Tuesday, March 29, 2011

Starfish Wars! The Great Debate in Sea Star Evolution!

Today: something a bit different. I thought that I would share a little bit about the details of a classic scientific debate among starfish scientists! What do we discuss? What sorts of topics are important?

Probably one of the most long-standing debates among scientists who study sea stars has been to understand the evolutionary relationships between those species living today and those which preceded them.
So, in the early 20th Century-many thoughts about starfish classification were also about evolution.

Which starfish would go at the "bottom" of the classification? (i.e., the oldest at bottom vs. youngest at top) What were the ancestors to those species living in the recent?

In the 1920s, opinions and DEBATE flared up as a heated exchange in the pages of the journal Nature between several of the most prominent workers of the day.

The participants in this discussion? Primarily it was between Professor E.W. MacBride (not shown) from the Royal College of Science in London and Dr. Theodore Mortensen at the Zoological Museum in Copenhagen (below)...
Originally, the discussion focused on the family Astropectinidae, which includes sea stars, such as this Astropecten, that burrow into sand, mud and other unconsolidated sediment like this..

But the discussion was later extended to other related members of the Paxillosida. An example is this related Mud Star (Ctenodiscus) here...

What are they fightin' about?
Astropectinids and their relatives in the larger order Paxillosida have a number of unqiue characteristics that suggested different evolutionary hypotheses to each of the two scientists. Simply put- Basal? or Ancestral? (i.e., "primtive" even though this term is not really accurate))

The Paxillosida is so-called because of the presence of structures called paxillae.
Paxillae provide a kind of surface protection above the papulae (i.e., the gills) which are present on the surface of the animal.

Mud and sand can cover over the paxillae but they don't clog up the papulae so that the animal can breathe...
Living in and around sediment-and SWALLOWING mud is often part of these' animals' repetoire!
Many Paxillosidans are voracious predators. You can see some of their eating habits documented here.

Mortensen (and indeed many of the other scientists of his day) interpreted many of the characters in this group as evidence that the group was more "primitive" (i.e., closer to a fossil ancestor).
(thanks to Echinoblog Art Dept. !)

This was opposite of MacBride who interpreted those characters as more "advanced' (i.e., specialized adaptations to the particular setting or lifestyle of the animals).
Let's see some of these characteristics-and see what the fuss was about!!

1. Absence of a brachiolaria larvae. It is common for many astropectinids (and presumably other paxillosidans-although data for all taxa isn't completely known) to LACK this one larval stage.
Mortensen and those who interpret this as a "primitve" or ancestral character would argue that the absence of this larval stage indicates that it is acquired later on by the OTHER starfish taxa.
And that this larvae was presumably NOT present in the ancestor of modern sea stars.

MacBride on the other hand argued that this was more evolved-i.e., a "lost" character as a result of specific modification. Presumably, an adaptation to the muddy/sandy/ sedimenty places that these starfish like to live.

2. Pointed Tube feet (or absence of suckers). Understanding of tube feet in starfish is actually a big deal with many isues pertaing to adhesion and so forth.. This interest continues to be a fairly active area of study.
Mortensen (and the belief of many) was that it was "primitive" to NOT have suckers on your tube feet in contrast to the -the way that most other starfish have. i.e., having suckers made you the more evolutionary sophisticated versus those without suckers.
MacBride argued that the suckerless tube feet were an acquired evolutionary feature (in other words-a loss or change from suckered tube feet) relative to most sea stars who have suckered tube feet (as above).

A point in MacBride's side has been that It turns out that SEM and subsequent work of tube feet shows that in the paxillosidans-these tube feet are actually pointed and not simply "suckerless".

It also turns out that in some Astropecten, there is a suckered tube foot present but only in juveniles.. So it does seem that the "pointed" tube feet develop and then give way to a the pointed tube feet in adults.

3. And finally... in the astropectinids surveyed-the ANUS was absent! And so the argument came down to the same back and forth: Was it missing because it was never there in the first place? (to be acquired by other sea stars later on?)

