Wednesday, May 20, 2015

What is Going on with cidaroid sea urchins and their WEIRD spines??

Following the last Okeanos Explorer cruise to Puerto Rico, a got a bunch of questions from folks about what the heck was going on with the very odd looking spines of this cidaroid urchin: Cidaris blakei?

Some Etymology: "Cidaris" the genus name for this species (and the other urchins in the group)? is Latin for a headdress or tiara for ancient Persian kings The species name "blakei" is named in honor of the USS Blake, which was the vessel on which the species was collected.

Now, "normally" (which is to say in the majority of other ciadaroid urchins) the spines are pointy or blunt. Surfaces might be more smooth (as in Cidaris cidaris)

or kind of textured like so...
Cidaroid urchins are unusual in that their spines LACK skin on the surface. Remember that ALL echinoderms display their ENDOSKELETONS. What you are seeing (spines, armor, etc.) are NOT a shell or exoskeleton. All the surfaces are covered by skin (an epithelium)

Because of this, cidaroid urchin spines are affected by the environment. So, epizoics and fouling faunas grow on them. Worm tubes, barnacles, etc. 
From one of my prior posts, these Antarctic urchins, called Austrocidaris, which actually can "seed" new areas with the animals that live on its spines.

So, what is Cidaris blakei doing with those wide, fan-like spines? Their unusual shape doesn't necessarily seem to display any obvious function. Defense? Some bizarre hydrodynamic function? perhaps to more effectively encourage growth of epizoics on the spines??
I also came upon this pic of C. blakei, which seems to grow a lot of critters on its spines...
from this echinoderm Scratchpad
MANY other cidaroid urchin species are known for having very unusual spine morphologies which I've shared with folks over the years. Here's a bunch for comparison

Broadly speaking, cidaroids are the only urchins that have these pretty weird spine morphologies. For what reason? Are they related to the growth on the spines?? (increase in surface area?) Defense? Reproduction? 

A curious bunch of animals. "About which, very little is known.."

Another one from the recent Okeanos 2015 Puerto Rico dive, this one was home to two serpent stars (I think.. Asteroporpa annulata)..

This one is called Chondrocidaris gigantea, a species from the Indo-Pacific, which was in the Paris collections...
Another curious species with very heavily serrated spines...
The above species seems to have a fairly heavy growth on their spines..
Another species of Chondrocidaris, C. brevispina, pic by Arthur Anker
Here's the mysterious Tylocidaris (formerly Psychocidaris) with these thickened "cortex" covering the spines... (see more here)

Some Haeckelian art showing odd cidaroid spines..
More strange treasures from the Paris Museum I've shown before.. BOTH of these urchins are different species in the genus Goniocidaris

But again, its unclear what they use the spines for..

Here's a second species with VERY different spine morphology

and there's a bunch of Paleozoic urchins that are worth checking out...


Wednesday, May 6, 2015

WHY FIVE? Mysteries in Symmetry!

It is May 5, 2015 as I write this. Thanks to a recent question from my Twitter followers (thanks @MissMolaMola!) I got it in my head that this month and indeed TODAY would be a great day to address one of the great fundamental questions in the evolution of echinoderms 

WHY DO Echinoderms have FIVE PART (pentameral) SYMMETRY???

Adult echinoderms have one of the more unusual features among "higher" metazoans in that adults have a form of radial symmetry called 5 part or pentameral symmetry. So, not only is the body radiating from a central axis but it does so in increments of five. This is in stark contrast to most other animals which show bilateral symmetry: two sides with paired limbs on either side as well as a head with centralized sensory apparatus. 

Most times, non-bilateral symmetry (radial and absence of symmetry) is associated with "lower" or simpler metazoans. Mainly sponges and cnidiarians (jellyfishes, corals, etc.). And indeed in much of the 19th Century echinoderms were considered as PART of this lower "grade" of animals called the Radiata (more here).  

But subsequent data over the centuries, first from development and later from DNA has verified without a doubt that echinoderms have complex tissues, organs and are actually more closely related to chordates (e.g., humans, fishes, etc.) than they are to "simple" animals. ALL adult echinoderms display radial symmetry (although as we'll see there is some modification).

So, therein begs the question: WHY go "back" to have a body type that is seen primarily in simpler animals??  and why five? 
(and in science "why" means what is the evolutionary reason, there is no metaphysics at play..)

The answer to this question is... elusive. But here are some facts. 

1. Larval Forms are Bilateral
thanks to Dr. Allison Gong for the pic
This is a fundamental part of echinoderm biology and part of what classifies them among the more complex animals. 

