Why is Pisaster ochraceus (aka ochre star) so many colors? AKA they are what they eat!

(image by Nancy Sefton, NOAA Olympic Coast page)

So, early in my career during my various internships and volunteer time, I spent a fair bit of time at the educational tidepools exhibits at the California Academy of Sciences and Monterey Bay Aquarium.

One of the most common questions I would get about the commonly encountered "Ochre Stars" (Pisaster ochraceus) that live on the west coast of North America.

"Is there any significance to the color?" (or some variant thereof)

Well, its taken nearly 15 years but FINALLY...I can answer this question! I thank a neat paper by Harley et al. 2006 in the Biological Bulletin, which is available via Open Access!

So, here's the story!

This species lives along the coast from Alaska to California, including British Columbia, Washington, and Oregon.

These animals have a brilliant and very distinctive suite of colors that stand out. These include

PURPLE...

(image from Wikipedia commons)

BROWN (or RED)

and... ORANGE...and in fact, the species epithet, "ochraceus" in "Pisaster ochraceus" or the common name "Ochre Star" refers to the yellow-brown color, which was probably the living color of the the first specimens that were described of this species.

It turns out that the colors DO indeed VARY with region. Different places along the west coast have variable colors. Of populations they surveyed from 31 sites in California (North & South), Oregon, Washington, British Columbia, and Alaska. (diagram below is NOT proportional)

Across the surveyed sites, they found that on the whole MOST of them were brown-reddish with a relative minority of orange colored members as part of the population.

Curiously, those in certain isolated channels..in Georgia Strait (British Columbia) and Puget Sound (Washington) were 95% PURPLE!!

(image from Wikipedia commons)

In addition to color, they further examined other factors: food, size, and injury. And ran them together with a cluster analysis.

And they got a diagram that showed overall similarity between members from each of the different sampled study sites.

(Fig. 3 from Harley et al., 2006)

There was a close association between all of the populations in California, Washington, and Oregon (seems like Alaska was omitted).

The Georgia Strait and Puget Sound populations (the purple ones) clusters together AGAIN.

COULD these purple populations be something new or different???

A logical question to ask at this point. Did this separate purple population or ANY population of this species have enough separation or structure to warrant consideration of a new species??

So, The study looked at population genetics of P. ochraceus.

That is, the amount of genetic structure was present in the various populations within the species across its distributed range.

(image by Nancy Sefton, NOAA Olympic Coast page)

Essentially, there was NO structure of populations across the range.

That is to say, that an individual from San Diego (southern range) and an individual from Alaska (northern range) were really NOT all that different. Gene flow between populations remained high (that is, no subset of the gene pool had been significantly isolated)

They found NO "obvious" relationship between color and each population.

So, to put it in much simpler terms- There is no color (or other) subset of this species that has become isolated enough that its about to become a separate species or even a genetically separated population.

WHAT's going ON with the PURPLE ones then????

One of the coolest conclusions of this paper was that COLOR in P. ochraceus is probably related to what individuals of this species ATE.

So, it turns out that individuals from California, Oregon, and Washington?

They enjoy eating The mussel Mytilus californianus

(image from Wikipedia commons)

...and now we get to the PURPLE ones from the isolated inlets in Georgia Strait and Puget Sound. What's different about these isolated areas that's different from the open ocean populations?

THEY AIN'T GOT NO MUSSELS!!!

Instead, they have ACORN BARNACLES!! (Balanus spp.)

(image from wallawalla.edu)

The immediate correlation seemed to be that this ecological/external effect (i.e., food type) was the reason why you get purple Pisaster ochraceus. It turns out that the mussel Mytilus contians carotenoid pigments, which are the same KIND of pigment that are responsible for the orange color in carrots!!
Harley et al. hypothesize that the mussels provide the pigment that yields the light orange/red color

(from MarineBio.net!)

and that those ochre stars deprived of mussels REVERT back to the bright PURPLE color!!! (note that mussels are absent in the pic below!)

They point out that the color still varies among individuals-some orange, some red/brown, and some in between. So, there might yet be an underlying genetic component to the variation in color.