Or because it was "lost" over time as an adaptation to the environment?

And so it goes...
The discussion/debates over the Paxillosida began in the 1920s and have been an ongoing part of the study of starfish evolution ever since..

The positions of MacBride and Mortensen have found new advocates and the discussion has itself evolved through the many different generations of biological thought-fossils had always been important-but have become a critical part of the discussion in the 1960s and 70s.
The 1980s and 90s saw the phylogenetic revolution and the application of cladistics to the issue. And the discussion continued.

Many of these are interesting stories in their own right..but for now-I am glossing over them and maybe someday I will be able to detail some of those accounts..

At this point, I had found that many of the cladistic and phylogenetic discussions advocating the Paxillosida as derived or "advanced" rather than primtive to be the one best supported by the evidence.

Enter DNA...
What do I bring to this discussion? A few years ago, myself and my colleague Dr. David Foltz at Louisiana State University at Baton Rouge began to work on this question-collecting from the huge diversity of starfish, extracting DNA from them and producing a "family" or phylogenetic tree of modern sea stars.

Part of this tree was recently published by the Zoological Journal of the Linnean Society.. and our tree shows some interesting results...

Now, keep in mind that for various reasons, we did not actually arrive at a solid answer of "what is the ancestral starfish".

Often times, these trees are heavily influenced by many parameters-how many different kinds of starfish are included, the kinds of genetic data used and how much genetic data were included.

That said-this is also one of the most comprehensive DNA trees for starfish that are currently available..

And what you see is that the basal taxa are in yell0w-the Forcipulatacea and the Velatida whereas lodged up in the higher branches (see red box at bottom) : The Paxillosida!

Also interesting-is that nearly all of the taxa that showed adaptations for burrowing were supported among the Paxillosida...
The tree above has also changed a lot of the underlying assumptions about many of the relationships among sea stars. The Velatida had not previously been thought of as being near the base of the tree. So many new angles have been brought into the discussion!

Now, I know for a fact that a new (non-DNA) paper that challenges this is in the pipe! How will this new data conflict or agree with the tree indicated above?

We shall see...

Tuesday, March 22, 2011

Tales from the Intertidal! Algae+Snail=Protection from Big Mean Starfish!

California's rocky intertidal (and subtidal) zone is a wonderous place!

A countless host of invertebrates and other organisms co-exist here-a rich ecosystem that is one of the most distinctive and well-studied marine ecosystems in the world.

Much of that richness comes from understanding the complex interaction between the species which live here.

The various "stories" or natural history that have resulted from the feeding, defense, behavior, and reproduction of MANY species has laid the foundation for well-grounded ecological understanding of the many organisms that live here. Few places in the world have such a well-understood and rich fauna.

Here is ONE of those stories!

Today's blog is based on research by Carol Thornber at the University of Rhode Island
in a 2007 issue of Marine Ecology 28: 480-486-Associational resistance mediates predator-prey interactions in a marine subtidal system.

Thornber began with an interest in the ecological effect of epibionts on ecological interactions.

Epibionts are organisms that live on the surface of another living organism, so a kind of commensalism. Examples might be barnacles on whales or perhaps the worms and other encrusters on these cidaroid sea urchins.
It turns out that there's a pretty neat little example of how epibionts affect an important interaction between the Brown Turban Snail and its sea star predators!

The Players
Thornber focused on the Brown Turban Snail: Chlorostoma (cited in the paper as Tegula which is a synonym) brunnea.
The Brown Turban snail is often covered with different types of crusty or coralline algae that give it a rougher "covered over appearance".

This isn't a brown turban snail...but it gives you the general idea.. A full "leaf" of coral is off to the right side..

The image below isn't the Brown Turban snail-but it gives you the general idea. Note the red/pink crustose algae at the top!

