The Rotulidae: Strangest of the Sand Dollars

Leave it to echinoderms to take even the best known of animals, like a sand dollar, and make it even STRANGER than you could have thought!!



But first things first. Sand dollars are highly modified SEA URCHINS that live on sandy bottoms, mostly in shallow tropical to temperate water places. Please make a note of it. (or go read this post here!)

Most sand dollars are either kind of disc-shaped such as this Pacific Dendraster excentricus.
                     






But then you get THESE weirdos!

See all of those flanges or finger-like projections coming off the edges?? Those are NORMAL for these sand dollars!  Let me introduce you to the sand dollars of the family Rotulidae! 

There are actually THREE genera of rotulid sand dollars, Rotuloidea (the oval one), Rotula (the one with holes) and Heliophora (the one without holes). And all of them seem to have some degree of these weird finger-like projections. Living rotulids seem to occur almost exclusively in the West Africa region (and some are fossils-see below).

This image is from Wikipedia! 

Let's go through and meet these three kinds of rotulid sand dollars!! By the way, if you want a scientific guide to ALL THE SAND DOLLARS including the rotulids, it is available as an OPEN ACCESS file here. A fine piece of work by California Academy of Sciences Curator and sea urchin/echinoderm expert Rich Mooi. 

I should note that all members of the Rotulidae are also found as fossils...
                          

1. The genus Rotuloidea. The first is this relatively simple looking guy.. Rotuloidea fimbriata. This species is only found as a fossil, occurring from the Miocene to the Pliocene (that's between 3-23 million years ago).  Found in Morocco.

Image from this Echinoids Gallery page

2. The second member of this fantastic sea urchin trio is the genus Heliophora! 

 That is ONE FANCY ass name! The genus is actually Latin for "Bearer of the sun" and you can sort of see why if you see one positioned as such (upside down). This genus includes two species.. Heliophora orbiculus and Heliophora orbicularis. 

This website recites a legend that Heliophora are actually coins left by mermaids! I wasn't able to verify it, but it does sound reasonable (as a legend of course. Mermaids aren't real.. :-) )

What is going on with all of these crazy flanges??? Its not really been shown exactly what they do. Studies suggest that they might be related to feeding or play a role in keeping the animal from being swept away. (more on that below)..

This species has actually been studied in the field by Joe Ghiold in a paper published in 1985 (note that it is Heliophora, not Rotula as indicated)

His work seems to suggest that these feed like other sand dollars, i.e. in the sand on the bottom. Ghiold suggests that the spines may further function to facilitate feeding. 

Also of interest is that the plates which form the perimeter of these animals seems to grow a LOT faster than the more proximal areas. 

Some members of the genus Heliophora get kinda CRAZY... 

This image borrowed from this Excellent French site: Sciences de la Terre et de la Vie. Photo by Coco CHATAIGNER

3. The genus Rotula. Last but not Least! The scientific name "Rotula"is Latin for "Little Wheel" There is one spectacular species here: Rotula deciesdigitatus. I believe the species name refers to "deci" or 10 in conjunction with "digitatus" or digit referring to the number of flanges or projections. 

So the name literally means "Little Wheel with 10 digits"

Note that in this species there are TWO BIG holes!! These are what's called LUNULES. While these have not been specifically tested, an earlier post I wrote summarized how these holes (the lunules) were thought to prevent sand dollars from being gaining "lift" and being "blown" away by water current!!

And that still does not explain the MANY weird "fingers" along the edge! and on only one side! 

I managed to snap a cool shot of a specimen in Paris awhile back showing the oral side of one of these with ALL the spines or "hair" present...

Living sand dollars are covered by "hair" which are in fact tiny spines that move food to the mouth. 

                             

Here's a video of a sand dollar (but not Rotula) with said spines in full motion!  This is the oral side where the mouth is located.  

Even for sand dollars, these critters are very odd! Many evolutionary and functional biology questions! What are the flanges for? How do they develop? Why ONLY in this group???  Is there something unusual about the environment that is associated with these flanges and shapes???

These sand dollars have been known since the 19th Century and yet we know next to NOTHING about them (relative to their more northern counterparts). These sand dollars are seen throughout hobby websites and are collected by enthusiasts! And yet we know very little about them.

 They present many tantalizing and interesting questions to the future marine and evolutionary biologists! 

What (and How) do Sea Urchins Eat? Sea Urchin Feeding Roundup!

from Wikipedia!