They add anecdotal accounts that some orange adults turn purple when held for long periods under laboratory conditions and that small individuals of Pisaster are actually not fixed on a color. So SIZE and maturity may also be important factors.

But the authors save the best for last. They also speculate that those factors that affect color are apparently stable over relatively long ecological time scales.
In some places, such as the famous Pacific Grove, California (home to Ed "Doc" Ricketts and Cannery Row), they were able to determine that Pisaster size, color frequency and diet have not significantly changed in over HALF a Century!!!

So, this famous population of sea stars has not undergone any real changes in prey abundance for the last 60 years or so!

When Sea Urchins ATTACK!!! Crinoid EVASION=Crinoid EVOLUTION!!

(image courtesy of T. Baumiller)

Ya' know what I love about today's post? When ya' got two great things that go great together!!!

Deep Sea Biology and Paleobiology!!!

This article is based on this brand spanking NEW paper by Tomasz Baumiller along with his colleagues in the new Proceedings of the National Academy of Science. Go here for the citation.

and has already gotten some play in Astrobiology Magazine....
You may recall, early in the Echinoblog, I wrote about the predation on stalked crinoids (above) by cidaroid sea urchins. (Click to see!)

To recap, both cidaroids and stalked crinoids are "living fossils", a term used to describe living animals that closely resemble (or are related to) critters that we are accustomed to seeing only as FOSSILS.

This makes them good analogs for inferring past ecological interpretations.

(Calocidaris image courtesy-D. Pawson, NMNH)

A few years ago Tomasz Baumiller and his colleagues, found that these sea urchins actually ATE stalked crinoids!!

This was surprising! Urchins as carnivores???? We generally think of sea urchins as algae-grazing, poop-producing, spiny balls. Sort of like the cows of the sea.

So, they pursued the matter!! Seeking out evidence from all corners of the EARTH that these Urchins were not cows, but the marine equivalent of ravenous, bloodthirsty BEASTS! And that evidence was FOUND!

1. Living Evidence. The authors looked at the shallow-water tropical urchin Eucidaris which was examined live in aquaria alongside the shallo-water crinoid Lamprometra palmata (along with some deep-sea crinoid bits as a further test of whether they would be appealing!)

Do the urchins go for it??

(image courtesy of T. Baumiller)

They DO indeed go for it!! Now, Eucidaris doesn't exclusively go after crinoids-but it DOES indeed like eating them up!!!

The picture above shows part of a crinoid (the small white bit) being devoured by the cidaroid urchin Eucidaris.

You can click here to see the movie at the PNAS website of this beast feeding!

Here's some pix!! Here's what's called a brachial-essentially one of the arms off the "chewed on" crinoid. (see here to see where a brachial goes)

(image courtesy of T. Baumiller)

..and a close up of the cidaroid POOP! An arm segment that's been digested and passed!

(image courtesy of T. Baumiller)

See the notches on that bottom segment?? Those are made by the JAWS on the urchin!

They leave a mark. This is important....

2. FOSSIL Evidence!!The authors wanted to see how far into the history of these animals this relationship may have lasted.

(image courtesy of T. Baumiller)

How could they tell? They looked for the NOTCH (from above).

They looked through 2,500 fragments of fossil crinoid stalks from five Triassic (Mesozoic) sites in Poland. More then 500 of these pieces (about 20% of the total) had these scratches and notches!!

(image courtesy of T. Baumiller)

Its entirely possible these marks could be explained through other reasons. Scavengers or perhaps environmental factors that erode or deform the remains?

But they argue that these fossil deposits were buried quickly-suggesting that the marks were made when they were alive as opposed to scavengers.

They also found that cidaroid spines and test fragments were present around damaged crinoid body parts at these Triassic fossil sites.

So, Yeah. They ATE them.

3. Did Urchins motivate Feather Stars into Adaptation Action?

So both crinoids and sea urchins (as well as most other marine animals) undergo a massive extinction event at the end of the Paleozoic.

The great end-Permian event (click here to get more info) resulted in near-extinction of most Paleozoic crinoids and sea urchins.

(this image from the Earth Through Time website)

But this was followed by a BIG rebound. In otherwords, they survived and RE-DIVERSIFIED.

