Pteraster & Kin- Starfish that FIGHT BACK with MUCUS!

Everybody LOVES mucus! Yes! Don't deny it!

What would our lives be without it? Dry, boring and unlubricated.

I thought I would kick off the Fall with one of my favoritest and weirdest of echinoderms.. Pteraster and its kin aka the Slime (or Mucus) Stars!!

Who??
Pteraster is one genus in the family Pterasteridae.

This is group of starfishes that includes 8 genera that mostly live in cold-water habitats, but are most diverse in the deep-sea. The genus Pteraster alone includes about 45 species that occur worldwide, from the Arctic to the Antarctic and all the deep, dark places in between... Many of them eat sponges. So, on the surface, fairly harmless..

We'll be focusing on one particularly well-studied species, Pteraster tesselatus that occurs on the west coast of North America between California and Alaska.

This is a sort of thick, pudgy looking beast that can have a diversity of color patterns...

(from AFSC NOAA photo guide)

What makes Pteraster and all of the Pterasteridae so unusual?

The surface of the animal is actually concealed below a tent-like covering [called a SUPRADORSAL MEMBRANE (SD in the figure below)] that extends OVER the animal's proper surface!!

That proper surface below the supradorsal membrane is where you would find the typical features on a sea star's surface, such as the gills and etc.. Here is a cross-section through the arm. Tube feet on the bottom.
Why is this important? Well, its through the supradorsal membrane that the SLIME comes out!!

2. The SLIME Star!!
So, let's cut to the chase-Pteraster tesselatus earns its common name "the Slime Star" owing to its ability to defensively SPEW MUCUS when annoyed by unwanted contact (and who wouldn't want to do that??) !! GOBS and GOBS of it.

Note to you grammar types... MUCUS= noun. MUCOUS=adjective!

(this photo from the Stony Brook mar bio page)

This picture illustrates a typical encounter with Pteraster. Pick it up, and the next thing you know...you've got a bucket full of mucus!

(from AFSC NOAA photo guide)

Much of the info presented herein was based on studies by James Nance (Texas A&M University) and Lee Braithwaite (Brigham Young University) from some neat papers in 1979 and 1981 in the Journal of Experimental Marine Biology & Ecology.

How It works!
Within the supradorsal membrane are a VAST number of mucus producing cells both on the surface and throughout pores present all throughout.

(this awesome photo from the Walla Walla University website)

When Pteraster feels threatened, water enters the animal via the bottom tube foot grooves and flows throw the body cavity and up into the cavity between the animals' surface and the supradorsal membrane.

Mucus is produced by cells in the supradorsal membrane.
An opening in the supradorsal membrane, called the osculum, which is normally open

(this awesome photo from the Walla Walla University website)

suddenly CLOSES

(this awesome photo from the Walla Walla University website)

....and all of the little pores in the supradorsal membrane OPENS!!

So, water pressure EXPELS the Mucous-Water mixture out of the supradorsal membrane!!
So..imagine suddenly stopping up the pipes in your bathroom....and watching it all go somewhere else...
ShABLURRRUUUUUBBBBPTTTHHHHHHHH!!!!!! (noise is simulated-Pteraster does not actually make this noise unless you REALLY want it to. Then, go ahead-knock yourself out!)

and you get THIS!

But WHY does it do that? Would could POSSIBLY offend such an innocuous creature? That just wants to sit around eating sponges all day?

On the west coast of North America, starfish ecology usually involves one of these two predatory "bad boys"...

Solaster..usually Solaster dawsoni, which feeds mostly on other starfish.

(One species of Solaster shown here eating a sea cucumber.. pic from the SIMON/NOAA/MBARI)

Solaster (diameter=24 cm) can apparently take up to FOUR DAYS to devour an undefended Pteraster (about 14 cm across)! And that's basically Pteraster being devoured alive!

OR the fearsome Sunflower Star..Pycnopodia helianthoides which usually just SWALLOWS their prey whole! This takes about 2 days to digest...

(photo by Allison Gong, UCSC)

Tests using mucus applied directly to food being eaten by either of these two species almost instantly resulted in the predator moving away VERY quickly.

Lab tests applying mucus to fish and other invertebrates, such as snails and etc. also results in avoidance or outright fatality following extended exposure.

The mucus probably contains saponins or something similar. I've talked about how 0ther starfish species use saponins as a means of discouraging predators here..

