Flukes in Cukes! Flatworm Commensals in Sea Cucumbers, Sea Urchins and Starfish!

Fantastic image by Chelsea L. Wood

Today we look at some neat examples of flatworms that live in echinoderms!

And to the flatworm and parasitic worm people reading this? YES, I know flukes aren't free-living flatworms. It rhymed! So go with it for now. thanks for your patience!

Flatworms aka the Platyhelminths (in Greek-literally the "flat worms") look like living carpets. They are mostly predatory, but may also feed on small organic particles and live all over the place. They can be parasites, such as tapeworms or free-living beasts such as the one featured in the collage below.

These include the familiar Dugesia-that you find in high school biology (brown with arrow shaped head and two weird eyes) to these big, colorful species that live throughout the tropical Atlantic and Pacific. There are some 4500 recognized species of free-living flatworms..

An awesome collage by Arthur Anker!

Papers that were used today:  this paper by George Shinn (1981)-Hydrobiologia 84: 155-162
another in Biological Bulletin 169: 182-198 (also by Shinn) and this one, from Canadian J. of Zoology 61(4): 750-760 which describes the species living in the sea cucumber

What's interesting about the ones that I'll be talking about is that none of these is exclusively parasitic (such as a tapeworm or a trematode). These are free-living species..but they live INSIDE the body cavities of echinoderms!  

Think of it as if you ended up living in the intestine or the body cavity of a whale. Lots of space there and potentially...a  lot of food. Plus protection from predators, the elements and a safe place to reproduce!

So, technically they aren't really parasites (where the host 'loses') they are commensals that are considered just kind of benign.

Image from WallaWalla University Inverts site! 

It makes sense. Sometimes, an animal with a huge internal body cavity can be a home. We've seen the classic pearlfish and even when clams that live inside the throats of sea cucumbers.

It turns out that there's something in the neighborhood of SEVENTY species of different free-living flatworms that live in echinoderms as hosts! A nice list of these can be found in this paper here.

Most of these hosts appear to be sea cucumbers with sea urchins and sea stars. Some in cold water but also in the tropics!  Crinoids and brittle stars seem to be among the minority as hosts for flatworms..probably because there's not much "living space" inside their body cavity. Or maybe they're just not as well studied?

Here are two well-documented worms from the North Pacific coast.. one that lives in several Pacific urchins and one in a North Pacific sea cucumber...

The Urchin as a "House" for flatworms! 
Urchins on the west coast of North America (in the Pacific Northwest) include the well-known purple sea urchin (Strongylocentrotus purpuratus)

Image by Joe McKenna

And the giant red urchin (Strongylocentrotus franciscanus)

Image by Dan Hershman

and the deep-sea Allocentrotus fragilis

Image by NOAA Photo Library

and even the southern California urchin Lytechinus anamesus

ALL of these are often inhabited by this beast! Syndisyrinx franciscanus

Image by Chelsea L. Wood

Syndisyrinx franciscanus lives in the digestive tract of its host and apparently, infested urchins have been found with up to 186 worms!!! (an average of about 29/individual)

Image by Chelsea L. Wood

This one is called Syndesmis dendrastrorum 

From the EOL page for S. dendrastrorum

and it lives in the common Pacific Northwest sand dollar Dendraster excentricus!! (seen below alive with spines)

Image by Patrick Warren

or perhaps more familiar if seen like this? Spines removed...

Image by Cheryl Moorehead

Even the familiar Pacific Sand Dollars can HAVE WORMS!!!  Ya' learn something new every day!

What do they do in there? Mostly, these live in the intestine feeding on the host's intestinal lining (the tissue) AND apparently also like to eat on the symbiotic protists (the ciliates) that ALSO live in the intestine of the host.  But this apparently doesn't create any detrimental effects on the host. So-commensal rather than parasitic.

Apparently, the worms produce egg capsules are released into the intestine of the host and released outside with the feces.  When the capsules are eaten by a new host, they become active.... probably a reaction to the intestinal fluid and proceed to live out their new life in the new host's intestine.

Flukes in Cukes! 
A free-living worm lives in the Pacific NW sea cucumber Parastichopus californicus.

Image by T. Van Nunnery

This beast is called Anoplodium hymanae, a worm that is named for the famous Invertebrate Zoologist Libbie Hyman

Image by Chelsea L. Wood

These are a little more aggressive than the ones that live in sea urchins.