ALL living echinoderms have a larval stage which is bilateral. These bilateral forms settle onto the sea bottom (or wherever) and develop pentameral symmetry as adults. 

This is an important part of understanding their evolution, since life modes observed at these early stages can sometimes be reflective of the early evolutionary stages of these animals. 

In this case, it shows that it shares the same morphology as other complicated animals in that it has bilateral symmetry in the same way that worms, arthropods, mollusks, and other "complicated" animals do. 

Thus, pentameral symmetry is a SECONDARY characteristic "on top of" the "basic" bilateral symmetry. It had to acquire this feature during its evolutionary development.  And so when we ask WHY do they have pentameral symmetry? We are also asking HOW/WHY did this unusual symmetry evolve in echinoderms??? Figuratively speaking, it adds an additional step to its evolution by appearing with this body form. What's the deal with that??

2. Are all adult echinoderms purely pentameral? 
You may suddenly realize "AHA! I GOT YOU!  SOME echinoderms show kind of BILATERAL SYMMETRY AS ADULTS!" Don't they???

Um. Well, yes and no.

Two notable exceptions: "irregular" urchins and sea cucumbers. Both are unusual in that most are detritivores or process sediment for food.  Therefore requiring movement in one direction.


Bilateral symmetry is associated with directed movement and so, its presence is often associated with organisms which show some kind of single-directed motion.

I've written about "irregular" urchins here.. basically a bunch of skeletal modifications that are part of the morphology in sea urchins that live/feed on sediment...(again, motion in a single direction)

What happens in "irregular urchins is that yet ANOTHER "symmetry" is overlain/"added" over the radial symmetry.. So, these animals go from bilateral (as larvae)
and eventually develop bilateral symmetry IN ADDITION to pentameral symmetry...  This is called SECONDARY bilateral symmetry.
Sea cucumbers  show bilateral symmetry (right and left sides) along their worm-like bodies. Presumably, again because they have a life mode which requires them to show directed movement in order to feed.

But you can see this five-part symmetry along the longitudinal axis of the body.. down the mouth or looking up the anus! Here we have anal teeth nicely showing pentameral symmetry!! (what are anal teeth? go here to find out)
So, all adult modern echinoderms show SOME kind of 5-part or pentameral symmetry. Even if it doesn't always look like it.

BUT was that ALWAYS the case????

3. Not all echinoderms were pentameral...
I've mentioned in past blogs that the echinoderms of the Paleozoic were mostly NOTHING like they
are today. 

Some such as this helicoplacoid (which I wrote about here) were actually asymmetrical! 
Others, such as this carpoid echinoderm laid on the bottom with one side...
Belemnocystites wetherbyi by avancna on DeviantArt

and indeed some of the fossils which have been proposed as among of the oldest known echinoderms (e.g. Tribrachidium) show three-part symmetry...
So, based on fossils like these, much of the older traditional paleontology suggested that echinoderms were evolving from different "grades" of symmetry.. 3-part to 5-part, etc. 

Modern approaches including  cladistics and more intensive scrutiny to characters were applied... and it turned out the "family tree" of echinoderm evolution was much more complicated than simply being different "grades" of symmetry....

Here are some phylogenetic trees from Mooi (2001)-a review of fossil echinoderm phylogeny... which show just how complicated the relationships can be. And that the question of symmetry in Paleozoic forms can be quite complicated.

Some of the oldest lineages look like they don't have pentameral symmetry but in some trees, asymmetry is an acquired characteristic. Something that evolves later rather than a feature of early echinoderms..

One of the lesssons from paleontology though: symmetry in echinoderms might be part of a changing/evolving body form through time rather than some discrete, adaptive event.

4. A crystalographic/developmental explanation? 
from Nichols 1967
Perhaps one of the most involved explanations of pentameral symmetry which was applied to LIVING echinoderms came from a series of papers outlined by echinoderm biologist Dave Nichols in the late 1960s. He followed up on an original notion by a previous worker who pursued crystallographic arguments:  
The pentagon is the only regular polygon for which the number of sides equals the number of diagonals...In all echinoderms whose development has been studied, the first plates to form include those at the apex of the animal-that is, at the pole opposite to the mouth. These plates are required to produce a body with basically a circular cross-section, and in order to reduce the planes of potential weakness across them, the sutures between them must be as few and as short as possible. Only with a pentamerous arrangement are these requirements satisfied.  from Nichols 1967, New Scientist 14, pg. 547 (italics mine). 
So, basically during development, Nichols arguments that the arrangement as seen above in "b"the theoretical  development of these plates that this is essentially the strongest arrangement of these plates. Four or six plate arrangements (a or c) presents a clear breakage plane whereas the 5-plate arrangement does not.

