Tuesday, March 13, 2012

Sand Dollars ARE Sea Urchins. Please make a note of it!

Sand Dollars
(photo by km6xo)
So, last week, I was contacted by an intrepid member of the public who was quite interested in finding out more about sand dollars.. But apparently, the curiosity of this fellow had been stymied by the internet!

GASP! Who would have thought that there could be a lot of questionable and apparently, conflicting information on the Internet!!!

A lot of info on these animals may be somewhat basic to the well-travelled marine biologist, but maybe not so much to the curious or intrigued student or aspiring beach scholar! So, I thought today I would explain some basic sand dollars "stuff" and clarify some of the mystery.

FIRST-some basic introductions.....
...

So, everyone is at least passingly familiar with sand dollars.

Those funny "dollar shaped shells" that one often finds walking along a beach down by the sea shore. How often have you seen this familiar sight?

Sand dollar at Lover's Key State Park
photo by Jdigeronimo66
So, let's clarify this first- and foremost.

Sand dollars are the skeletons from ANIMALS.

Specifically, they are Echinoderms, which is the group that includes starfish, sea cucumbers, crinoids, and of course... sea urchins.

When sand dollars are alive, they are covered with a "fuzz" and look like this..
Eccentric Sand Dollar (Alive) - Dendraster excentricus
photo by Cheryl Moorehead
But following a little erosion and/or natural "cleaning" of the "fuzz" what you get is the internal skeleton:
Eccentric Sand Dollar (Dead) - Dendraster excentricus
photo by Cheryl Moorehead
And what you are seeing? is an INTERNAL skeleton (aka an endoskeleton) This even includes the spines (i.e., the "fuzz") because ultimately all of that (body skeleton+fuzz) is covered by a thin layer of skin or epidermis.

The "fuzz" are actually the SPINES on a very strange looking sea urchin!

Sand dollars are ANIMALS, specifically they are sea urchins! (Class Echinoidea) 

Sand dollars belong to a group known as the Clypeasteroida. There are some 75 genera of sand dollars, 29 living and 49 fossil (following Mooi 1989) with quite a few species.

Most sand dollars live in tropical shallow-water places (e.g., Africa, Singapore, Indonesia, the Bahamas, etc) but a few do live in cold to temperate waters (e.g.,
Dendraster excentricus on the west coast of North America)

Sand dollars are NOT shells. Proper shells are deposited by some kind of organism and are external to an animal's body.

And while we're discussing this, please note that sand dollars have ENDOSKELETONS rather than exoskeletons. That is to say, they are covered by skin and are considered "inside" the animal's body. A Sand dollar skeleton is known as a TEST.


So What makes a sand dollar a sea urchin??

Typically, we think of conventional sea urchins as looking kind of like this...
Sea Urchin
A big sized, ROUND ball covered by spines. These sea urchins often graze on algae an live out in the open on reefs or kelp beds. Often in large numbers.

These have historically been referred to as "Regular" urchins. They have long, well-developed Spines and well-developed teeth as part of a elaborate jaw called Aristotle's Lantern. You can see all of these features in this video...


Now, in CONTRAST....
Sand dollars are members of a specialized sub-group of sea urchins that are often referred to as the "Irregular Urchins" These urchins differ quite a bit from the so-called "Regular" urchins because they show a suite of adaptations to living in sandy/muddy/ bottoms!
In "irregular" sea urchins.. specifically sand dollars the following changes occur...

1. The body (i.e., the test) changes from round and radial (in regulars) to flat and bilateral (in irregulars) like this...
Sea urchin shell

(photo by Electropod)

to something more like this, which you'll note has both a left and a right side..
Sand dollar on the beach

(photo by Avian Cetacean)

2. Spines in "regular" urchins are usually elongate and pointed. But those in "irregular urchins" (esp. sand dollars) are short and specially modified to help in moving sediment..like so...




Here's what a single spine looks like under SEM close up. Not pointed but with a more blunt tip...

3. The special jaw apparatus "Aristotle's Lantern" is modified!

In a "regular" sea urchin, the Aristotle's Lantern or Jaw (seen here from the inside with the rest of the body removed) is used to feed on algae and its positioned as such..
Here's a neat video that shows the oral surface-and you can see the jaw's teeth in action emerging from the mouth


In contrast..here's the jaw (aka the Aristotle's Lantern) from a sand dollar. The top has been cut away and you can see it from the inside (mouth facing bottom). Its been modified into basically, a "crushing mill" for grinding up sand and so forth.
Here's a picture to show you more of what the "jaw" is like in other species.
the starfish within

The individual pieces of the jaw (aka the Aristotle's Lantern -which are often broken) is probably what you hear rattling around inside when you pick one up off the beach...

