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Tribrachidium Classification Essay

How should I go about choosing my topic?

Begin by reading the explanations below.  Examples of each are provided below!

  • Division Essay: find a topic that people might tend to underestimate or over-simplify.  In other words, choose something that the average person might not know much about, and therefore can't really understand how complex or interesting that topic really is.  Your job in the essay will be to break your topic down into meaningful and important categories.

  • Classification Essay: think about the categories we place things in everyday and the characteristics of those categories.  The topic you choose should allow you to argue that something has been misplaced.

How should I organize this essay?

As you write, keep these guidelines in mind: 

  1. Your thesis statement and introduction will need to explain why these divisions/ classifications should matter to your reader.
  2. Your thesis statement and introduction MUST define or explain the category you plan to discuss (i.e. A sport is a competitive, physical activity therefore cheerleading should be considered a sport.)
  3. You should organize your body paragraphs so that each division or category has it's own paragraph or section. (i.e. cardio exercise is paragraph 1 and weightlifting is paragraph 2, etc.)

Division Essay Examples

If you want to lose weight, simply saying that you're going to "exercise" everyday may not be the most effective way to do so.  Exercising is more complex than many people realize--attaining your goals will involve understanding how different types of exercise can help you achieve your goals.   

Types of Exercise

  • cardio: burns calories and strengthens your heart (running, using an elliptical or stair-stepping machine, etc.)
  • weight lifting: tones muscles, increases physical strength, burns fat (using weights or weighted machines)
  • recreational/sports: depending on the sport, can provide both cardio and toning benefits (cycling, tennis, kayaking)

We could also narrow this topic down a bit further and write about the important differences between different types of cycling.

Types of Cycling:  stationary (exercise) biking, road biking, mountain biking, recreational biking

Classification Essay Examples

To write this type of essay, we'll need to think about things that should or should not be placed in a particular category.

Example: Batman (that's our topic!) is not a superhero (category people place him in), but is simply a local vigilante (category he belongs in).

Ask yourself: Why do I think that...?

  • Does not possess super powers (powers most humans don't possess).  
  • Chooses to be a hero, rather than being "chosen" by others/other forces.

Example: Cheerleading (That's our topic!) should be considered a sport (It belongs in the category, "sports"). 

Ask yourself: Why do I think that...?

  • cheerleaders go to "practice" and must be in good physical shape
  • cheerleaders work together toward a common goal
  • cheerleaders must "try out" for their squad and often compete against other squads

Sample Papers

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 aclear 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....


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! 


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...

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


For the next 3 weeks I am visiting the marine invertebrate collections at the Iziko Museum in Cape Town, South Africa! 

Here is one of my earlier posts showing South African asteroids! 

Things are keeping me busy.. so here are some moments that give you some ideas of life on the road!

Spines and arm from the South African goniasterid, Calliaster acanthodes...

The mountains surrounding Cape Town as the fog clears...

The original catalog of the South African Museum invertebrate specimens! (mostly now databased)
This nudibranch, described by Dr. Terry Gosliner, named for Nelson Mandela...

This lovely walk through the Company Gardens outside the Iziko Museum...

Similar to the New Zealanders and the Australians, it seems that Cape Town also loves its fresh, hot meat pies!

More to come! 



Greetings to everyone from Cape Town where I've been for the last two weeks! One more left to go!

Sorry if I've been a bit quiet and off Twitter. What have I been up to? What have I been learning?

1. Identifying Starfish!
So, it turns out that the South African Iziko Museum's invertebrate zoology collection is probably the largest one of all the collections of its kind in Africa. It includes a huge number of specimens from multiple places throughout the region. So, all major groups have been building up on their shelves.

BUT sometimes, groups need the attention that a specialized scientist, such as myself can offer: in this case the taxonomic skills to identify the many shallow and deep-water species which occur in the very diverse and wonderful waters of South Africa and nearby locales in TWO different habitats: a cold-temperate water setting on the west and south coast AND a tropical water setting on the east coast. Its unusal.

The last starfish/echinoderm expert to visit Cape Town was in the mid 1970s. Specimen collection has been proceeding at a semi-regularr ate for this entire time. And so, there's a LOT of them.

These represent valuable specimens with applications to everything from ecology to natural resource management.

I've identified HUNDREDS of specimens.. some rare. some new. More on this to follow.

2. Learning about South African Starfish!
So, along with what I've been learning from specimens, I've also been in contact with an extensive network of South Africa's marine biologists, citizen scientists, and other ocean-themed folks who are likely to know about echinoderms in the region.

I've been able to experience everything from pictures of living animals to general tips about local species. And yeah.. stuff that will hopefully find its way into a published paper...



3. Observing that temperate water South African marine habitats look astonishingly like those from central California!!

Thanks to some time at the Two Oceans aquarium and talking to colleagues, I have been reminded how stunningly similar (identical) the kelp forests of temperate South Africa can be!

Most folks think of Africa as a tropical locale but I can tell you (especially since I'm here during their winter) that Cape Town gives Monterey, California a run for its money for kelp, urchins, rain and food!

A kelp forest (Macrocystis) off Cape Town


versus a kelp forest (Macrocystis) in Monterey, California