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September 17, 2013

I'll Remember Me: Sponges

Remember the sky you were born under,
know each of the star's stories.
Remember the moon, know who she is.
Remember the sun's birth at dawn, that is the
strongest point of time.  Remember sundown
and the giving away to night.
Remember your birth, your mother struggled
to give you form and breath.  You are evidence of
her life, and her mother's, and hers.
Remember your father.  He is your life, also.
Remember the earth whose skin you are:
red earth, black earth, yellow earth, white earth
brown earth, we are earth.
Remember the plants, trees, animal life who all have their
tribes, their families, their histories, too.  Talk to them,
listen to them.  They are alive poems.
Remember the wind.  Remember her voice.  She knows the
origin of this universe.
Remember you are all people and all people
are you.
Remember you are this universe and this
universe is you.
Remember all is in motion, is growing, is you.
Remember language comes from this.
Remember the dance language is, that life is.

-Poem by Joy Harjo of the Muscogee Nation

Sponges (Phylum Porifera) are multicellular organisms that live in marine and freshwater environments with no specialized organs--and yes, some are harvested, sold at stores, and used to clean our dishes and bodies.  They have no specialized reproductive, respiratory, nervous or digestive organs, but they do have several specialized cell types that form layers in the sponge.  (The different cell types end in the suffix -cyte in the diagram below.)

Sponges attach to hard substrates like rocks and suck water through the pores in their body-walls filtering out organic nutriment and blowing the rest out the top of their chimneys (or more correctly, their osculum).  There is fossil evidence that places sponge-like animals back in the Precambrian at 650 million years ago.  Evolutionarily speaking, sponges are considered to be representatives of the first multicellular animals and who are thought to have descended from single-celled protozoans called choanoflagellates who can live individually or as colonies. 

The ancestor of multicellular organisms (like humans) might've been a group of colonial choanoflagellates whose cells became more dependent upon each other and could no longer dissociate and survive individually in the environment.  The ability to form into a multicellular organism has some interesting consequences and implications like the fact that the individual cells must cooperate and communicate with one another, depend on one another and recognize each other; they sacrifice some personal autonomy, if you will, for the benefit of the multicellular organism.  But just think of what chaos would ensue if the cells of your body suddenly said, "Screw this!  I want my freedom!" and then just left one after another.  For one, the cells of the human body don't survive well on their own in the environment and would thus die.  Secondly, we would disintegrate and die if all of our cells took off to start lives of their own.  The destinies of the multicellular organism and each cell thereof have become inseparable.

Sponges, however, still retain traces of the ancient transition from "colonialism" to multicellularity in their biology.  They have no neurons nor a nervous system, yet they possess a form of collective memory and a form of organizing drive. 

As I said before, sponges are composed of just a few differentiated cell types.  With a few species you can break a sponge down to its individual cells in a jar and each cell will dedifferentiate to an amoeba-like state.

But that isn't the end of the story.  These cells remember where they came from and given enough time they will crawl around, searching, until they come into contact with each other and they will begin to aggregate and specialize back into the cells they once were and reform the sponge again!  It would be like if your body was blended up into a soup of all your dissociated cells, but then, with time, all the cells reassembled and reformed your body again. 

Furthermore, sponges can distinguish "self" from "non-self" and exhibit a kind of immune response with adaptive memory despite a lack of brain.  Experiments were done with Callyspongia diffusa, a species of sponge, where an arm of the sponge was cut off and then grafted back on to the same sponge, but in a different location.  There was no rejection; The arm grew normally in its new position.  But when the experiment was changed and an arm from one sponge was cut off and grafted to a different sponge, rejection ensued.  The foreign arm would atrophy and die.  The sponge could consistently tell "self" from "non-self," repairing "self" and killing off "non-self."  What's even cooler is that when the experiment was redone with the graft from a foreign sponge, after being left to "rest" for 12 days, it would reject and kill the non-self part even faster; It displayed a kind of memory by consistently acting faster when re-exposed to the foreign sponge arm.  This response is similar to our immune response to donor organs.  When a person receives a donor kidney the organ has to be checked for many potential characteristics that might cause the recipient to reject it because our immune system can so sensitively distinguish between what is part of our body and what is not.

What drew me to this "sponge story" is how remarkably complex things can behave despite being seemingly boring and simple.  Sponges don't have brains or neurons yet they know who they are; members of the same species living in close proximity can tell themselves apart from all the others.  Not only can they tell "self" from "non-self," but they can also be torn apart cell-by-cell only to rebuild themselves.  Sponges are the animal with the closest resemblance to fossils that are over 650 million years old.  All the animals that've descended from that common ancestor 650 million years ago have retained the remarkable traits alongside the sponges and have built upon them.  The animal immune system with adaptive memory response and the interdependent, cooperative communities of cells that differentiate colonies from multicellular organisms can all trace their lineage to a primitive animal that lived in the ancient oceans 650 million years ago; an ancestor whose innovations survived all those eons of time, through many forms, and which can still be found in us.

Biology of the Invertebrates, 6th ed.
by Jan A. Pechenik

Evolution: Making Sense of Life
by Carl Zimmer and Douglas J. Emlen

-Seth Commichaux

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