Plus, you'll often see it used in "inspirational" art and the famous "Legend of the Sand Dollar" postcards and related paraphenalia like this one...
The "doves of peace" are the broken fragments of the Aristotle's Lantern, i.e, the jaw the animal used to crush and grind sand. The "Star of Bethlehem" is a neatly dissected, complete jaw from the inside of the sand dollar.

5. The Body shape has changed as the body as evolved from that of a "regular" to an "irregular" urchin.
The above tree is used from Mooi, 1990 in Paleobiology!

The overall shape has seen a flattening out in the upper right part of the tree where we see sand dollars relative to their more globose relatives.

If you want to read an excellent paper on the evolution of sand dollars, I would suggest checking out this paper by Rich Mooi in Paleobiology. Its from 1990 but has many interesting bits!

Finally..
Here's a kind of loosey goosey summary diagram  by Echinoblog Art Dept!
So again..
The "Regular" urchin or ancestor:
1. Grazes on algae or other items. Many live out on reefs or on kelp beds.
2. Test (the body) is round, globose and pentameral (that is -radial in 5 directions)
3. Spines are elongate.
4. The Aristotle's Lantern is larger and generally, is used to graze off bottoms

BUT if you compare the SAME features in Sand dollars and other "Irregular" urchins...
1. Lives on sandy or other bottoms with lots of sediments or mud.
2. Test is often flattened, and bilaterally symmetrical
3. Spines are shorted and appear "fuzzy" on the surface
4. The Aristtotle's Lantern is flattened and specialized for grinding sand.

Remember that the above differences are morphological ADAPTATIONS that are specifically tied to living and digging through the sandy, bottom habitat.

The spines and Aristotle's Lantern see clear modification for a specific lifestyle... In many ways, this is a beautiful example of how morphology has changed as adaptation to a specific life mode.

Sand dollars have many NEAT adaptations to living on sandy bottoms.

For example, this one and this one are used to keep it from getting washed away..


Cloning in sand dollar larvae as a defense!

Sand dollars are basically, REALLY strange sea urchins! Please make a note of it!

16 comments:

Jenny said...

Excellent info!

Anonymous said...

Yes. But why the scary music in the sand dollar time lapse movie?

HuntandGather said...

Thank you, great information. I really loved the videos as well.

Anonymous said...

I found a sand dollar fossil behind my house in Puerto Rico. I would like you to see it. Where can I send you the photos?

ChrisM said...

is this one? Mellita quinquesperforata is one of the more commonly encountered species in that area:

http://www.echinoids.nl/Echinoids/Mellita-quinquesperforata/Mellita-quinquesperforata.htm

feel free to host it on a photohost and post in the comments!

Anonymous said...

Mine is like the first photo you have on this page. I sent the photos on the link you gave me.

Anonymous said...

Thanks for sharing this info. My son just got a couple of fossil urchins for christmas and we couldn't figure out why they looked more like sand dollars.

Anonymous said...

Very impressed with your researched knowledge. I learned a great deal and you answered my question...thank you

New Beginnings said...
This comment has been removed by the author.
marsh said...

bsebyalGlad to know we didn't take home a live one. Thanks.

Anonymous said...

Excellent blog! I stumbled across it in my search to learn how sand dollars breathe. I have discovered they breathe by diffusion through tube feet. But i've seen differing answers online to whether they can breathe out of water when they're on the sand at low tide. Can you help me with this?

ChrisM said...

I would think that gas exchange works best if there is water present, so they would need to be submerged. Its not likely that they will do well out of water for very long.

Anonymous said...

very informative. well done. I learned quite a bit about these creatures which I've been curious about. thank you.

Diane Tomblin said...

We were at beach this weekend and found 3 things that look kinda like sand dollars but they are not flat.... Is this a sand dollar or something else??? I have only ever seen the flat ones before now.... I told my kids I would help them find out what they are but have not found a picture on Internet any where.

ChrisM said...

Hi Diane,
if you are on the SE coast of the US, then what you probably saw was the sea biscuit, Clypeaster
which looks like this

http://science.kennesaw.edu/~jdirnber/MarBioBelize/Lectures/MarineInverts10/Clypeast.jpg

these have a body form which is much more swollen. Hope that helps!

Diane Tomblin said...

Thank you so much ChrisM you have been very helpful and I appreciate the info very much!!!