that rarely kill their human host but leave the host alive and well enough to interact with other potential hosts, sniffling and sneezing for a long time, thus spreading the parasite or disease while keeping the host happy enough to not die with the parasite. Many of the most common parasites of people are just that kind of low key parasites, using up just quite limited amounts of resources of their hosts, thus achieving a long period of mutual life facilitating spread of the parasite or disease. Lice, flees, many roundworms and a number of other intestinal human parasites are examples of such interactions. In contrast, gruel and horrible for those few that gets it, a very potent disease such as Ebola will have severe difficulties of spreading to a large number of victims, as victims simply die before spreading gets significant. In marine systems, many of the so-called commensal interactions that we meet are probably weak parasitic interactions with the commensal/parasite taking little energy from its host in exchange for a long life on the host. Examples that come to mind are flatworms living on many corals,
the fish lice living on fish mucus, the crustaceans parasitizing wire coral gobies.
Actually, the wire coral gobies that so often are hosts of crustacean parasites themselves are probably mild parasites on their wire coral hosts, killing of polyps on the host in order to create a safe egg laying site for the eggs of the gobies. And let us not forget the Cymothoa isopods probably parasitizing anemone fish.
So next time you get a cold or get an internal parasite, it might just be reasonable to thank your ancestors and the disease organism for their coevolution during millennia, currently keeping you alive from a disease or a parasite that in many cases would have been very problematic had you encountered it some tens of thousand years ago.
Some diseases and parasites have gone even further in their effect on us and changed our behavior, thus spreading the disease or the parasite even more efficiently. Many parasites have intermediate hosts that need to be eaten by a final host in order for the parasite to mature. There are several examples of parasites in intermediate host altering their intermediate host behaviour such that the host is far more likely to be killed and eaten by the final host of the parasite. Sexually transmitted diseases such as for example Syphilis, a disease that can keep spreading from its victims for many years, can be expected to alter the sexual behavior of its host, thus facilitating the spread of the disease. And after all, many of us have experienced exactly such a behaviour altering effect of a disease, as one of the symptoms of ordinary colds is sneezing, which is a behaviour that efficiently spreads the disease. We have few studies addressing that kind of potential relationships in marine ecosystems, but there are a couple that I will return to in a later blog post.