This species reaches the body cavity by penetrating the wall of the intestine..usually through the respiratory trees (feathery structures colored in blue in the pic).  I've briefly mentioned this area as where some sea cucumbers can feed via their butt!



The eggs are spread out via the anus with the feces until they are devoured by a host.

Similar to the ones in urchins, the larvae hatch in response to digestive fluids in the intestine of the host. Get into the intestine, move to the respiratory trees and then further move out into the body cavity of their new host!

Image by Chelsea L. Wood

Starfish got worms too!
So, there aren't a lot of records of flatworms that live in/among sea stars. Six were recorded in asteroids..and oddly enough, the one below was not included. So maybe its something new?

Description of this pic indicated the cold-temperate North Atlantic asteriid species Leptasterias littoralis. Is this a commensal flatworm moving within the tube foot groove? Moving around on the surface?  Something new? A convergence of two species by chance?

Image by Nick Sleptov

For more worm-starfish relations?

Go to this pic of Echinaster callosus and look closely at the short, striped things crawling on the swellings!    this one has a tighter shot that shows them a little more easily..note the brown squares with the white stripes.. (and includes a shrimp to boot!) WOO!  Acoel flatworms? 

How many remain to be discovered?? 

How many starfish species are there?? Where do they Live? How long have they been around? Five Points about Sea Star Diversity!

Images here from the Encyclopedia of Life

This week, something about the many different KINDS of asteroids (aka sea star or starfish!) that are found throughout the world. Data for this, is from a paper I wrote with Dan Blake last year for PLOS One. You can download it for free here!  based on info at the World Asteroidea Database.

I can't tell you how many times I've read something about starfish in pop culture or even in pop sci-and the writer assumes that starfish are the SAME all over the world! As if there was only one type.

This gets to be annoying..so let me just make sure everyone knows: There's a LOT OF DIFFERENT KINDS OF STARFISH. And they live in different places all around the world.

Similarly, I've talked about my field work in Antarctica and in the middle of my story, someone stops me to ask "Wait, there are starfish IN ANTARCTICA??"

So here are five things about the diversity of sea stars that will razzle your dazzle and make you a favorite at echinoderm-themed invertebrate zoology parties (esp. those where people are ignorant of starfish)!

1. How Many Starfish Species Are there?

At the moment? There's about 1900 species of accepted (living) starfish in the database. That runs a bit lower than the larger numbers quoted elsewhere (such as here),  but I assure you, that I am doing everything I can to kick that 1900 up a notch every chance I get!

Here's a post where I keep track of many of my new species!

More importantly, that's 1900 species in 36 FAMILIES. That means there's 36 different distinct groupings found all throughout the world in different habitats at different depths in different settings. So, its not just 1900 different kinds spread out all over the place- its 1900 species divided up into 36 distinct body types adapted to living in many different ecological niches!! (note that 1900 is a rounded figure due to questionable status for some names)

For those who need to know: the full list is in the PLOS paper listed above. So feel free to click and go!

Of the many weird kinds, there's Tremaster, as I discussed here awhile back.. 

There's the deep-sea brisingids! Crazy lookin' things that live in the deeps! Here

This Antarctic monster! Labidiaster annulatus!            The Slime Stars! Yay Mucus!

Predatory Sun Stars!     And just for good measure- here's a neat assortment from Hawaii! 

2. Where Do Starfish Live? 
All echinoderms, including sea stars, live in the ocean-on the sea bottoms (although their larvae swim in the water column). They are among the few groups of animals which live exclusively in marine habitats.

Starfish live in all the oceans! Atlantic! Pacific! Indian! Arctic! Southern!

And in answer to the question "Do starfish live in Antarctica?" and "Under the Ice?"  Yes!

The video below is from the famous "brinicle" video released awhile back showing starfish (Odontaster validus, probably) getting around some hardcore Antarctic ice! (narrated by 30 Rock celebrity Alex Baldwin!)

And many starfish also live in the deep-sea (below 200m).

Of the 36 living families of sea stars, nineteen of them occur exclusively in the deep-sea (436 species)! Four families are mostly deep sea but with some shallow-water members (1191 species) and several families include large numbers of deep-sea members. Only 8 families live ONLY in tropical habitats (218 species).
 It turns out that MOST starfish live in cold-water (or temperate) settings, such as the deep-sea or in the polar or near polar regions!

