|(photo by km6xo)|
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|
Sand dollars are the skeletons from ANIMALS.
When sand dollars are alive, they are covered with a "fuzz" and look like this..
|photo by Cheryl Moorehead|
|photo by Cheryl Moorehead|
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...
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...
to something more like this, which you'll note has both a left and a right side..
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...
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...
Sand dollars are basically, REALLY strange sea urchins! Please make a note of it!