Tuesday, April 13, 2010

Starfish "Bones"! Understanding evolution & adaptation from starfish ossicles! (aka "bones")

(what is this thing? see below for the answer!)

So, during a tour I gave the other day, I was explaining just how it was that something as offbeat as studying starfish evolution IS actually pretty similar to anthropology and human forensics.

Or, to put it more simply...what I and some of my colleagues do is really NOT that much different then what they do on the TV show "Bones"...The premise of the show is that the namesake "Bones" (=Dr. Temperance Brennan) can reconstruct a history of what happened to an individual by looking at and analyzing their bone structure and INFER the history (and thus, the criminal events) of what happened to that individual.
Believe it or not, a starfish (or even a brittle star) is NOT all that much different. Let's take for example, this goniasterid starfish, Ceramaster.
Asteroids (and other echinoderms) are made up hundreds of thousands to millions of small calcium carbonate bits infused with tissue that are made into a living 3-dimensional box aka the BODY of the starfish.
Kind of like those 3-D puzzles that you see around...
These individual pieces, called OSSICLES or sometimes more informally-PLATES compose the body of not only starfish but all echinoderms. It is the arrangement of these plates that allow us to identify and classify them.

Ossicle architecture and arrangement are analogous to the way bones are understood in humans (and other vertebrates). And by knowing where these "bones" (=ossicles) are located, we can use them to reconstruct an animal...and (within limits) its history.
This comes in quite handy for paleontologists who study fossil starfish!

Fossil starfish remains are often broken apart and disarticulated (i.e., they are discovered as individually separated pieces). Trying to put them back together to resemble the formerly living animal can be quite a challenge.

There is a HUGE literature of European starfish paleontology that focuses exclusively on individual ossicles like this. One great website with images of individual ossicles and animals can be found here.

In some places, you can literally find bucketloads of individual ossicles! Sometimes they are so abundant, they can actually be used to identify stratigraphic rock layers. Very RARELY are starfish fossils ever found intact.

Okay...so ossicles are nice...but how is that ANYTHING like being the starfish version of Temperance "Bones" Brennan ???

What can you tell about a starfish as it lived from dead pieces?
If we observe how the SAME series of ossicles in parallel position on the animals' body, we can see how those ossicles series change across different species. In vertebrates we would look at how those same BONES have changed across different animals

In other words, how have those bones/limbs have EVOLVED or ADAPTED to different environments.
We can do the same thing with starfish!!
What you're seeing below is the above Ceramaster with its surface body wall completely removed.

That's the mouth at center with each of its five arms projecting from what's called the mouth frame.

Each arm series (kind of like an arm or a leg in a vertebrate) is made up of a very distinctive series of ossicles called the ambulacrals. The ambulacrals are often specific to different kinds of starfish species!
Let's take this Pacific Asterias amurensis for example...
If you take the top off..you see that there's a similar but different structure.

The individual ambulacral plate/ossicle series are made of of much thinner, smaller pieces as you can see here...
These suggest that there is much more flexibility and range of movement in the arm, which is what you would expect for Asterias, which feeds on mussels and other shellfish versus Ceramaster which is more of a less active, sit-on-the bottom type deposit/opportunistic bottom feeder.

In contrast, we look at a very opposite feeding type...the brisingid, filter feeder Novodinia antillensis. I've written up how these feed here...

But long story short, they hold their arms up into the water column to pluck tasty crustaceans and other tiny food out of the current as it flows by....

The ambulacral ossicles on Novodinia look like the vertebrae from a backbone!! These permit the arms to be very long and stable but ALSO to FLEX! and curve into the water.
And finally...just to show you another extreme, here's a pic of Culcita novaeguineae, a living tropical Indo-Pacific "cushion star"
And here is what happens when you cut one open!

Much of the inside is filled with water (i.e., empty) but you've got the massive armored body wall and you actually need a "support pillar" inside the body cavity that connects between the body wall and the ambulacrals.
The mouth frame is pretty thick and robust...and the ambulacrals actually have kind of a special cavity for the tube feet to pull up into when the animal doesn't want any of its soft parts exposed!

The weird insides of this cushion star are actually pretty typical for ALL oreasterids.

So, you could infer from the individual ossicles or even a badly damaged specimen that you've NEVER seen alive before, the possible KIND of life mode it had! Not always...but it is one way to do the job!

For MORE starfish forensics goodness! Check this post from Echinoblog 2008!

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