So among those deep-sea starfish species, which ones live most deeply?  As it turns out several starfish groups occur in the deep abyss below 5000 meters!

One of the most commonly encountered deep-sea asteroid groups at that depth is the Porcellanasteridae.

Image from the EOL page

They usually live buried in mud, devouring massive amounts of sediment, presumably absorbing digesting various goodies present in it.  This feeding mode is similar to the more shallow-water mud stars, which I profiled here.

Another group known to occur down to 6000 meters? Brisingids (go here to learn more) in the family Freyellidae...

Easily half a dozen families of asteroids are represented at the 5000+ meter depth range! Its unclear how many go deeper than 6000 however...

3. Which Group of Sea Stars is Most Diverse? (i.e., Which one has the most species?)
What's weird about this question is that the group with the MOST number of species is probably the group you are least familiar with...  Enter:  The Goniasteridae! 

Ding!Ding! This group rings in at a whopping 256 species in 65 genera! Deep sea! Tropical! Antarctic! Shallow! Goniasterids are everywhere (but mostly in cold-water places, in the deep sea)! 

But usually, in out of the way places, so they don't really turn up in places where people encounter them. So not many folks think of them you ask  "what does a starfish look like?"

Goniasterids vary in size and shape as you can see.. but some get REALLY large! Such as this monstrous Mariaster giganteus from Japan !!   (held by my colleague Yoichi Kogure!)

Goniasterids are diverse-and there remain MANY different species yet to be discovered....They are ecologically important in deep-sea AND shallow waters.

I'll probably do a full post on this group at some point-but in the mean time here's a bunch so far!
Tosia-the biscuit star: hidden species and brooding behavior! here!

New genera and species of deep-sea corallivores! here!  and some video!


4. How Many Undescribed Starfish Remain to be found? 

Image by Island_girl

Its hard to make estimates about stuff that doesn't exist yet!  How many more species are out there to be discovered?  What is my best estimate???

Sea stars/starfish are big, obvious creatures. One would think that we have found most of them...but you'd be wrong!

One subject group: The Goniasteride has 256 species in 65 genera. Out of the total # of genera and species:  14% (approx n=9) of genera  and 12% (approx n=31) of species were discovered ONLY recently (since 2001).

I have further unpublished data on many MORE new goniasterids! Which suggests that the total number of newly discovered genera could go up to 37% and the number of new species could go up to 32%!!

These are rough estimates from one group. Most people might expect that new species would be found in inaccessible and poorly studied places such as the deep-sea or perhaps in distant tropical lands such as New Caledonia.  But another possible source may already be right in front of us....

Cryptic species are those which are distinguished by genetics or reproductive differences, some other source of evidence OTHER than external morphological characteristics to demonstrate they are separate species. Molecular genetics shows great potential as a tool to discover diversity where cryptic species are concerned...

Again, the case of Tosia-the biscuit star: hidden species and brooding behavior! here! is a great example.



5.  How Long Have We Observed Starfish in the Fossil Record? 

Hudsonaster USNM 40882, early asteroid from the Ordovician.  Copyright Dan Blake. Image via www.tol.org/Asteroideahttp://tolweb.org/Asteroidea

Starfish are old. And their history will doubtlessly fill another post on the Echinoblog some day. A general account of the fossil history of the Asteroidea can be found here.  But the short version is this..

Some very early ancient forms (i.e., predecessors to modern day "proper" asteroids) are observed in the Paleozoic (540 to 250 million years ago!).

But we really don't start to see modern asteroids until the Mesozoic, that is early Triassic....where the record is poorly preserved...

Left to right: Trichasteropsis weissmanni (specs. MHI 843/1, SMNS 3173/5 and Noriaster barberoi (MPUM 8420) on far right. Images copyright Dan Blake, Images via www.tol.org/Asteroidea

But basically, from then on, ALL modern asteroids continue on to the Recent. In other words, starfish which are alive today are part of the same lineage which has been around since the Triassic (250 to 200 million years ago). A distinct, separate lineage from those in the very, much older Paleozoic. 

But just to give everyone a "landmark" to gauge what I'm talking about:  YES. Starfish are OLDER than dinosaurs. 

