As you may have surmised by now, I’m interested in interactions between unlike organisms, or symbiotic relationships. These can take a variety of forms and so I thought it worthwhile to distinguish between the three main types, as referred to by biologists, naturalists and the like. It’s also worth noting here that a symbiotic relationship is a long-term (or comparatively long-term) association, differentiating it from passing interactions such as those between a predator and its prey.
My favourite type (you don’t have a favourite type of symbiosis? … sort it out) is the mutualistic relationship, the kind that most people connect with the term ‘symbiotic’. This essentially means that, over the course of their association, both individuals benefit from their interactions with one another (this is not to say that such a relationship is without potential cost, rather, overall the benefits to both are greater).
Remember Finding Nemo? The classic example of a mutualism is the well-documented relationship between clown fish and anemones, in which the clown fish reduce the parasite-burden of the anemones and, in return, are sheltered from predators by the anemones’ tentacles. It’s also thought that the bright colours of the fish attract smaller fish for the anemones to consume and that the fish additionally benefit by ‘hoovering up’ the fishy leftovers; they didn’t mention that bit in Finding Nemo, of course.
Most people are aware of the second type, parasitism. The parasite in an interaction is one that benefits from an association to the detriment of the other, the ‘host’. A parasite may have more than one host throughout its life-cycle. It is the most ‘selfish’ form of symbiosis and abounds throughout the natural world. It is often in the best interest of a parasite to keep its host(s) alive until it is ready to move to the next stage in its life cycle; in some cases, this includes manipulating the body or mind of the host to its advantage.
Euhaplorchis californiensis, a tropical flatworm, has three hosts: horn snails, killifish and various killifish-eating birds. It therefore relies on what is essentially an entire food chain to survive. Horn snails eat the flatworm’s eggs, which hatch and develop inside the snail’s body and make it infertile (by eating its gonads, in case you were wondering). The flatworm larvae, which have spade-shaped heads and ‘swimming tails’, then leave the snail and head out into the water, where they attach to the gills of killifish and make their way inside, all the way to the brain. There, they form cysts across the brain’s surface, which alter the fishes’ behavioural responses. It is thought that, when an infested killifish spots a predator, its response (rather than the normal behaviour, i.e. hiding) is to ‘flash’ to the surface and flop around. Unsurprisingly, this raises the chance of the parasite-infested fish being eaten by a local shore-bird, thus allowing the flatworm to complete its life cycle by breeding inside the bird. The eggs fall to earth in the birds’ faeces… and the whole thing starts again. There is a nice little video about this particular parasite here.
I chose this example partially because I think it’s fascinating and partially because it is a good demonstration of the variability of parasite-host interactions. All of the species here are disadvantaged by the parasite’s presence, but to differing extremes. Although the birds may not appear significantly harmed, carrying parasites (by definition) has a detrimental effect on health; the worm is taking its sustenance from the birds’ own supplies, which can lead, for example, to their lacking in vital nutrients or becoming susceptible to other diseases or parasites. The snails become infertile, a clear reproductive disadvantage (although I’m sure any man reading this would be more immediately concerned about the prospect of having their gonads eaten). The fish, of course, lose their lives. Again, some might think that even this is preferable to the slow chomping of gonads, but that’s not for me to say.
Parasites are vastly numerous and variable; humans, as I’m sure you know, carry a fair few themselves. In some cases, it has been proposed, humans can also take the role of a parasite; although people are often offended by this classification, it is not completely without grounds. The situation is generally more complex when humans are involved, because while one human or group of humans might maintain a practice that could be considered parasitism (for example, the draining of bile from live Asiatic bears), this behaviour is not typical of humans; there is an ethical and cultural dimension to our own practices that, one would assume, does not exist amongst flatworms.
A brief look, now, at the third (often forgotten, I think) type of symbiosis. This is called ‘commensalism’ and is inhabited by the unsung (though not always unnoticed) creatures of the symbiotic word. A commensal is one that benefits from its association with another species without significantly helping or harming their associate.
The human-rat relationship (and here I mean the most common kind, of rustling in the night and skittering in the gutter) is a classic case of commensalism; wherever humans go, there go rats (see their world distribution map, below). Rats live alongside humans and benefit from our shelter, our warmth and our refuse; but they are not, in themselves, parasites upon us. Don’t be fooled by the plague and similar diseases; these are carried by fleas, which are parasites on the rats, but the rats do not benefit from our disease; they are simply a part of its transmission, as water is to cholera.
Rats are successful, in significant part, because humans are. Although rats are probably the most common example of a human commensal, as I’ve already noted, different individuals form different kinds of symbioses with humans: the sewer rat may be a commensal, but the fancy rat – the same species, just somewhat altered – can be a pampered pet. On the other side of the coin, there is the laboratory rat, also the same species.
So, I’ll finish on more stable ground by introducing you to another commensal, a new friend you might not previously have met, which goes by the name Demodex. Demodex mites (there are about 200 types) live around the hair follicles of mammals and two species (d. folliculorum and d. brevis) live on humans, specifically on your eyelashes and eyebrows. Yours? Yes, quite possibly; it is understood that about half of all adult humans have them. They eat skin cells and secretions and occasionally wander around at night (yes, really), but for the most part do no harm, certainly not alone, though occasionally a significant population can cause irritation to the skin. If you are so minded, you can study your eyebrow hairs under a microscope to see if you have your own commensal collection. I haven’t got round to that, to be honest, but if I did find them, I think I’d name them all Dexter.
It is clear from these examples that the lines between the types of relationship we classify as ‘symbiotic’ are not clear; similarly, it is not always clear when a relationship is lengthy or involved enough to be classed as symbiotic. This returns us to my previous post about classification, and I would stress that this is a ‘rough guide’ only; the world is really too complex to be so simply divided. Still, I personally find that these particular divisions provide a broad frame of reference for the various associations and interactions in this blog (and elsewhere in life; nature programmes, the garden, the office…). I hope they serve you and your Demodex just as well.
(If this has inflamed a dormant fascination for parasites, you might be interested in this blog. If you just want to see another creepy parasite altering brain activity, this is a brilliantly disgusting example).