SEA URCHINS! Who doesn't love em? The spiny balls of the sea! We eat em! They're important to marine ecosystems all around the world! They're often visually stunning and they do all sorts of neat and wacky things!

But sometimes I get ahead of myself and forget the basics! Basic questions that everyone has aren't often obvious and so this week a quick summary overview:  What and How do sea urchins eat???

(for info on starfish feeding see this)

Urchins feed primarily with a unique jaw-like structure known as Aristotle's lantern. It looks like this in most urchins but is modified in so-called "Irregular" urchins such as sand dollars, sea biscuits and etc. A bit about this here.

Here is the jaw in action, with the tips being extended from the mouth as the animal rasps away on the bottom it lives on...

For the FULL effect go here to LISTEN as the urchins graze: teeth to reef..  HERE is the accompanying summary account & background.

Believe it or not. Sea urchins have among the most diverse feeding modes within the Echinodermata. Here is a roundup... COUNTING DOWN!

5. Herbivores and Grazing
This is the one everybody knows about and the feeding mode with which most people are most familiar. Many sea urchins prefer kelp and various other seaweeds and marine "plants." There are several species found in cold-temperate water habitats.. California, New Zealand, Chile.. to name a few, and all these places have sea urchin species in abundance.

Here is the famous purple sea urchin, Strongylocentrotus purpuratus engaged in some kelp feeding!

Image by Todd Jackowski

Urchins are ecologically important in kelp forests. Removal of predators (and control of the population) can lead to a circumstance known as "urchin barrens" where sea urchin abundance goes out of control. In those circumstances, even sea urchin poop can become a serious consideration (here)

Here is a nice Shape of Life video that shows feeding by the Purple sea urchin Strongylocentrotus purpuratus.
Echinoderms: Urchin Time-lapse: Eating Kelp from Shape of Life on Vimeo.

Note that while many sea urchins feed primarily on kelp, they are not obligated to do so.  Their diet permits them some nutritional flexibility.....

4. Omnivory and Scavenging

Most people (even several biologists) don't realize that sea urchins can be pretty flexible in their diet. If kelp isn't available, they will obtain whatever nutrition happens to be available.

Some large examples of food here, but feeding also includes microalgae (such as diatoms), encrusting algae, moss animals (i.e. bryozoans) and etc.

Here we have what Strongylocentrotus spp. in the Arctic or sub Arctic feeding on what looks like a wayward or moribund jellyfish..

Image by Alexander Semenov

And in the tropical Indo-Pacific (Lembeh, Indonesia), Astropyga radiata is feeding on some nice dead fish (and who doesn't like a nice dead fish every so often?)

Image by Christian Loader

Up until recently though, feeding in sea urchins has been thought to be relatively passive and opportunistic.  Whatever comes along is good to go!

Most people don't think of sea urchins as aggressively chasing down and pursuing ACTIVE prey...

This notion was recently shown to be incorrect...

3. Predation
Probably one of the most dramatic deep-sea/paleontology events in the last few years was the discovery that, not only could stalked crinoids (echinoderms with a ring of feeding tentacles and a stalk) crawl BUT they did so with some urgency!


It turns out that they were running from Cidaroid SEA URCHINS. I've written up these papers here.

Calocidaris -a cidaroid urchin. Image by D. Pawson

This predatory act was observed on video between shallow water species. Here is a shallow water cidaroid urchin attacking a feather star...

WARNING! Those who do not wish to watch crinoids being brutalized and devoured should avert their eyes!


Further evidence on deep-sea crinoid skeletal pieces is here. That notch is a scar left over from where an Aristotle's Lantern has gotten to this animal...

Urchin bite on a crinoid stem by T. Baumiller

There have also been recent accounts that leaving sea urchins in large numbers under artificial circumstances (sea urchin farms) leads to cannibalism! (here)  Sea Urchins eating OTHER Sea Urchins!!

2. Deposit Feeding/Sediment Feeding
Whew! That was quite a violent section for sea urchins, wasn't it???  Let's go to something a bit more majestic!

One of the NEATEST stories in sea urchin evolution is how a major sub group, the "irregularia" aka the sand dollars, sea biscuits, and spatangoid  ('heart urchins') sea urchins all evolved from a more open lifestyle with the feeding modes shown above (herbivory, predation, omnivory) to a specialized series of body forms that involve plowing through sediment/mud/sand in order to obtain food. Deposit or sediment feeding.  A more involved post of this story can be found in this post.

