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

(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 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 endoskeletons 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 again-an INTERNAL skeleton (aka an endoskeleton) because ultimately all of that "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! They are bona fide members of the class Echinoidea aka the sea urchins.

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. Shells are external and although deposited by an organism, they are essentially outside the 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)

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 as drawn 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..

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!

The Origami Echinoderm Tree of Life

This is REALLY neat! Overall, the tree is phylogenetically accurate and the origami representations are AMAZING!
Although obviously, still missing sea cucumbers and feather stars (the author has a cool origami stalked crinoid!)

My thanks to reader Tentaculus for sending me to the original source page on origami animals (and esp. showing different origami echinoderms), including one or two Paleozoic echinoderms! Go to this by clicking here: (http://t-usuda-origami.blog.so-net.ne.jp/archive/c2300418772-1)

A scientific mystery! Self-Fertilization in Parvulastra exigua

Photo by Nuytsia@Tas on Flickr

Scientists LOVE a good mystery. And biology is full of them. But some are not obvious and are found among the tiniest of creatures....

Case in point, is this tiny little beast called Parvulastra exigua (called Patiriella exigua in older literature)

(photo from Biodiversity Snapshots at Museum Victoria!)

This tiny species (often about the diameter of a dime or small coin, 1-2 cm across) is present widely across the Indian and Pacific oceans as you can see in the dark brownish pink colored areas on the map below... Primarily in Australia and on the east coast of Africa (Indian Ocean)

(from the World Asteroidea Database listing for P. exigua!)

That little starfish occurs over a HUGE AREA.

When we see invertebrates occurring over an unusually large region we typically assume that its ocean borne larvae are dispersed via ocean currents, carrying the "babies" of this species over a huge area.

Larvae transported in this way are referred to as planktonic that, is they swim and are dispersed via ocean currents. Think of it as similar to the way pollen is blown through the air and carried to far away lands..

But as it turns out, Parvulastra exigua doesn't have planktonic larvae!!

Note the red box below.. There's the animal plus its corresponding bottom-living (non-planktonic) larvae.

A mysterious paradox then enters the scene!

How did such a widely distributed species get around if the larvae DON'T GET carried around on ocean currents??

A recent paper from Maria Byrne's lab at the University of Sydney by Sergio Barbosa and his colleagues in the new issue of Marine Biology adds some new insight into this big question.

Parvulastra exigua has many reproductive strategies!
A survey of the biology of P. exigua shows that it has a pretty diverse tool kit of reproductive strategies!


For example, the eggs of P. exigua can be fertilized by the direct release of sperm onto the egg masses!

Now, this may not sound remarkable, but remember that nearly all starfish reproduce by directing their eggs and sperm into the water!

So, this kind of reproduction is ACTUALLY a kind of PSEUDOCOPULATION. Hmmm...where have we seen that before? Go here and see some of my earlier posts on this weird pseudo-sex behavior.

BUT As Dan Savage often says "But wait! That's not all, there's MORE..."

Pseudoexigua exigua are also what's called simultaneous hermaphrodites!! and one funky thing that simultaneous hermaphrodites can do? is to SELF-Fertilize!

Could this have some bearing on the paradoxical distribution mystery?

Could having the powers of outcrossing AND self-fertilzation explain how this tiny starfish gets so far around the ocean??

Barbosa and his colleagues sampled egg masses with evidence of self-fertilization from 5 populations across Australia and here is what they found!!

(Fig. 2 from Barbosa et al.)

The top graph (Fig 2a fr. the paper) shows the % of egg masses with evidence of self-fertilization. The one locality on the far left (from Clovelly) shows that highest abundance of egg masses produced from self-fertilized individuals.

The lower graph shows % of developing embryos in egg masses that had evidence of "selfing" (self-fertilization). The overall "average" % of developing embryos within egg masses that showed fertilization was 8.9% of the total..

So, a significant number of the population can self-fertilize! Some populations do so more than others!

Some Reproductive Dynamics!

