It is challenging to determine the exact age of a seagrass meadow, but we estimate the Shark Bay plant is around 4,500 years old, based on its size and growth rate.
To find out how many different individual plants are growing in a seagrass meadow, you have to test their DNA. We did this for meadows of ribbon weed seagrass called Posidonia australis in the shallow sun-drenched waters of the Shark Bay World Heritage Area, in Western Australia.
The result blew us away: it was all one plant. One single plant has expanded over a stretch of 180 km making it the largest known plant on Earth.
We collected shoot samples from ten seagrass meadows from across Shark Bay, in waters where the salt levels range from normal ocean salinity to almost twice as salty. In all samples, we studied 18,000 genetic markers to show that 200 km² of ribbon weed meadows expanded from a single, colonising seedling.
Another examples like this is are Aspen tree glades and the fungus Armillaria ostoyae.
How does evolution work in these cases? Perhaps its more “intra-organismal” competition between smaller units than the organism as a whole (cells? genes? genomes?). How adaptable are mega-organisms? How much adaptive genetic change can they see over millenium?
This raises the question of what counts as an individual. Should you consider a clonal or asexually reproducing species as a single individual, or does physical separation make the difference? One difference would be the absence of meiosis, but how big a deal is that? It’s claimed that asexual species tend to be more vulnerable to extinction than others; then again, bdelloid rotifers have been very successful for a very long time.
For plants and fungus, this might allow an individual to monopolize resources or a favorable micro-climate, excluding competition. That doesn’t mean there wasn’t any competition to start; the organism got so large because it won.
I’m sensing that we are quickly heading into arbitrary definition land.
There are criteria worth considering. Are the different parts of the colony physically connected in any meaningful way? Is the plant colony strategy giving the colony a selective advantage in any meaningful way? What can the colony do that an individual piece can not do?
I have a funny feeling that the definition of individual used by the authors of this paper would define members of a clone that are separated from each other, with no tissues connecting them, as being part of that one big individual. This is a very functional and practical definition, because it gets your work into the popular science media.