Some starfish deposits from the Cretaceous are very well preserved and so, when T. rex was running around, starfish were there, doing their thing.  Thanks to some excellent fossil preservation, we know that some of them looked like this...

Metopaster parkinsoni

Fr. Discovering Fossils UK

Also M. parkinsoni, I believe...

Calliderma schulzei

Image by Claire H

But, fossil goniasterids were highly diverse and the number of fossil species is very high. The actual number of fossil "species" is difficult to compare with living species owing to differences in what geologists called "species" versus what biologists observed in living animals. Uh.. trust me, its complex and I'll get into it another day.

But yes. Starfish have been around for quite a long time. Longer than humans (and primates). Longer than dinosaurs. 

To sum up this week?

1. There are about 1900 species of starfish and that number is climbing.
2. They live everywhere in the ocean! Especially in cold-water habitats like the deep-sea!
3. The family Goniasteridae is the MOST diverse (i.e. most number of species)
4. MANY more species of asteroids remain to be discovered!
5. The fossil record shows that starfish have been around for a LONG time, since before big reptiles roamed the Earth!

More mysteries with Porania pulvillus! Yay for Crowd Sourcing!

Thanks to this weeks' blog, a new mystery photograph from Diver/photographer George B, who sent me an image of this strange swelling on Porania pulvillus. Image was taken from the west coast of Scotland at 20 m depth.

Is it feeding and have a distended disk (as other starfish do here)? Does it have a parasite? Some kind of tumor?  Not sure it has ever been reported before!!!

This is a neat highlight, that even though you have a species which has been recognized since 1776 and lives in the North Atlantic where many divers and scientists are active, many of these species remain poorly known.

It also highlights the value of crowd sourcing. Images on Flickr and YouTube add so many more eyes and observations...

Thanks again!

Natural History Meets Data Mining! A case study in Porania pulvillus

Image by Geir Friestad via Flickr. Norway I think.

I've always been a big fan of how the vast body of available images and crowd-sourced information can be beneficial to scientists but also interesting and engaging to the citizen scientist!  From tracking down videos and images of rarely seen benthic ctenophores (here) to helping to locate invasive brittle stars! (here)

As I've said in the past-images and videos collected by everyone..from amateur divers on Flickr to pics aggregated on websites such as the Encyclopedia of Life contain MASSIVE amounts of potential observations (and thus data)!! 

But of course, the images have to be recognized first.  How often do discoveries like this happen?  Sometimes new species are found on Flickr (such as this new lacewing species). 

I've blogged about how crowd sourcing and citizen science has discovered new and unpublished (or at least very poorly known) commensal behavior in sea stars, fishes and their commensals (here!) and today another discovery!  

But not in the exotic Indo-Pacific but in the cold to temperate water familiar North Atlantic!! 

Our subject is a starfish called Porania pulvillus (aka the "red cushion star"-one of many), one of many summary accounts of this species can be found here.  It is an uncommon but recognized species in the North Atlantic from both the North American and European sides...  Porania is a member of the Poraniidae which is an unusual group occuring mostly in deep-sea and cold-water habitats, occurring from roughly 10 to 300 (often deeper in some places) meter depths . Surprisingly little is known about them...

The story starts with a picture...

Yesterday, I received an inquiry from my colleague at EOL- Dr. Jen Hammock who forwarded me a link to the picture below from the SERPENT Media Archive!  SERPENT is a coalition of industry and academic organizations that work to share images of the deep-sea with the public..(here) to see more.

 She asked me "What's going on here?" "Is this feeding on sea pens a thing?"

from this EOL page via SERPENT Media Archive

Indeed it was a "thing!"

But let's take a quick step back and get started with all the players! 

What we know...

First: the species..... Porania pulvillus! A nicer pic is here: 

Note that the white "dots" above and the finger-like yellow bits below are the papulae- aka the gills! This is how the animal "breathes".. Image by Mark Craig

In 1915, zoologist James F. Gemmill described what he observed to be filter feeding using the numerous tiny cillia covering the body of this species.

Dr. Gemmill was quite an authority..being an  M.A., M.D. D.Sc, F. Z.S."!!! And is pretty well known historically for a number of significant contributions.  He dropped little red carmine particles (basically a  red dye made up of a fine powder) on P. pulvillus and watched them move over the surface..

