<a href=showthread.php?s=&postid=11679586#post11679586 target=_blank>Originally posted</a> by KingDiamond
I think that your above example is a good reason to keep in mind that individual poylps are individual animals. When we talk about the age of a "coral" most are refering to the colony as a whole. Polyps on a colony probably die frequently (eaten by predators, killed by other corals ect..) but can be replaced by new members of the colony. When compaired to a social animal like ants you can see that individuals are not as important as the whole. If an individual ant dies in a colony (save the Queen) it really is not missed because of the constant reproduction and growth of the colony. Coals (individual polyps) probably don't live all that long, while the whole colony can live for many many year.
An individual of a species is typically defined as a genetically unique member of that species.
A colony of ants (or other social animal) includes many individuals. They are all very close relatives (all sisters with ants/bees), but they are individuals nonetheless. They are produced by sexual reproduction and are genetically distinct. A coral colony is completely different. New polyps are produced as asexual buds and in most corals the polyps maintain tissue connections to all other polyps. This is all genetically the same tissue and the same individual.
If some polyps are killed--say the colony gets partially buried with sand--some of the tissue that makes up that colony will die but the individual remains. As the coral grows it may resheet over the skeleton, but it may just as easily sheet over rock, over coralline algae, or over other objects.
While it is true that a single polyp can be separated from the rest of the colony to produce a new colony, the same can be done with many other organisms without us considering each little piece a separate individual. We don't consider every leaf of a tree a new individual. We wouldn't say a 100 yr old deciduous tree growing back its leaves in spring is only a few weeks old, though that may be the age of its leaves.
And for that matter, consider that in living things the very atoms that make up our tissues are replaced regularly. If asked I'd say that I am 24 because I was born 24 years ago. If you examine the chemical makeup of my body, however, you'll find that most of the atoms in my body have been part of my body for only a few months. You'll also find that essentially none of the atoms I was born with remain in my body. Does this mean I'm only a few months old? Certainly not!
I will grant you that due to the colonial nature of corals and the propensity of some species to reproduce asexually due to fragmentation muddies the waters. However, in massive corals where fragmentation is uncommon the age of the coral is, by convention, typically thought of as the number of years before present the planula that produced that colony settled. In branching, foliaceous, etc. corals that are more easily fragmented we'd like to apply the same definition, but the practicality of things is such that it is sometimes difficult in the field to determine if adjacent colonies were produced by different sexual recruitment events (two different planulae) or if the colonies resulted from fragmentation of a single colony. Knowing the difference is very important, howver.
For example, we see that throughout its range the genetic diversity of Acropora cervicornis is not terribly high primarily because fragmentation and reattachment is usually more common than sexual reproduction. Thus, on an entire reef there may only be a few individuals (= genetically distinct) though there may be many colonies. This is similar for Acropora palmata in the West Caribbean/Atlantic whereas sexual reproduction is a more common source for new colonies of A. palmata in the East Caribbean.
We might be tempted to call clones of the same organism different individuals, but from a biologically relevant perspective we really shouldn't. We really should use different terms. In fact such terms are typically used. Genetically different individual of corals are called genets. Different colonies that are genetically the same (= the result of fragmentation) are called ramets.
Genetically distinct members of a species are what we usually mean when we say "individual" and probably what we should save this term for. To refer to genetically indistinct members we should probably talk about "ramets" or "clones" because, while separate and living separately, this are by many biologically relevant measures the same "individual."
Chris
Some parts are more important to a colonial species than others. The polyps are individuals the other tissiues associated with the coral (stolon, sclerites ect...) are important but are the result of the colony of individuals growing.
