The Gorgeous Sea Urchin Skeleton: An SEM Odyssey

Image by gary.brake

The Scanning Electron Microscope (SEM) is a wonderous device. Simply put, electron beams provide a highly detailed almost surreal image of the surface they are directed at.

But what happens we direct an SEM at animals that are ALREADY kind of weird? The striking beauty of sea urchins is revelaed!

The sea urchin skeleton. Also known as a TEST.  One of my many regularly repeated caveats- these are NOT shells. These are underlying skeletons which have a layer of skin which is typically removed to reveal the more aesthetic skeleton...

Here is an example from a cidaroid sea urchin.. Those round knobs or bosses?  Those are where the spines articulate with the body...

Image by Gripspix

Here's a nice macro shot of aforementioned "boss" (=the knob which connects with the spine).

Image by "Nervous System"

But what happens when we focus a Scanning Electron Microscope (SEM) on these surfaces?  (note that these images are a different species from the one above).

Image by Monkey.grip

Here's another view looking down. Note how the skeleton is actually porous! Echinoderm skeletons aren't simply inorganic calcium carbonate, they are actually infused with tissue...

Image by gary.brake

Image by Studio Jonas Coersmeier

Another view from a different perspective...

Image by particlesixtyfour

Image by monkey.grip

Closer...

Image by particlesixtyfour

..and closer still!

Image by particlesixtyfour

right on top of it!

Image by particlesixtyfour

But wait! What about the SPINES???   I'm not sure which species these are from.. but they give you a good idea about the fine topology that one might not realize is present from simply looking at a spine with your eye... 

Image by particlesixtyfour

Closer!

Image by particlesixtyfour

And finally... CLOSEST!!

Image by particlesixtyfour

And just to cap off the whole odyssey through a sea urchin spine... here is a cross section THROUGH a spine magnified 150X!!!   The final three images below are from the Biology Dept. at the University of Dayton! 

Here are the featured spines...

 and together to show perspective...

Who Eats Who? Figuring out Feeding in Deep-Sea Starfish

thanks to MBARI!

People are always fascinated by how animals eat and, I think, the weirder the better. Starfish have one of the most distinctive feeding modes of all animals and I think, that's why people get fixated on how they consume their prey.

The feeding posts on the Echinoblog are among the top ten highest hit and the research literature is full of comprehensive studies detailing the feeding ecology and behavior of a great many starfish species. 

As one might expect, the great majority of feeding studies were initially those of shallow-water species which were easy to observe. Feeding is important to understanding marine ecology in many systems. So, its not just some casual trivia that comes in handy at cocktail parties.

But what about deep-sea species? Understanding the role of predators in these often inaccessible systems is important.  In cases, such as with deep-sea coral, its important to understand what role these animals play, especially given how little we know about the individual animal's importance.

How does one figure out the role..and indeed, the importance of these species in such far away and forboding habitats??

Today's Echinoblog post features work by Ms. Katie Gale at Memorial University who resides in the lab of Annie Mercier (and including Jean-Francois Hamel).

Data from this paper was recently published in the new issue of Deep-Sea Research studying the trophic biology of various deep-sea asteroids from the coast of eastern Canada. (here).

1. The Starfish

Fig. 1 from Gale et al. 2013

Ms. Gale's research focused on seven deep-sea starfish species from the North Atlantic:
A. Ceramaster granularis
B. Ctenodiscus crispatus-aka the mud star (learn more about it here)
C. Hippasteria phrygiana (learn more about it here)
D. Leptychaster arcticus
E. Mediaster bairdi
F. Novodinia americana-a brisingid
G. Zoroaster fulgens (more about Zoroaster here)

but also had some feeding notes on this weird guy.. Tremaster mirabilis! (about which very little is known)

Most of these species occur in a primary range of about 500 to 1500 meters, but some can get relatively shallow. Most are difficult to study and live on the deep-dark sea bottoms...

2. Figuring out feeding
There's generally TWO ways to study feeding:

• Directly: i.e., you watch a species consuming its prey (or whatever food) and voila! You have a direct feeding observation.
• Indirectly: You have something which provides inference about what the animal has already consumed. Look at the gut contents or something similar...

In the old, old days, figuring out the feeding ecology of deep-sea species was difficult. Specimens were collected via net-and usually brought up badly damaged. The animals were mostly dead and had often emptied all of their gut and stomach contents. Rarely did you have an opportunity to see the animal interacting with any possible prey items. Any interactions you might have spied could have been caused by the trawl net scooping up any and all of the bottom fauna...

In contrast, I think Ms. Gale et al's paper has acquired some great information using some modern techniques and good ol' fashioned detective work!