• In other simultaneous hermaphroditic invertebrates the evolution of "selfing" may serve to Eliminate unfavorable genetic material
• create a way to ENSURE that a population of "your" species persists in times when there are low populations or when there are low numbers of males and females. This strategy has been observed in other species of sea stars and may be a likely explanation here.
• Given the difference in "selfing" among the five populations surveyed-it seems possible that this is a feature which varies across populations. Why did some of the other populations LACK "selfing" altogether? Why did the Clovelly population have such a relatively high % of it? Environment? Innate population differences?

In biology, we often look to "outcross" or to spread our genetic material across other individuals in a population in order to "reshuffle" or diversify the individuals in the gene pool, thus providing more material for selection.

BUT we have exceptions! Sometimes, "selfing" (i.e reproducing with oneself as a simultaneous hermaphrodite!) and yes, I've always wanted to write that sentence Can give a reproductive advantage. How?

Suppose you are the ONLY member of yourself in some remote location? Creating further progeny from yourself is one way to start a population!

This species, for example, is called Parvulastra vivipara-another hermaphrodite. This species actually BROODS its young (i.e., it has little baby starfish that live on the adult!). P. vivipara produces young but often in isolation for YEARS...

(Photo by Nuytsia@Tas)

There are challenges here. Is there enough diversity between your eggs and sperm? How many fertilized eggs mature to the juvenile stage relative to "out-crossed" individuals?

The important point here is that this mixture of reproductive modes..the pseudocopulation, the outcrossing, the "selfing" and so on are all important if not critical to understanding the widespread distribution/ success of this species as it becomes distributed across the oceans!

Sand Dollar Dissolving Before your very eyes!

Just a little extra... A Pacific sand dollar (Dendraster excentricus) left to dissolve in vinegar solution as an example of ocean acidification.

Hard to say if skeletal dissolution would happen exactly like this, since it would likely affect the larvae and other aspects of life history..but its an interesting video to bear in mind...

more soon!

Bacteria in the Belly of the Beast! Prokaryote Diversity in Abyssal Sea Cucumbers!

(photo from the USNM Invertebrate Zoology Collection)

This week a follow up to an earlier blog I wrote in October last year about microbes in the guts of deep-sea holothurians!

This new paper is by Teresa Amaro, University of Aveiro in Portugal and her colleagues and has been recently published in Deep-Sea Research I vol. 63: 82-90. (she also wrote the earlier paper that I wrote about above)

This paper focuses on this lovely echinoderm champion-the sea cucumber Molpadia musculus-which occurs throughout the North Atlantic, buried in deep-sea (circa 2000-5000 m depths) mud.

(photo from the USNM Invertebrate Zoology Collection)

Molpadia are infaunal deposit feeders-that is they live buried in the mud, head down with mouth directed into the soft, muddy goo... (the mouth is shown below-this is the part that faces into the mud! Lovely, yah?

(photo from the USNM Invertebrate Zoology Collection)

Their "tails" presumably face upward to permit the feces to vent out on the surface, following ingestion of the mud/sediment...

(photo from the USNM Invertebrate Zoology Collection)

Molpadia can occur in very high biomass and in great abundance on the deep-sea floor with some 220 individuals per square meter!!

Bacteria and Archaea on the Deep Benthic Floor!
So, you've got a big, living dirt-eating vacuum cleaner buried in the mud of the deep-sea floor. What are they doing down there?? What are they interacting with? Are they Eating?

(photo from the USNM Invertebrate Zoology Collection)

For years, it had been noticed that there is actually quite a lot of microbial life present in the oceans. Scientists have been using methods known as DNA fingerprinting to recognize the amount of life that is present on deep-sea bottoms.

The diversity we focus on here, emphasizes two of the major Kingdoms in the overall Tree of Life-The Archaea and the "Bacteria" or the Prokaryotes-those with less developed cells relative to the eurkaryotes. Note in the diagram below.. "Eurkaryota" refers to organisms with full and complete cells, such as algae, protists and all the animals.