What about other Pterasterids?
So, I've mentioned other deep-sea starfish related to Pteraster..such as this Hymenaster from
the deep sea North Pacific, which is practically MADE from clear gelatinous goo!
...and here...(about a foot across)
And if you pick them up? You get the mucous love!
or to say it another way.. "It slimed me!"


But this simple behavior begs a lot of interesting questions.
If the slime is defensive then WHAT PREYS on the other species around the world in the deep sea?
What about the tropical Pacific Euretaster?
Is the slime more or less toxic where specific predators are around?
How has this affected the long term evolution of the Pterasterids?

All questions for another day. Pteraster is interesting for MANY reasons that I haven't even begun to address...

Why Are there NO Bat Stars (Patiria miniata) in Wash. & Oregon?? What does genetics tell us?

(Patiria miniata from marinebio.net-a great educational resource!)

Today, a 2009 paper from Carson Keever and others from UC Davis and Mike Hart's lab in UC Davis from the journal Evolution! (click to see) about understanding the history of populations of the familiar intertidal/shallow-water Bat Star, Patiria (formerly Asterina) miniata!

We start with a mystery.

If you look at the entry for bat stars in any intertidal guide to marine invertebrates, the range for this species is reliably cited as "Southern California up to British Columbia and Alaska..." But what often gets left out, is that there's a strange disjunction or absence of these starfish along a segment of the coast between Oregon and Washington!
The first step was to look at the genetic relationships between the bat stars along the coast.
Is the disjunction the start of new distinct species (i.e., isolated populations) ?? We've seen this kind of method applied in a previous post about the color morphs of Pisaster (see here).

The authors extracted DNA from 423 individuals from 14 locations from Alaska, the Queen Charlotte Islands, Vancouver Islands, and California (from Fort Bragg to Santa Barbara).
DNA from these specimens can be analyzed and compared so that they essentially assess the overall similarity between them and among them.
Plus, they can use DNA to determine how the genetic material of the sampled animals flows between and among populations. They can boil this down to a hand-dandy intuitive diagram that shows where the genetic populations of bat stars on the Pacific Northwest coast occur relative to one another...

(Fig. 1 from Keever et al., 2009)

The circles represent the samples taken. The larger size of circles represents the number of specimens sampled. The colors correspond to distinct haplotypes (the aforementioned genetic material) from each original region. Thus, red corresponds to specimens originally from California, green from Vancouver, and blue from Alaska/northern Haida Gawaii (in the northern part of the Queen Charlotte Islands).

Each circle (i.e. the samples) has multiple colors, suggesting some mixing of populations in each region. You can see distinctly different colors (i.e., the haplotypes) between the more northern populations [i.e., blue from Alaska and North Vancouver (Haida Gawaii)] versus the red/green/blue populations in the southern regions (California and Vancouver Island).

The genetic "break" between the Northern populations versus the Southern populations may have been caused by past oceanographic barriers, such as the west to east North Pacific Current.

The North Pacific Current may form a barrier to the larval forms of this species as it effectively keeps the two populations apart..

On the other side of the range...
Based on the data on gene flow, it turns out there's a LOT of genetic mixing between southern populations, especially between California and southern Vancouver.

(modified Fig. 3 from Keever et al., 2009)

So, what do we know about the OR/WA disjunction??

There was no direct evidence that these genetic populations showed isolation or the beginning of distinct species across the Washington/ Oregon Disjunction (i.e., the no bat star zone). In fact there was 10 TIMES higher the rate of gene flow across the range disjunction relative to the gene flow in the Queen Charlotte Sound. (Note that a high rate of gene flow generally suggests coalesence of a population rather then a separation).

So, from a genetics standpoint, the WA/OR disjunction is NOT the starting point for two separate species.
Is it glaciation?? During the last ice age, glaciers extended out to sea and could have affected the distribution of marine life. Could this have been responsible for the distribution we observe?
It turns out..no. The authors rule out glaciation as a possible mechanism.

Unfortunately for this idea-the genetic structure of Patiria miniata populations don't match up with the disjunction.

Without a clearer explanation, the authors can only speculate that there was a "recent local extirpation" which would have broken up Patiria's range along the coast....perhaps something more ecological then historical or geological. At this point, the Washington-Oregon disjunction remains a matter of "well, we know its not that..."

But stay tuned! you never know what will come up next!