He goes on to apply this structural explanation to various living echinoderms, but unfortunately, even Nichols admits, that this idea was experimentally untestable.

5. Some insight from Evo-Devo! 
Some of the more intriguing clues into the "How did pentameral symmetry evolve?" are almost certainly going to be found from the field of "Evo-Devo", which is short for "Evolution & Development". A multidisciplinary field which integrates genetics and developmental biology. Which genes "turn on" or express certain characters??

One paper by Arenas-Mena et al. (2000) from Andy Cameron's lab at the California Institute of Technology in Development  shows expression of the Hox cluster of genes in the purple sea urchin, Strongylocentrotus purpuratus.
Basically, what this colorful digram shows is the gene represented by color and what part of the larval form is being "expressed" or developing in the larvae. So, there is easily a LONG description of how each gene triggers a different body cavity or other structure to be expressed or to be formed but long story short: These are all features "tracked" from a bilateral larvae which are observed in the pentameral body form.

There has undoubtedly been more work on this topic, but honestly, this was about all I was going to gather in the time I had and its a VERY involved field!

So, developmental perspectives give us SOME perspective into the process and its a start, but ultimately there remain a LOT of questions.
  1. Is pentameral symmetry evolutionarily adaptive?
  2. How is it relevant to the calcium carbonate skeleton? If at all? 
  3. Under what conditions does pentameral symmetry evolve from an ancestral form with bilateral symmetry?
  4. How would this shift/expression be observed in early echinoderms? Like crinoids? 
  5. Does the "5 part crystal stability" theory have any support?
Another fundamental aspect of echinoderms we know practically NOTHING about! Understanding of these types of evolutionary changes has important ramifications for many fields.. ecology, paleontology, developmental biology and even astrobiology! 

Thursday, April 30, 2015

Deep-Sea Sea Cucumbers! Transparent Guts Galore!

Wednesday (today) was the last day of the Okeanos Explorer's Oceano Profundo expedition, exploring the deep-sea habitats off Puerto Rico in the tropical Atlantic. In the next few months, they will transit through the Panama canal and onto Hawaii! 

Among the most commonly encountered of the animals they encountered were swimming sea cucumbers! I've written about swimming sea cucumbers here.  Most of the ones we saw were benthopelagic, which is to say that they live on the bottoms but swim when needed/desired. There's really only one that is truly pelagic... 

Several different morphotypes (species?) were seen..

1. This transparent one...
Here it was as it was taking off...

Is this the same as this one??

 Were those the same as these? An Enypniastes like species (possibly Amperima or Peniagone?)
It left MANY nice traces!
2. This "sea pig" like one.. Elpidiidae? 

3. The following ones were all "Benthodytes-like"
This like one had a clear body and with an highly convoluted gut! 

This one was a solid purple...
4. and there was this whitish purple beauty! 
5. and finally there was this weird pink one we saw yesterday....

This last week had SO MANY exciting observations, so maybe there will be some extra posts soon....

Wednesday, April 22, 2015

Five Important Facts about Paulasterias and the Paulasteriidae! NEW species! New genus! New Family!

Another new week and another new discovery that I'm overjoyed to share with everyone! A NEW paper I've published in the prestigious Zoological Journal of the Linnean Society! link here This paper was co-authored by colleagues from the British Antarctic Survey (BAS), the National Oceanography Centre at Southmapton, in the UK, the Monterey Bay Aquarium Research Institute (MBARI) and Dave Foltz from Louisiana State University. 

This whole paper is basically a monument to scientific collaboration! Basically, I was contacted by the BAS to study sea star specimens they had collected from a new hydrothermal vent system in the Scotia Arc, (Antarctica) which is famously the habitat of the "Hoff Crab"   and as it turns out, they encountered MANY different types of animals from this area. Barnacles, "hoff crabs"...and even sea stars (as we can see from this Wired gallery!)

They turned them over to me for study and while analyzing their DNA we found that they were actually closely related to ANOTHER sea star which I had collected during the MBARI 2009 Pacific Northwest Expedition!!  

and off we go....

1. FIRST starfish from a Hydrothermal Vent Habitat!!
UPDATE: New Video!
There aren't many echinoderms that live in these types of environments, such as brittle stars, sea cucumbers but this species is the FIRST starfish/sea star to be found in association with a hydrothermal vent habitat! 

Why? Hard to say, exactly but it probably has to do with the fact that most echinoderms can't process toxins very well. Their "body fluid" is basically sea water. This is probably why there are no freshwater or land echinoderms.