Direct Observation  So, the most obvious and direct way to observe feeding is by watching it!  These days, submersible robots aka ROV's (Remotely Operated Vehicles) are one of the main platforms for these types of observations. I've done some similar work in the Pacific (here)

Fig 9A from Gale et al. 2013

This is Hippasteria phrygiana, a widely occurring cold-water/deep-sea coral (and cnidarian) predator, but Gale et al. observed several other species from the deeps, about 500-1100 meter depths,  of the North Atlantic.

Several species were observed as predators for the first time, whereas others were confirmed. For example, Novodinia is a brisingid with a documented suspension-feeding mode and we saw more of that in Gale et al. 2013.

Gale et al. also reported feeding for Tremaster for the first time! Feeding on coral...

Laboratory Feeding Experiments
As a complement to the direct observations, Gale also performed several laboratory feeding experiments and was able to observe several direct feeding moments!

Predictably, Hippasteria fed on various cnidarians, including sea pens and other deep-sea cnidarians.
Whereas Ceramaster fed on sponges..

But not all the prey allow the predators to just...eat them. Some, like the sea anemone Hormathia

nodosa 

Image from Natioal Museum of Northern Ireland via EOL

and the deep-sea coral Flabellum alabastrum 

Image from Fisheries & Oceans Canada via marinespecies.org

used their tentacles (which all have stinging cells) as a defense against the oncoming hunger dogs!  And this was effective against the more timid Ceramaster but not against Hippasteria. Flabellum was fed upon by Hippasteria VERY quickly (in 18 minutes)..

Indirect Evidence. This is where some newer techniques shows us some cool ecological stuff!!

Stable Isotopes!!
Here is a video that explains the basics of stable isotopes but basically what it comes down to is this: elements like nitrogen (N) and carbon (C) undergo changes as they pass through different ecological levels in the environment.

In doing so, they become kind of like a "fingerprint" for a particular kind of ecological role. So, for example, species with a stable isotope N (Nitrogen) value of about 16, but w/ Carbon value of about -14 (Hippasteria, Ceramaster and Mediaster) are higher within the overall trophic relationship among these asteroid species.

Fig. 6 from Gale et al. 2013

You've seen Hippasteria, but here's Ceramaster granularis

and Mediaster bairdi

Image by K. Gale

All the other species, including Novodinia americana, Leptychaster arcticus, Ctenodiscus crispatus and Zoroaster fulgens display lower values which would be consistent with their previously thought of feeding modes as suspension feeder (the brisingid) and deposit feeders/detritivores (mud stars, including Leptychaster and Ctenodiscus) and Zoroaster.

Gut Contents & Prey Items!
One other indirect way of looking at food items?  Gut contents.  What were they eating?

Ms. Gale did a LOT of work looking through the guts of many starfishes.. Much of how they fed is based on detective work.  For example, many animals such as deep sea gorgonians and such, after being digested leave only skeletal bits called sclerites.

Fortunately, these can be used to identify the animals with some accuracy. Curiously Hippasteira also had some crustaceans in its gut..

Figure 3 from Gale et al.

One of the subject animals, Zoroaster fulgens has been one of the more mysterious deep-sea starfish species in my experience is an infaunal predator, that is, a species which eats animals living in bottom mud and sediments.  A related zoroasterid called Doraster is shown here with a snail in its mouth with snail food in the red circle

Gale et al reiterate the importance of the feeding ecology of many of these species...

• Hippasteria is a widely occuring asteroid which likely affects coral populations
• Ctenodiscus-the mud stars occur in LARGE numbers, up to ~6000 individuals per hectare and influence the sediment as they move around through it feeding on mud..

Fr. Arcodiv.org

• Suspension feeding asteroids such as Novodinia capture food from the water column that would ordinarily not be made available to bottom feeders

not N. americana. Image by NOAA National Ocean service

I've recently discussed some recent observations of a seemingly innocuous species, Porania pulvillus as a predator rather than a passive ciliary feeder. Understanding deep-sea ecosystems is an exciting endeavor, who knows what we'll find!  Simple things like feeding are intriguing and interesting-but poorly known. What will the important impacts of these species be down the line?

But even BASIC knowledge such as this is a complex and time-intensive process. It starts with work like this...

URCHIN BARRENS! Aka the Trouble with Tribbles (=sea urchins!) Post!

Image by Annie Crawley

Sea urchins are among the best known, most heavily published on, and most "important" of echinoderms. People eat them and they are studied in marine ecology pretty heavily. Most marine biologists I know think highly of sea urchins. They're pleasant animals with an unusual appearance

But the truth is, no matter how adorable or fuzzy, useful and/or cute an animal may be, TOO many of them is nothing but trouble! True for Star Trek tribbles and for sea urchins!
(disclaimer: Tribbles are science fiction, sea urchins are not)

*Tribble factoid: Someone has ACTUALLY given tribbles a scientific name: Polygeminus grex! don't believe me? go see Memory Alpha!)