Many of the older taxa went away-and this was followed by new body forms, which exploited those which had gone extinct. This "re-set" of animal diversity is called the Mesozoic Marine Revolution.

So what happens to the cidaroid urchins? They started getting STRONGER and more effective jaws.
And Crinoids?? CRINOIDS BECOME MORE MOBILE!!!

Some got into FLOATING....(see here for more on this-not everyone agrees on interpretation of these)

(this image modified from Seilacher & Hauff, 2004)

how to CRAWL....

Some lost the stalk and learned how to SWIM....

Based on this, the authors suggest that there was a sort of "escalation of arms".  Armament went "up" in the predator and so Defense correspondingly rises in they prey.

 A sort of Mesozoic Arms Race is at play.... One driving the other.
Thus, the GREATER mobility seen in crinoids was A REACTION TO THEIR PREDATORS...Namely...cidaroid sea urchins!!!

You get a bigger jaw? We're gonna swim away! Get armor! Whatever it takes.
Thus, this relatively straightforward predator/prey relationship has conceivably driven the evolution of this group for hundreds of millions of years.

Wow. Tasty.

Mystery of the Mud Star: Ctenodiscus REVEALED!!

So, everyone knows that various animals (and other living things) have adaptations to where they live. And certainly, many if not ALL of the various things I describe on the Echinoblog are an ongoing story of how these unique animals have evolved to deal with the environments they live in.

What's always curious is that echinoderms evolve in strange ways, which is what makes them so cool! Sometimes, there are such clear cases of HOW critters have such specific adaptations to living in certain habitats that its pretty amazing.

One case in point? The "Mud Star" Ctenodiscus crispatus ! (along with sister species C. australis and C. procurator)
Ctenodiscus lives primarily in MUD , mostly on continental shelves around the world. It is WIDELY distributed in the Arctic and subArctic as well as the subAntarctic and cold-water regions adjacent to these areas.

As you can see, where they occur, they are present in huge abundance. Single collections have resulted in hundreds of individuals!

(image from NOAA Ocean Explorer 2005)

That means that it lives in cold water regions almost ALL over the world. If that's not an evolutionary sucess story, then I don't know what is.

So, let's break it down...
Two of the important things that starfish need: They need FOOD. And they need to BREATHE.The material for this blog is taken from work by Malcolm Shick here and here.


1. Ctenodiscus is an non-selective deposit feeder.
Simply put, they open their mouth and put ALL the mud in it. They get all sorts of sediment with good organic goo out of it, as its slowly digested and processed.
The animal is often found GORGED with mud filling the entire disk.


2. The Real Story: HOW Ctenodiscus BREATHES.
So, here's where the COOL stuff starts. HOW does a starfish BURIED in mud get WATER so that it can exchange gases over the body surface and BREATHE??

It does THIS:

(Fig. 10 from Shick et al. 1981)

Here's what happens: Ctenodiscus is buried near the surface of the muddy bottom, but water flows in and out of the various channels around the surface of the animal!!

How does the water current flow???

The body surface of starfish (and other echinoderms) are COVERED by a very thin ciliated skin. Cilia are tiny, microscopic, hair-like structures that cover the surface and when they beat, they can create a current flow over the body.

So, that is what happens normally in ALL echinoderms. How does THIS species, which lives buried in mud, create such a STRONG current that it actually can circulate water over its body surface??

It has TURBINES!!!
What? Where?? HERE.
See these horizontal and lateral channels between the spines? Those are special structures known as CRIBIFORM ORGANS. These channels lead into deeper channels that are further divided into pockets-ALL covered internally with cilia.

(Fig. 10 from Shick et al. 1981)

These cilia beat, creating a current that flows through these channels around the animal and over the papulae (i.e., the gills) so that Ctenodiscus is properly aerated.

Its like a bunch of canals ALL over the surface of the animal and water flows through every one of them. Acting like sort of a fluid insulation that brings oxygen to the surface of the starfish so it can respire.

