Re: In colonal marine organisms, what is the individual? the colony as a whole?

Hi Sylvia,

You've hit upon a very thorny question in modern zoology, at least for those of us who study colonial animals. In fact, I have been thinking about the very question you've asked for about 15 years now, ever since I was an undergraduate at UC Davis and took a course in invertebrate zoology from a professor who studied coloniality in hydroids and ascidians. And it still fascinates me, probably exactly because it's such a controversial issue.

As you've already realized, with regards to colonial marine invertebrates the term "individual" could arguably apply to either a genetic entity or any of the clonally produced zooids making up the genetic entity. I think it might be useful to define some of the less ambiguous terms that apply to clonal and colonial marine invertebrates. I'll define these terms as I use them, but I think the definitions are acceptable to most zoologists.

-genet – a genetic individual; all of the tissue derived from a single zygote
-ramet – a (usually) clonally produced unit; in unitary (i.e., non-clonal) animals, a genet consists of a single ramet; in clonal and colonial animals, a genet consists of more than one ramet
-clonal animal – an animal that grows via asexual replication; the resulting ramets eventually detach from each other; e.g., sea anemones
-colonial animal – an animal that grows via asexual replication; the resulting ramets remain connected to each other and are somewhat interdependent; e.g., hydroids, reef- building corals, bryozoans
-zooid – in colonial hydrozoans, bryozoans, and ascidians, the asexually produced ramet that is replicated many times to form the colony; there is usually some physiological or neurological connection among zooids within a colony

Beyond these few terms things get conceptually blurry. Zoologists can agree that genets of colonial animals are composed of zooids, but is it the genet or the zooid that should be considered the individual? Biologists have been pondering this very issue for over a century; for example, T. H. Huxley, President of the Royal Society, addressed the idea of animal individuality in 1849. Today, 150+ years later, we still have no universally accepted definition of "individual" as it applies to colonial animals.

As I was considering an answer to your question, I came up with a definition that works for me; it is deliberately nonspecific and biased towards my own tendency to think of things in terms of evolution, but it may also suffice for you. I would consider an individual of a colonial invertebrate to be the unit on which selection acts. This definition allows for "individual" to apply both to a single ramet in a clone of separated sea anemones (which may experience very different ecological conditions) and a hydroid colony made up of dozens or hundreds of interconnected zooids which act as a single ecological entity.

I hope this helps. If you'd like to continue a discussion of coloniality in marine invertebrates, you can reach me through the MadSci site. I'd be happy to clarify anything that doesn't quite make sense to you, and to share my thoughts on colonial animals.

Allison J. Gong
Mad Scientist