(Image from Wikipedia)

The prior blog that I wrote about this showed that there different "kinds" of microbial life present not only on the sea bottom but actually IN the guts of these deep-sea sea cucumbers.

Amaro's newer paper now shows that there is an astonishing diversity of microbes associated with the gut contents in Molpadia musculus! And details the distribution and diversity of the kinds of microbes present.

Amaro and her colleagues surveyed the abundance of different prokaryotes both in the sediment (measured from sediment cores) and at different points along the gut of Molpadia and found this....

(Fig. 2 from Amaro et al., 2012)

First (top-Fig. 2a) , that the actual amount of prokaryotes (this includes both bacteria and archaea) was highest in the top 1 cm of sediment.

Second (bottom-2b) That after survey the abundance along the animal from the gut to the end, the greatest amount of prokaryotes was actually found in the oesophagus (i.e., the front end of the intestine) of the animal.

The latter result likely shows the result of digestion with most of the food (i.e. bacteria) being digested as it proceeds to the midgut and so on...

Both of these results make sense. Prokaryotes are settled out on the surface and the oesophagus (the front end of the intestine) is where all of that enters into digestion.

What kinds of bacterial diversity did they find?
The authors counted the diversity of bacteria in terms of Operational Taxonomic Units (OTU)s-that is a general term that refers to the different types of organisms or species present.

They found 28 to 71 OTUs in the surrounding sediments but about 33 to 105 in the sea cucumber itself!

(Fig. 4 from Amaro et al, 2012)

The authors further compared the abundance and composition of the bacterial OTUs (i.e., the diversity) between the sea cucumber and the surrounding sediment.

Now, if the surrounding bacterial were food-one would expect the same diversity in the sea cucumber as in the nearby sediment.

BUT, as it turns out, about 40% of the bacterial OTUs were uniquely associated with the gut contents and were ABSENT from the surrounding sediments!

Some Further Dynamics from the paper...

1. Prokaryotes decrease in abundance along the gut. From the above Figure 2 we see the abrupt decrease of abundance of prokaryotes from the oesophagus to the hind parts. This was a 6-fold reduction from the oesophagus to the hind gut.

This suggests that GREATER than 80% of the total were digested by the animals.

2. Archaea vs. Bacteria in the Hindgut. They also found that among the prokaryotes-bacteria accounted for only about 55% of the total number found in the oesophagus but only 35% in the hindgut, BUT archaeans actually increased in the hindgut! Why? The authors suggest that the different kinds of prokaryotes are used differently across the intestine..

3. But do the prokaryotes keep Molpadia fed? Well, in a word: No.

It turns out that these bugs only contribute about 0.5% of the total organic carbon used by individuals in the study. The authors conclude that Molpadia, at least, does not rely in any significant way, on prokaryotes for their food requirements.

4. So, if not food-then what are they for?? As indicated above- the DNA fingerprinting identified the fact that the "bugs" in the surrounding sediment were significantly different from those in the sea cucumber itself.

So the authors conclude that the animal carries the prokaryotes around in its gut as commensals-possibly to facilitate digestion in a manner similar to how termites and some mammals (such as ourselves) carry around a prokaryote fauna to facilitate digestion. (think of it as yogurt for sea cucumbers!)

And so there we have it. Some deep-sea sea cucumbers are just big bags filled with prokaryotes that feed on mud and ooze!

What can the Internet do for Biodiversity? Let's study COMMENSALS!! Some biology can't be studied from samples!

(photo by MerMate)

The Internet is amazing!

But often one gets the impression that a lot of people haven't quite realized the potential for discovery.

What can something as simple as humble as a photostream or video channel do for science?

Especially for something like exploring biodiversity? For looking at the many different kinds of animals -some that have yet to be discovered?

What makes the Internet so potentially important is YOU. Everyone out there. A nearly infinite amount of eyes, cameras, videos, talent, and interest.

What does that mean? That means that some people have the good (nay excellent!) graces to take pictures (and/or videos) of moments and/or species in places that many scientists just aren't or can't be. I believe the buzzword is "crowd sourcing".