Tribbles are actually a GREAT introduction for today's topic: SEA URCHIN BARRENS!

What are Sea Urchin Barrens??  These are places where a sea urchin species' abundance increases dramatically to the point where the urchin devours EVERYTHING in its path, effectively leaving all else 'barren' except for more hungry sea urchins.

Image by Annie Crawley

Images above by AndyOlsson

This is not far removed from the imagined "ecology" of Star Trek's tribbles (A good essay applying real population math about tribble populations can be found here, but this image from the famous ST:TOS episode hopefully gives you the general idea!)

Image from TrekNews.net

The gist of it is simple:  TOO MANY URCHINS and they EAT TOO MUCH. But unlike tribbles (which were eradicated by Klingons-yes I know they're not real), in the case of sea urchins, we can actively study the ecological interactions and conditions which have caused the populations to explode in number.

Image by AndyOlsson

Here is a video showing tons and tons of Red Urchins (S. franciscanus) on a barren in Southern California. Thee bottom is essentially devoid of all but more hungry urchins!



What causes urchin barrens? 
Um. Its complicated but the common thread seems to be that there is an association between barrens and the absence of sea urchin predators.

In many of the papers I've read about Northern Hemisphere species, the loss of a major sea urchin predator seems to be one of the immediate attributed causes of the runaway population growth, but as we've seen with other species such as the Crown of Thorns (Acanthaster planci) the story is often complicated....

Most of the studies involve temperate-cold water urchins in the Strongylocentrotidae, specifically Strongylocentrotus purpuratus (purple urchin), S. franciscanus (red urchin), S. droebachiensis (green urchin) and S. polyacanthus.  Literature was abundant, but this paper by Nathan Stewart & Brenda Konar provided much of (but not all) the info for this post.

In one of the most familiar studies from the Pacific Northwest coast, the main predators were sea otters (in many cases, I assume Enhydra lutris-some papers did not mention species).

The fundamental ideas outline the notion that as sea otter populations decline, predation pressure decreases and with nothing to keep the populations at a controlled level sea urchin populations dramatically increase and began to devour kelp (and really everything else!)  to the extent that they effectively clear the bottom.

Image by Santa Monica Bay Restoration Foundation

Image by Annie Crawley

In Stewart & Konar's paper, individuals from these population explosion urchins were compared against "healthy" urchins which occurred naturally in kelp forest habitats.  Some dynamics:

• Urchin densities were SEVEN times greater than those elsewhere
• Kelp forest (vs. 'barren') urchins were larger and more robust
• "Barren' urchins were smaller with less tissue
• "Barren urchins had little to no reproductive tissue compared to kelp forest urchins

Different species of Strongylocentrotus (as well as other urchin species!) live in different places and have different predators!

On the North Atlantic coast, there is a similar population explosion of the Green Urchin, Strongylocentrotus droebachiensis, which from the look of it, is pretty severe

Here's a video that shows just WOW... a lot of them..

I have briefly written about the impact of this many Green Sea Urchins. They all POOP! This actually has a pretty serious ecological impact. 

Some, such as this paper, have proposed that these population increases have been caused by the loss of lobsters (Homarus americanus) which feed on green sea urchins. But in all liklihood, as the system is better understood the more complicated the explanation.

Image by AJmart

Other predators, such as wolf eels and starfish, also feed on green sea urchins and well.. it can get messier...

Now, in the Southern Hemisphere we have a similar, parallel situation with a completely different family and species of sea urchin: Centrostephanus rodgersii (Diadematidae).

Image byEdward Vella

Climate Change Enters the Picture! 
A paper by Ling et al. 2009, in the distinguished Proceedings of the National Academy details  a scenario with some important dynamics

1. The range of the urchin is dramatically expanded because of increasingly warm waters in/around the eastern Tasmanian region.
2. The lobster Jasus edwardsii is one of the primary predators of Centrostephanus and has been heavily overfished. The BIG lobsters that would feed on urchins are taken for food leaving the urchins to run amok!

Its important to note how significant the human factor has played into these dynamics. Climate change and overfishing are thought to be the primary agents responsible for urchin "barrens" in these circumstances.

This issue has been conveniently summarized in this video...


The takeaway lesson: Predator loss seems pretty strongly associated with urchin "barrens" aka population explosions. But all sorts of environmental factors, including warmer waters, and multiple predator interactions can be important..

So we have a LOT of sea urchins. Couldn't we uh..just eat them? 

Yes. 

BUT, you can after all, only fish so much. After you've taken the lobsters, the urchins and the kelp what else have you got left? A good answer seems to lie with good sustainable fisheries management..but we shall see how this works out...

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!