BREATHING in MUD? But what happens when there's NO OXYGEN??
Where you've got mud, you sometimes get a condition called hypoxia. That's when the oxygen just doesn't reach a certain level of the sediment and you see a build up of a toxic chemical called hydrogen sulfide. That' that level of mud that turns black.
Ctenodiscus has a high tolerance to hypoxia, possibly one of the best known for any echinoderm (but there isn't much known about other taxa).
What happens? The mud star has what's called an anal cone aka an epiproctal cone (see above).

(Fig. 10 from Shick et al. 1981)

The cone is basically an outpocket of the body. Its extended UPWARDS through the sediment.

(Fig. 2 from Shick 1976)

Observations of Ctenodiscus under hypoxic conditions led to the illustration above. Basically, its thought that the cone gets more enlarged as hypoxia and hydrogen sulfide increases.

The extension of the cone extends through the surface, with the tip at the surface. For your typical 6.0 cm diameter animal, these animals can have a cone that can attain 3 to 4 cm and extend 2 to 3 cm above the mud. It can leave this extended for over an hour. As the picture suggests, it can move around and push through sediment as the mud shifts, and etc. So, it can move around.

This also serves to make the top surface of the animal thinner, allowing easier gas exchange and opening up a channel to the surface water above the sediment surface!!

Yowza.

MYSTERIES of the mud stars revealed!! Success comes in many forms!

Is it Love? Mutualism in Leather Stars (Dermasterias) and Scale Worms (Arctonoe)!!

(this image from Pt. Lobos.com!)

So, with all this talk of Invertebrate Blog War, I thought I would veer away from all of that and instead talk about the more positive relationships that are observed between two different phyla of invertebrates.
This is based on a paper by Wagner et al. 1979 that details the relationship between two species that are found all along the west (Pacific) Coast of North America.

One of them is this species, a sea star, Dermasterias imbricata, aka the Leather Star or the Garlic Star.

(this image from Pt. Lobos.com!)

AND along with it is a worm. A member of the Polynoidae called Arctonoe, species- A. vittata.

(these images from the WallaWalla invertebrates site)

This worm lives in the tube foot grooves of Dermasterias. They basically take residence in the spaces between the tube feet and make themselves at home in all of the open space.

(this image from Pt. Lobos.com!)

This relationship was observed by many of the early Pacific coast naturalists and was labelled commensalism, that is two species that lived together but neither species had either loss or gain. Kind of a "roommates that pass in the night" kind of arrangement.

Here's a cool video that shows the worm ON the underside of the leather star and in/around the animal's tube foot groove.


These worms are commensal in MANY other marine invertebrates. That includes this limpet (in the red circle)

as well as other species of sea stars, and the giant gumboot chiton that lives on the west coast of North America...

These worms have apparently been shown to be chemically attracted to their hosts.

This doesn't really come as a surprise. If they live in the "house" that one of these species provides, then they should know how to find it.

But how far gone is the relationship between this worm and its host???

Wagner et al. wanted to know the extent to which Dermasterias (the starfish) were attracted to the worms in a "Y-shaped" aquatic maze. It was basically given a choice of the worm in one arm (A) versus control (B)..

Which arm of the maze would Dermasterias choose?

The worms have been shown to move toward their sea star host, but in the total number of trials... Amazingly in 16/20 trials, the sea stars also moved towards the WORM!

They also changed variables. They switched out the commensal worm with a free-living worm, other food items, the sea anemone Anthopleura. And even more amazing??
The sea stars PREFER the commensal OVER its favorite food item!

Another curious dynamic?? The stars will take the worms REGARDLESS of which host it lived in! They removed one from a limpet, placed it into the maze, and voila! The sea star STILL likes the worm !
So, its not so much a commensal relationship but a MUTUAL one!! Both of them get something out of it!

So...Why? What makes the starfish WANT a worm living in its tube foot groove and in/among its parts?

Wagner et al. speculate that this is tied to the worm feeding on either mucus, detritus or other prey.

Curiously, they notice that the worms might be hanging out on starfish that LACK pedicellariae, which are pincers or clamp-like stuctures that starfish use to remove surface detritus/defence etc.
As an end note, they caution that the experimental results aren't necessarily an indication of what is "real" in the wild. So, one hopes that someday, someone will pick up this study and follow up on these curious relationships, not to mention the other species that harbor these worms!