Feeding in these animals is intrinsically connected with their life mode and body shape. Here are some videos that show some of these animals plowing through sand...sometimes just to get around but maybe also to feed?

Japanese sand dollar plowing through sand...but check out the food going to the mouth at 1:05
.

Note all the spines moving through the sediment...


Again.. check out the spines!


1. Filter/Suspension Feeding
So, now that I just got done telling you that sand dollars and their relatives are deposit feeders.. I will immediately point out the exception!  Perhaps the most UNUSUAL feeding mode in sea urchins is filter feeding, i.e., obtaining food from water currents using some kind of sieve or screen.

Urchin morphology tends to be...counterproductive where this sort of feeding is concerned.. Except in TWO unusual examples.....

Dendraster excentricus-is the so-called "Eccentric Sand Dollar"which lives along the west coast of North America.  So named for the very erratic pattern of its feeding grooves on the oral surface.

Image by fiveinchpixie

Image by Peter_r

Note an oddity in its feeding posture.. these animals are tilted at an angle into the water current standing on its "side" in the sand. Other sand dollars such as the ones listed under #3 lie flat on the sandy bottom.

Dendraster uses its tube feet, pedicellariae and spines to pass along food caught from the water currents to the mouth.. More info on this species to be found here..  This is a pretty commonly encountered animal, but really when you look at them in this fashion, they are freaky deaky! 

Dermechinus horridus  So yes. Saved the BEST for last! 

Dermechinus horridus is a strange deep-sea sea urchin which has a body, literally shaped like a cactus, with sharp, needle-like spines to boot!  It is among the oddest of the sea urchins known.

Image by NIWA

But what's even more strange? Its been postulated that this species captures food from the water!! In other words...a suspension feeder! (I wrote this up here)


Here is some of the FIRST available video of this species in its natural habitat occurring next to brisingids, which are suspension feeding asteroids...


And although I don't wish to steal his thunder, let's just say that info from a recent International Echinoderm Conference presents some intriguing possibilities!

Image by NIWA

Spiny balls! Predators! Diggers! Eaters of Kelp and Purveyors of fine watery nutriton! Huzzah!

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!

Solid Sand Dollar Survival: Enter the Weight Belt!!

So, when last we left the Echinoblog......

We looked at Sand Dollars (highly specialized sea urchins in the order Clypeasteroida) with lunules (i.e., the holes or notches in their body) and how those lunules were used as mechanisms to prevent sand dollars from being picked up and washed/"blown" away by the water currents around them.

But if you don't have lunules, then how do you cope?

Answer: Get a WEIGHT BELT!
This part of the story begins in 1973 (I would have only been 3 years old at the time!) with a paper in Science by famous echinoderm biologist Fu-Shiang Chia which is most simply summarized by their abstract:

Juvenile sand dollars (Dendraster excentricus) selectively ingest heavy sand grains from the substrate and store them in an intestinal diverticulum which may function as a weight belt, assisting the young animal to remain in the shifting sandy environment. The sand disappears from the diverticulum when the animal reaches the length of 30 millimeters.

That's pretty much as simply amazing as it sounds (Plus, how often do sand dollar papers make it into Science?).

Several of these "un-lunuled" sand dollars apparently have a very specialized intestine which has a bunch of tubes and pouches in the peripheral part of the animal's body.

In, at least some species, this part of the intestine is filled with selectively heavier sand grains!

(Dendraster excentricus from SFSU biblio page)

Chia found that in Dendraster excentricus, a cold-temperate water sand dollar from the west coast of North America (California, Washington, Canada, etc.), that there was a high percentage (78%) of high-density magnetite sand grains in these intestinal "weight belts" versus a presence of only 9.8% of the surrounding sand!!

(Image from Geology.com)

However, Chia's study focused on juveniles and it wasn't clear what the ramifications were for OTHER sand dollars or for that matter for adult Dendraster!

Weight Belts & the Evolution of Sand Dollars!

In 1996, a neat paper by Rich Mooi and Chang-Po Chen was published in the Bulletin of Marine Science which surveyed weight belts across the many different types of sand dollars.

How many sand dollar taxa have a "weight belt"?

Was having a weight belt more of an incidental evolutionary response?

Or were these actually a broad phylogenetic character event? or Something that evolved across a whole GROUP of sand dollars?