#1-How common is it to find Brown Turban Snail (i.e., C. brunnea) shells covered by algae? and how does this relate to the abundance? Or in other words does more starfish correlate with MORE or LESS coraline algae on shells ??
#2-Does the presence/absence of algae covering the shell influence feeding preferences of two predatory starfish??

Specifically..the intertidal Pisaster ochraceus
and the shallow-water subtidal sunflower star Pycnopodia helianthoides!
In Answer to the first question ...
-Apparently >60% of of snails were at least 75% covered with one or more species of crustose algae. A minority of snails (35%) wer completely covered. So covering of shells with the red/pink algae is pretty common.
Fig. 2 from Thornber's paper shows the correlation of density of Brown Turban snails relative to the density of Pisaster and Pycnopodia.

She found that the density of the snails was driven by the abundance of predatory sea stars-specifically Pisaster !!

Lab Experiments (2nd question)! How will Pisaster and Pycnopodia feed on algal covered shells??

Based on her Fig. 3, it turns out that shells that LACK any kind of the algae included (coralline and crustose) are the MOST preferred by the two predatory stars tested!

The bar graph below shows the greatest % of C. brunnea was fed upon when the shells were bare.
Pisaster ochraceus was nearly THREE times as likely to consume BARE C. brunnea than those covered by algae.
Pycnopodia ate FOUR times as many BARE snails as those covered with crustose coralline algae.
The results support an interesting idea-the crusty/red algae that is present as an epibiont on the shells seems to discourage predatory starfish from feeding on the snails!

Thornber discusses some dynamics...

1. The algae may provide camoflage against visual predators, such as crabs, octopus, fish, etc.

2. It is unclear if the distribution of the algae is somehow influenced by the snail or some other non-random factor.

3. What makes the algae so "discouraging" to starfish? Possibly the production of chemical defenses..

Or it could also be that the rough surface itself creates an unpleasant or irritating sensation to Pisaster and/or Pycnopodia.

The "mollusk escapes starfish predator" theme is an old one. Its an important ecological interaction that has been at the center of many ecological surveys.

From a purely behavioral escape point of view though, it joins the ranks of such noteworthy strategies such as this...

clam escapes from sunflower star!

and this! Scallops escape!

Tuesday, March 15, 2011

Putting Tags on Starfish! Where do they go? What do they do? How fast do they do it?

This week-a neat paper from Miles Lamare and team (who are mostly from the University of Otago in Dunedin, New Zealand) about using electronic tagging to study individual movement in the temperate-water New Zealand asteriid starfish Coscinasterias muricata in a 2009 issue of the Journal of Experimental Marine Biology and Ecology (click here).

As much as the idea of tagging echinoderms has come up (and this is actually one of the most common professional inquiries I receive) its not been done very much.

Largely, because its very difficult to stick a pin, tag, flag or any kind of inorganic device into an echinoderm without any number of deleterious effects, including:

-seriously affecting its behavior
-killing it
-causing the arm/spine/limb to be cast off and regenerated and/or variations thereof

.......and so on.

But technology has progressed and the authors introduce the usage of this handy little electronic tag (the DST-milli electronic tag) made by Star-Oddi. The tag is programmed with electronics that record water temperature, and depth every 5 minutes.
The authors attach these tags to the animal with a metal wire to which the tracker tag is attached so that the animal looks like so...
Figure 1 from Lamare et al.

They tested the behavior of the species and essentially, their findings indicated that there were no apparent behavioral changes in feeding and etc.

So, what happened next? They chose 3 individuals with the goal in mind of tracking their movements relative to their feeding behavior and environmental factors. The paper mainly addresses the kinds of data that can be obtained with the tag (as opposed to Coscinasterias behavior) but they did discover a lot...

1. Where they went! and how fast! The Data
So, as it turns out Coscinasterias muricata is fast.

Individuals vary-but on average they seem to be capable of 15.6 meters/day.