These starfish aren't "primary" vent fauna, such as big vent worms or clams that can manufacture food out directly of toxic sulfide..  These exist at the edge of the community feeding on the animals that ARE part of the primary vent community. BUT that said, they are pretty important (see below)

this handy chart from THIS paper by Leigh Marsh et al. in PLOS One!  shows this relationship...

2. What do they eat? Barnacles & HOFF crabs! 
A study of the food web among the fauna at the Scotia Arc site (in this keen PLOS One paper!) showed that they occupy a fairly important role in this community as one of the top predators.

Among their food? The weird stalked barnacle, Vulcanolepis The zonation above is probably not that discrete but it does suggest that animals on the "periphery" probably move inside and among all the inner zones, although there are probably several of these that are at the "edge" of their overall zone...
and the very abundant "hoff crab" 
There's obviously a LOT we still have to learn about the ecology of this habitat! But as far as understanding a habitat 2000-2500 meters down, in the Antarctic??  Knowing the top predator is a good start! 

3. NEW family, new genus and two new species!
So, we have a weird, deep-sea habitat with weird inhabitants. What does the starfish look like??
The skeleton is pretty reduced. A fairly soft and fleshy body wall.

As I had mentioned earlier, my colleagues and I were ALSO studying this beast... an innocuous looking 6-rayed sea star from the North Pacific!! Collected by myself and MBARI during the 2009 Pacific Northwest Expedition! 

Astonishingly, almost EVERYTHING starfish from the area studied during the expedition was new!! 
An  analysis of the molecules from these two species showed that they were in fact, closely related to one another!!

The truth is that there were not a lot of external characteristics that could have been used to have done the same analysis.

AND molecules also revealed that these two species were part of a lineage or clade SEPARATE from other known species!
HENCE! NEW names were needed! NEW SPECIES! NEW GENERA! and ultimately a NEW FAMILY was needed to properly describe these animals!!

This makes sense. "New" ecosystem and you have new species which compose that ecosystem. Another reason that taxonomy is important! 

This makes these species one of the FIRST new families of sea stars to be described since the 70s! (or possibly the 90s, it sort of depends on how you look at it)

4. Unusual and yet related to something familiar...
So, a lot of this might seem kind of alien to everyone, so here's a little something that I think everyone can relate to..

It turns out that these two starfish species are members of the Forcipulatacea, which is the larger group of starfishes to which familiar, intertidal species belong! Read this account of their unusual evolutionary tree! 

This familiar Pisaster ochraceus 

and Asterias rubens..

Are both distant relatives of these two weird deep-sea species!  What character is shared between them? Features such as these pedicellariae! Tiny wrench shaped claws that cover the body...
5. Named for some deep-sea biologists! 
So... WHAT TO NAME THEM?? As a taxonomist, one of my super powers is that I can honor a person, place or thing by converting their name into Latin, thus immortalizing them into the history of science!

As a matter of good practice, its considered more informative to use descriptive terms, but ultimately species names are at the discretion of the author.

The Antarctic species had been discovered by the British Antarctic Survey and these expeditions had heavily involved Dr. Paul Tyler from the Southampton Oceanographic Centre and so it was decided that the Antarctic species would be named Paulasterias tyleri!

Which basically translate's to "Paul Tyler's starfish" (kind of)...

Dr. Tyler is a HUGE name in deep-sea biology, having co-written one of the most important books in deep-sea research in addition to hundreds of articles on deep-sea ecology and invertebrates!

Professor Tyler was recently been made a Member of the Most Excellent Order of the British Empire (MBE)!!(here

The North Pacific species was discovered under the auspice of my colleague and friend Craig Mcclain over at Deep-Sea News!

And so, I deemed that this second species would be named for him! honoring him was critical!
Otherwise would these species have EVER been discovered if it were not for him???
  Craig has written a GREAT post on Paulasterias mcclaini over at Deep-Sea News here
This discovery is only the START of many MORE questions!

  • How does this species tolerate even a little bit of the toxic sulfide in the water?
  • Do these have defenses given that they are basically little six to eight-rayed fleshy, water bags?
  • How do they capture and eat a hoff crab?? 
  • What is the relevance of this group to the diversification and evolution of forcipulate sea star?
  • How do members of this family become so widely distributed??
  • How many more of these are out there somewhere??
  • IS this 6-rayed Atlantic starfish seen by the Okeanos Explorer the same thing???
My special thanks to Katrin Linse, Jon Copley (@expeditionlog), Leigh Marsh, Dave Foltz, Alex Rodgers, Dave Clague, Craig Mcclain, Lonny Lundsten and Linda Kuhnz!