Exploring the blurry line between colony and individual
Written by: August 3rd, 2011 // The Realm of Bizzare//
I found this great post on the Portuguese man-o-war, known as the bluebottle in Australia, over at Deep Sea News the other day. It’s eating a fish!
The post also said:
Remember this species is colonial and made of four different polyps or zooids, working in unison and dividing labor. The bladder is a single polyp called a pneumatophore. The long tentacles are dactylzooids used for fishing. The dactylzooids bring the fish up to another set of zooids, gastrozooids, responsible for digestion. Last, there is set of zooids, gonozooids, in charge of reproduction.
So it looks like a jellyfish, but it ain’t. It’s a colony of four specialists working together, each with their own nervous system but incapable of living by themselves.
As I was doing a bit of research about bluebottles and how they sting even when dead and dried up, I came across an interesting question. How do they reproduce? If the gonozooids are responsible for getting jiggy with it, don’t they just make more gonozooids? Where do the rest of the polyps come from?
Well, no one really is a hundred percent sure. I guess that’s fair enough, studying a swarm (a navy) of man-o-wars during mating season doesn’t sound too good. But here’s what they think.
A gonozooid from one man-o-war will make sperm which combines with an egg from another man-o-war gonozooid. Hey presto, you’ve got fertilisation and one embryo – which will become the bladder polyp at the top. That embryo divides several times, then reproduces asexually to make more zooids, which bud out of it. The budding polyps will become either tentacle, digestion or reproduction individuals.
That’s where I got confused. Does this mean that each of the zooids actually come from a single polyp? Are they just differentiated forms of the original polyp, specialised for their particular role? How is this different to a human embryo producing heart cells?
One explanation uses phylogenetics – comparing organisms to see how similar and different they are. Each zooid is similar to solitary Cnidaria (the phylum that includes jellyfish, coral and bluebottles), so can be considered an individual in its own right and a bluebottle as a colony.
But if we define an individual as something with similarity to other individuals, then all the cells of a multicellular organism would be individuals. Are individual humans really colonies of individual human cells? Really, the microbes on and in you outnumber your human cells 10 to one, so you’re more like a walking microbial factory anyway.I think we have a very human-centric model for defining individuals, which is not surprising really. But most species on the planet don’t reproduce like we do, the boundaries between individual and colony are much less clear.
Take aspen trees, which can grow by seeds (sexually) or by underground runners which sprout a tree-clone (asexually.) Over time the runners can decay separating the trees. How can we tell if the trees are individuals or clones, and if we can’t, how do we study adaptation and natural selection?
Tasmania has these Huon pines that are the oldest genetically identical stand of trees which has lasted 10,000 years. Each tree lives about 2,000 years, but the original tree renews itself through genetic clones. Tassie also has the oldest genetically identical plants, clones of King’s lomatia estimated to be at least 43,000 years old.
Strawberries do it too, as do fungus. A single specimen of Armillaria solidepes was found in Oregon the size of 1,220 football pitches and estimated at 2,400 years old. It’s one of the largest organisms in the world.
Where does the individual end and a colony begin? Looking at all the bizarre stuff out there, I can’t help but wonder if we’re the weird ones.
Clarke, E. (2010). The Problem of Biological Individuality Biological Theory, 5 (4), 312-325 DOI: 10.1162/BIOT_a_00068
Read it at the homepage of Ellen Clarke
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