Sources like Flickr and YouTube thus become great treasure troves of potential discoveries.
Moments and relationships caught of the live animals doing their thing!

So, today I thought I would use some videos and images to demonstrate some commensal relationships between sea stars/starfish) that most people don't often see because they are best seen in the wild when the animals are left to their own devices.

But then, some incredible individual took a picture or video of it!

And in conjunction with a little bit of narrative (on my part) there is another layer of appreciation of these images as more than just aesthetically pleasing-but a representation of something in nature that everyone can enjoy. GO!

1. Fish Living commensally among the oreasterid Protoreaster nodosus

This video was exciting! I'd never heard of fish living in and around asteroids and have yet to discover prior accounts.

Was this unique or does it happen all the time? Are fish juveniles tied to sea stars? Is this important to their life cycle?

Note also the tiny little brittle stars living on the spines! Probably Ophiactis savignyi-a species of ophiuroid that may literally live everywhere!
(photo by Mermate)

2. The benthic ctenophore Coeloplana living on the surface of Echinaster luzonicus
(photo by Arne Kuilman)
So, what are these?

Ctenophores are usually found swimming in the water column like this (intro to the group in the video):


But some of the really WEIRD ones live on tropical bottoms and look like this.. (note the feeding tentacles emerging out of the two top "lobes")

But some of these benthic ctenophores are these worm-like forms and they actually live ON sea stars, such as this Echinaster luzonicus

(photo by Arne Kuilman via Flickr)

I don't know if they are necessarily rare..but people don't see them often. They can be difficult to see and who thinks to look for what is basically a bottom-living jellyfish??

On top of all this- the benthic ctenophores themselves almost never preserve well. So scientists have relativelly little data on them. When I learned about these in Invertebrate Zoology way back when, there was only a footnote about these critters.

If we were lucky-we MIGHT see one in our lifetime. Now-everyone can!

(photo by "MerMate" on Flickr)

Here's one close up..

(photo by Mark Atwell via Flickr)

3. This looks like a polychaete and/or a flatworm living on another specimen of what looks like Echinaster luzonicus (Taiwan? or the Philippines?)

(photo by Star Tsai via Flickr)

4. And finally,the shrimp Periclimenes that lives in and around various big tropical sea stars.

They're all called P. soror in the pictures but there seem to be so many-I'm not entirely sure its the same species.. But here's a bunch...
This one living on the ophidiasterid Linckia laevigata in Komodo, Indonesia
(photo by Fiona Ayerst)
this one on the Crown of Thorns-Acanthaster planci..

(photo by Diver Ken)

(photo by Olin Feuerbacher)

This one looks like a shrimp living in/around the uncommon oreasterid Halityle regularis

(photo by Shaun-in-Munich)

On/around the mouth of a "cushion star" so..Culcita?

(photo by pummkin)

In and around Echinaster luzonicus (oreasterids aren't the only ones with shrimp!)

(photo by MerMate)

If you had collected any of these shrimp without the asteroid (or vice versa) it would have been difficult to make the animal-animal association. What function do the shrimp serve? What do they get out of it? Protection?

These images may seem just like holiday snaps-but to some scientist-they may demonstrate some new species/behavior/relationship!

So, if you're in some exotic part of the world and have some weird, neat picture that you've never seen before let a biologist who knows a thing or two-take a look! You never do know...

And uh... for those with more...mercantile motivation, scientists don't do it for the money (and mostly don't have a lot to give away).

Valentine's Day-Feb. 14th The Echinoblog Twitter Feed gets SEXY!

Greetings!

So tomorrow as an Echinoblog EVENT! I will be Tweeting as MANY Weird marine invertebrate (and some vertebrate) SEX videos as I can! Sea stars! Nudibranchs! Slugs! Worms! Hermaphrodites! Pseudocopulation! Penis Fencing! Giant Sperm! Yow!

Check out the new Echinoblog Twitter Updates in the side column

Enjoy these pics of the tropical Archaster typicus caught in the act of PSEUDOCOPULATION!! More on that tomorrow..