The Surprising Evolutionary Tree of Forcipulate (Asteriid) Starfishes!

Have ya' ever wondered how different things go into the BIG picture?

Well, that's what I do for a living. Try to figure out the BIG starfish picture!

My research looks at placing different species within the broader, larger context of that animal's evolutionary history and discovering new species is part of that overall direction.

But how do they all fit together? how does that work?

One of the main sources of scientific data for trying to determine relationships between organisms has been its morphology. Or in other words its overall body shape and how different structures on the skeleton have changed across different species..

I have written a little bit about how this works with sea star endoskeletons here.

So, different groups of whatever animal or organism that are studied all tend to have various dynamics and levels of difficulty at figuring out these evolutionary relationships (and consequently, the groups or classification that they belong to).

And the "higher up" you go (i.e., beyond the species up to families and orders), so too does the level of difficulty!

One such group that has always vexed scientists has been the Forcipulatacea, also known commonly as the Forcipulate asteroids...

This confusion is kind of ironic since it is this group that includes such familiar species such as Asterias and Pisaster which have been some of the most heavily studied species in marine biology!!

But sometimes, diversity is a big pain in the ass!!

How do you figure out relationships between species that are as different looking as this

to this?

To this?

Believe it or not..all of the species above are actually related (albeit distantly). These all belong to the Forcipulatacea, one of the largest, most diverse groups of asteroids living today. But HOW are they related?

Our starting point began with a big, historical mess. The classification had historically included several groups that were confusing and difficult to define..

This week, a NEW paper written by myself and my colleague Dave Foltz at Louisiana State University was published in the new July Zoological Journal of the Linnean Society takes a new look at the classification and evolutionary relationships within the Forcipulatacea and shows us that not all is as it seems...

What was done...

So, basically we sampled DNA from ALL the major groups within the Forcipulate asteroids with the aim of reconstructing the broad evolutionary history of the group!

Forcipulates are diverse echinoderms. There are some 365 species in 77 genera in 7 or 8 families. We sampled from almost all the families and almost all of the genera.

These included species from all over the world.. Antarctica, South America, Canada, Alaska, Japan, Australia, and New Zealand.. We also got species from shallow-water intertidal (like the familiar Pisaster) and from some species in the deepest of oceans (from several thousand meters). Data was collected and analyzed using various computer programs...

What the Prior Morphology-Based Classifications would have said...

So, before I go into the actual results, I thought I would explain a little bit about how the historical methods work and how DNA provides another way to test and study the relationships between different starfish species!

So, basically in the past, I and many of my colleagues would have used morphological characteristics from the external and internal skeleton. My colleagues who work with fossils are limited pretty much ONLY to morphological data.

A tree from morphology would clump all similar-looking species together.. So, in a VERY superficial sense... the tree of relationships would look like this...

So, for example, something like this Pycnopodia

might be MOST likely to closely clustered with this Australian Coscinasterias because of the multiple arms..

as would this formidible Japanese Plazaster borealis

But SOMETIMES, the external appearance of animals can be deceiving!! There exists a phenomena known as convergence!! Which happens when animals develop a pronounced external appearance in reaction to the life mode in which they live.

So, dolphins, salmon and sharks are all distantly related from one another-but superficially they can all LOOK ALIKE because they all swim through the oceans!

If we're lucky, then the organisms we're studying show this convergence more clearly..but often times its NOT. And as a consequence external appearance can be VERY misleading..

And its for this reason that a LOT of the work on the Forcipulates (and particularly on the largest subgrouping-called the Asteriidae) is so difficult.

One important detail about this tree

The morphology would provide us with evidence to support or cluster starfish with similar appearance together.

THEN we could look at the biogeography or evolutionary relationships of different animals based on where they lived..

So, there would be Australian species separated into different groups based on the extensive morphological features shown above...

THAT is what we would expect to see in a lot of these kinds of evolutionary studies...

.... But what we got was different from what we expected!!

THE RESULTS

There was a lot about the overall final tree that wasn't surprising. A lot of the more fantastic and weird-looking deep-sea stuff , such as this zoroasterid was supported and came out pretty consistently where we predicted..

One bit of setting straight was the family Stichasteridae. Our molecular data separated this group out and re-established a classification/grouping that French scientists had established in the 18th Century!

Interestingly, members of this group (e.g., Stichaster) have what may be a parallel Southern Hemisphere ecological role in South America/Australia/New Zealand relative to the northern Pisaster or Pycnopodia which occur NOWHERE in the Southern Hemisphere!

but the one big subgrouping within the Forcipulatacea called the ASTERIIDAE..the family that includes well-known starfish like Pisaster (aka the Ochre star) was always the most troublesome for past workers trying to work out..

The morphology of the Asteriidae gave us a lot of good leads-but ultimately, it just got to be a big morass. A lot of the various genera either look very different or a LOT alike. And understanding the classification of the group became kind of a slow trudge owing to the scope of the problem.

Fortunately, our results were very encouraging and unexpected!

Groupings within the Asteriidae didn't cluster by external appearance, they clustered based on WHERE THEY WERE FOUND.

Lineages/groups were associated with different discrete regions...

For example, there is a lineage of Asteriidae that occur at high-latitudes, around the Antarctica/Southern Ocean!!

Like this Diplasterias brandti (shown here brooding babies in its mouth)

May superficially resemble Pisaster or Asterias, but actually displays NO immediate relationship! I argue that these and their relatives pretty much diversified when the Southern Ocean became isolated by the Antarctic Counter Current...

Its sister group are a lineage of asteriids that live in the Equatorial Tropics!

This is actually a pretty widespread group that extends from Mexico to Hawaii, Northern Japan and even to the tropical Atlantic in both shallow (e.g., Coscinasterias, Astrostole, Astrometis, etc.) and deep water (e.g., Coronaster)

here's the Australian Coscinasterias

the southern California/Baja California Astrometis sertulifera

(thanks to CSU Fullerton for the pic!)

and Finally, one of the largest and most diverse groups is the BOREAL Asteriidae!

This was probably the most interesting development for me, as a kid who grew up in California.

ALL of the well-known California asteriids, such as the big Sunflower Star (Pycnopodia)

Pisaster ochraceus!

The very tiny Leptasterias!

And the very large Evasterias echinosoma!

the uniquely temperate Asian taxon Plazaster borealis

are ALL part of the SAME evolutionary lineage that occurs (well mostly ) ONLY in the North Pacific and the North Atlantic!!

So, for those who have ever wondered? Pycnopodia. Pisaster. Evasterias. These are all starfishes that are basically endemic to the west coast of the United States!

They are found nowhere else in the world.

So, even though you have one big multi-armed guy starfish like the Sunflower star, Pycnopodia, it has closer relationship to something "normal looking" like Pisaster than it does to ANOTHER multi-rayed (and simlar looking) starfish species, say in Australia or Antarctica!

So even though, you have many species that SEEM to be more closely related superficially, the genetics tells us that different lineages are separated by the various different geographic regions!!

I think that's pretty nifty. And I didn't even get to talk about some of the OTHER parts of the paper!