This still pales in comparison to the west coast sunflower star (Pycnopodia helianthoides) which is practically a starfish lamborgini at the rate of 3 meters (possibly 2 from other sources) a minute as outlined here-I'll let you do the math)

And that's MUCH faster compared to The Crown-of-Thorns (Acanthaster planci) which can go 2.3 to 4.3 m/day versus the much smaller and slower asterinids (i.e., bat stars) Patiria pectinifera (1.5 to 3.7 meters/day) and Patiriella regularis (5.7 meters per day).

Why is speed important? Read on below...

We see from the data above that different individuals of this species can travel quite a bit and over a pretty broad vertical distance (four to 14 meters). So, what's going on?

2. The Salinity Story
It turns out that C. muricata displays "vertical migration" which is to say that it TRAVELS back and forth from deeper to shallower regions in order to forage for its favorite food, the blue mussel, Mytilus galloprovincialis, which live in the intertidal.
Their direct observations showed that of the ndividuals observed, 92% of these were present in water LESS than 1.0 meter! with fewer and fewer numbers being present throughout deeper adjacent regions.

But based on the tracking info, two of the individuals showed some variation. One occupied deeper water below the mussels and then migrated into shallow water for short periods of less than a day and at intervals of about five days. Another occupied shallow depths for the first week before moving on to deeper depths.
Another important behavior they noticed was how physical factors, such as salinity affected the behavior of the observed individuals. As a species, Coscinasterias muricata is VERY intolerant of low salinity and experiments suggest that individuals WILL die in a relatively short period if salinity is not maintained.

So, if there's freshwater runoff or a storm-it will retreat from shallow water.

Figure 6 below charts the position of each of THREE individuals relative to salinity, temperature, height above the "mussel line" and the amount of rainfall (on the bottom) versus the day during a two week period.

NOTE in the top "blue" box and the bottom box. The depth inhabited by the animals DECREASES when rainfall is highest (i.e., the dilution of salinity is lowest).

Salinity also gets higher as you get deeper...

So, as a physical influence, the salt water layer influences how C. muricata forages for food! An influence that is apparently very significant to the animal's biology..

The authors note something of particular interest... that the animals begin their descent BEFORE the deepening/formation of the low-salinity water layer!! This suggests that there are other physical cues (e.g., changes in wave action) to the animal, so that so that it KNOWS when to move!

This is kind of neat, because these are the KINDS of cues that are not obvious to us. Asteroids have a very different world of perception.
It turns out the aforementioned speed is also an important factor in understanding how the salinity motivates the animal's movement.

The speed of the animal..observed as between 1.78 and 23 meters/hour is faster than the rate at which the low salinity layer deepens during the weather!!

In other words- the starfish can outrace the "low salt" layer as it approaches!

I think most people interpret movement as a function of something observable-like predation. But in this case-maybe not so much?

Undoubtedly, the authors probably have more (or could have more) than this-but the paper's intent was mainly to see what they could learn using these new tags. Foraging behavior for an animal like this is important to marine ecologists. You can't exactly follow a starfish like a panther or a racoon.

And if our recognition of Pisaster (same family as Coscinasterias by the way..) as a keystone species is considered important to ecology-then understanding things like foraging patterns is also potentially significant to what's going on in marine ecosystems.

And just on a pure curiosity level wouldn't YOU like to know the secret lives of starfish?

Tuesday, March 1, 2011

Springtime Sea Urchin SPAWNING videos! from Cell to Settlement!

As we draw closer to Springtime all thoughts turn to expelling our gametes into the water!

Here's some new sea urchin videos expelling their echinoderm love into the water to meet MORE love!

The gametes (reproductive cells like sperm and eggs) are expelled by urchins as the white clouds you see below.. The gametes are draw together, meet, and fertilize in the water-leading inevitably to a juvenile settling out and growing up...(the last video)

Some Fire Urchins (Astropyga) from Lembeh

Hawaiian Tripneustes gratilla

here's what happens from spawn to fertilization in a very nicely narrated animation!

and the BEST for last... video of a new settled juvenile sea urchin!!