Technically, yes. The glochidium is a larva of the freshwater mussel, and attaches to fish, but it does not draw nutrition from the fish; it is more analogous to a burr, using the fish as a dispersal vector.
If fleas and ticks are to be considered parasites because they eat prey in units of less than one, then there are parasitic vertebrates by the same token: lampreys and vampire bats. A lamprey is pretty much just a vertebrate leech. I was definitely not thinking along those lines, though. The part about living on or in the host was part of my thinking.
The main direction my question was going towards is the reasons for the absence of some possible combinations.
Agree with you about the lamprey. Perhaps some combinations are not possible because there is such a huge difference in parasite/host responses. Plants cannot parasitize vertebrates because the host immune response is just something they are unable to deal with? Basically, Iām guessing. Then there is environment to think about - a salt water organism cannot live on or in a freshwater or terrestrial host (and vice versa). Basically, Iām just throwing out some ideas. I know nothing about a plantās immune responses. Itās quite a fascinating question though.
Partly this is a matter of how one defines Plants. Green algae are actually plants and do sometimes parasitize mammals, including Homo sapiens, Prototheca being the most well known example but not the only one.
Totally agree with that! Haldane also said - āFrom the fact that there are 400,000 species of beetles on this planet, but only 8,000 species of mammals, the Creator, if he exists, has a special preference for beetles.ā
As an evolutionary biologist, this is the definition of parasitism that I grok. If the interaction is beneficial to one species and detrimental to the other, and does not cleanly meet the definition of predation, the one that benefits is a parasite.
For example, imagine a counterfactual world in which humans who keep cats therefore have fewer kids, but cats who are kept by humans live longer and have more kittens. In that world, cats would be parasites of humans.
There are bacterial parasites of other bacteria. See for example Bdellovibrio.
It is also worth noting that there are many documented cases of interactions that are only sometimes parasitic. For example, some human diseases are caused by generally beneficial members of our microbiomes in people with suppressed immune systems.
Thatās interesting. Being Devilās advocate again, if fleas, bedbugs or lice feed on human blood, but do not affect the humanās overall function (ignoring possible disease transmission), would you consider them to be parasites? BTW, Iām actually interested to hear your response, not just stirring the pot!
Yes, according to the definition used in ecology and evolutionary biology, if the blood-suckers have exactly zero effect on the hostās survival and reproduction, they are commensals rather than parasites. In practice, of course, having fleas, bedbugs, and lice can affect survival and reproduction through disease transmission, decreased attractiveness to potential mates, immune suppression, and so on, even through these parasites rarely directly kill anybody. But that time in 1985 when lice spread around my elementary school and my parents had to use the special shampoo? Those were probably commensals.
Agreed. One of the best examples of the effect on fitness of blood-sucking parasites is playing out in North America right now where the expansion of winter tick range (presumably because of climate change) is correlated with significant stress on moose populations. There are documented cases of moose dying from the effects of blood loss related to heavy tick infestation.
Sometimes the formal definition of a word does a poor job of capturing what we mean by it. Consistently lethal parasitic microbes do meet some formal definition of parasitoid, but are clearly not what are meant. Other definitions of parasitiod specify that the parasite must be an animal, which I donāt like because if there was a plant that did that to another plant (or to an animal) I think we would still rightly call it a parasitoid. What we really mean is: any organism whose parasitic method of reproduction is sufficiently similar to that of the ichneumon wasps that we want to use the same word to describe it.