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

photo by Cheryl Moorehead

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?

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..

photo by Cheryl Moorehead

But following a little erosion and/or natural "cleaning" of the "fuzz" what you get is the internal skeleton:

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 organisms (such as mollusks) 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...

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...

(photo by Electropod)

to something more like this, which you'll note has both a left and a right side..

(photo by Avian-Cetacean Press)

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...

(this image is from Mooi, 1990 in Paleobiology!)

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 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 in the open. Several species live 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 Aristotle'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..

Brittle stars that live ON sand dollars..

Cloning in sand dollar larvae as a defense!

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