(from Mooi & Chen, 1996)

It turns out that a WHOLE bunch of sand dollars have weight belts! The black in the figure above shows where sand-filled weight belts were present. Incidentally, it turns out that SOME sand dollars with lunules DO have weight belts (although they seem more weakly developed to me...)

(from Mooi & Chen, 1996)

Also, as it turns out, weight belts are a unique feature (i.e., a synapomorphy) of a major clade (i.e., evolutionary group) of sand dollars called the Scutellina, which includes not only 'un-lunuled" but also the "lunuled" sand dollars.

As it turns out, the weight belt is strongly expressed in juveniles of both types of lunuled and "non-lunuled" taxa..So it seems that weight belts start out as a mechanism for keeping baby sand dollars from getting 'washed away'...

Unfortunately, there was is not yet data for a clear cut conclusion of how much weight belts compensate for resisting hydrodynamic forces in "non-lunuled" sand dollars vs. "lunuled" forms
but expression of weight belts IS an important factor in different species.

For example, in Dendraster , the shallow-water species, D. excentricus, which lives in high-energy environments retains its weight belt at GREATER sizes then does D. laevis, a deeper-water form that would presumably not be as vulnerable to high-energy currents.

So, two things help keep sand dollars stabilized where they live

• lunules that essentially break up the hydrodynamic flow and
• weight belts that keep the body forms stabilized and "weighted" down on the sandy bottom.

So, in this time of financial crisis! We may all continue to look toward Sand Dollars as a good way to keep our economic markets well-anchored and safe from being blown away!!

What are those Holes in Sand Dollars for? The Airplane-Sand Dollar Connection!

What are those holes in Sand Dollars for???

So..probably everyone has SEEN sand dollars or knows what a sand dollar is... (just fyi..they are highly modified sea urchins!).

Most live in shallow tropical to temperate water environments all around the world. They largely live in unconsolidated sediment on organic detritus and when they die, their bodies (also called a "test") frequently wash up on beaches and such..

They come in a wide diversity of shapes and forms. If you live in colder type northern climates, you think of sand dollars as looking like this:

(Echinarachnius parma from the NHM database)

BUT if you live in tropical climates, sand dollars look more like this...

These are familiar to the tropical beach-goer and names such as "keyhole sand dollar" or "keyhole urchins". One of the most obvious features are the presence of BIG holes or prominent notches on the sides of the test (aka the body). These are called lunules.

This is their story.

The data and results for this are taken from a neat paper by Malcolm Telford (1981-Hydrodynamic Interpretation of Sand Dollar Morphology. Bull. Mar. Sci .31(3):605-622) who takes a biophysics/ hydrodynamic interpretation.

Our understanding starts with this basic tutorial on (oddly enough) how airplanes fly.

While watching this, replace "airplane wing" with "sand dollar" 

Let's take a peek!

Okay...So, what's going on here? 

Sand dollars, such as Echinarachnius, tested with wind tunnels and dye+water flow simulators where lunules were ABSENT showed a relatively high coefficient of lift!

What that means is that the same pressure differential that makes airplanes fly is accomplished more easily where lunules are absent. 

Essentially the hydrodynamic flow moves over the discoid form of a sand dollar like Echinarachnius smoothly and easily.

But what happens with a "keyhole" sand dollar such as Encope or Mellita??

Image by Jani Oravisjärvi

Lunules REDUCE lift either by interfering with hydrodynamic flow or by "relieving" pressure from the under surface!! i.e., they INTERRUPT and LOWER the coefficient of lift!

To put this simply..they KEEP THE SAND DOLLAR FROM BEING "BLOWN" away!!

One quotation from his paper sentence sums up the important bit very nicely

The lunules of Mellita reduce lift by providing channels to bleed off excess pressure from the oral surface. This is shown by the clear flow of water (which was marked with dye)...through the lunules.

Telford used three different sea urchin tests as models, and each one had its own specific modifications and stories..

Encope grandis from Echinoids.nl

For example, the edges of the body and around the lunules of this Encope for example are actually modified into pressure drainage channels (i.e., places where lift is further weakened)!

Subtle modifications, such as the absence of tube feet in these areas are also present.

After seeing how lunules and notches can affect and help these animals from attaining "lift-off" one has to wonder then, what happens with critters like these rotulid sand dollars with the odd notches and lunules below??

Image by Bellwizard via Flickr

One of these days I hope we find out!

But how do the urchins WITHOUT lunules solve this problem?? Go here to see all about sand dollar weight belts!