A quick reminder about basic biology of this (and all starfish). The epidermis COVERS the body! The epidermis is in turn covered by tiny hair-like structures called cilia, which are usually in constant motion.

These little particles were moved over the body and apparently headed towards the mouth. Here's his diagram showing the little arrows apparently indicating how water was flowing to the mouth.

Fig. 2 from Gemmill 1915

And this description as a "filter feeder" has stuck. Even a relatively recent paper (here) from 2005 describes them as "suspension feeders." But are they truly???  Is that all there is to the story?  Could they be predators? Opportunists? 

A paper in 1973 (here), one of the first SCUBA accounts documented that they fed on "soft corals" such as this Alcyonium digitatum-the so-called "Dead Man's Fingers"

Image by Christine Howson via Flickr

The paper reports that this species feeds on brachiopods and even tunicates!  Here's a nice one showing feeding on tunicates...

Image by Tony J. Gilbert

Perhaps P. pulvillus is sort of an opportunistic predator in addition to the "suspension feeding" with mucous threads?

Is this Porania pulvillus getting ready for a big banquet? Or is this the echinoderm version of those "1 dog, 1 pig, and a cat" pet adventure stories?

Image by www.ilreporter.com

But getting back to the picture...
It turns out that what we are seeing here is probably something unreported! NEW!!  A previously undocumented feeding interaction!!

From this EOL page via SERPENT

So, that Porania pulvillus was FEEDING (that's what it appears to be doing) on this sea pen (called Pennatula phosphorea). Image was from 124 meter depth in the North Sea, west of Shetland.


Here's a nicer pic of the sea pen...

Image by Jarle Strømodden via Flickr

Again-this was a NEW discovery! What does it mean for this animal's ecology? Does it mainly feed on yummy cnidarians?  Is the species more opportunistic than thought?  More predatory than previously thought??

And of course.. there's the impact on the bottom fauna.... I've written about goniasterids that feed on deep-sea coral (here) and here's a video about them from MBARI a few years back...

These types of interactions are important owing to our interest in deep-sea corals and their role in biodiversity and conservation of deep-sea ecosystems..

But after looking at the pic above from the SERPENT pool on EOL, I discovered several more...

Some further interesting feeding observations of Porania pulvillus, such as this one of it hunched over this rock and possibly feeding on the polychaete worm Pomatocerus sp. (which makes up the tubes). And of course, there could be hydroids or other encrusting goodies on there as well...

EOL page link here SERPENT Media

Here were a couple that showed P. pulvillus CLEARLY hunched on and probably feeding on something-but it was unclear exactly what it was.  Dead sea pen maybe??

From this EOL page working w/ SERPENT Media

From the EOL page here SERPENT Media

And here we have P. pulvillus feeding on another mysterious gooey blob...

From EOL page by SERPENT media

The one below was labelled as "Porania pulvillus" but at least most of these look like the goniasterid Ceramaster granularis, another species with a large disk and triangular arms...These look like they're hunched over the sponges.  Looks very much like feeding...

But either way with whichever species, here's ANOTHER great example of a behavior that hasn't been published on as yet.. 

From the EOL page SERPENT Media archive

Bear in mind-if these things had been thought of as primarily suspension feeders for years and years-and it turns out they're big time predators???  That would represent a big shift in understanding the ecosystem/food web in this area. Sometimes it can all be in the details..

So.. really ALL kinds of neat legitimately important observations from this pretty vast pool of data fr. SERPENT/EOL!

And there's MORE??

With my appetite teased I spread out a little more and went looking on Flickr!

Feeding on some green stuff! Algae? Hydroids? Image by Gordon.Milligan

Or maybe gettin' ready to go after those tunicates in the upper right hand corner?


Feeding?  Or getting ready to spawn? 

Image by Mark Craig via Flickr

So, I've spoken about how poorly known the "spawning on tippy toes" posture is... Here was the link to that post

What does this say? 

What I see when I look at these pictures is inspiration.  ANY of these observations- the feeding or the spawning posture might be the beginning of someone's research project...or be relevant to one. 

It seems strange to think -but this blog has more on the "natural history" of this species than has been published since that last account in 1973!  Many of these pictures have been floating around the internet for YEARS. Funny to think of "natural history" and "data mining" as terms that go together here, but here they are indeed a strange marriage....

Thanks to Jen Hammock, EOL, the SERPENT Media Project and all the photographers cited!