Stichaster australis-A Pisaster-like parallel species!!

(Image from Wikipedia!)

This week's blog is a strange collision of starfish and comic-book/science fiction geekery!

So, one of my favorite sci-fi notions is that of the multiverse or parallel earth-essentially that you can have a universe that is JUST LIKE the one you live in but differs in minor to substantial ways.

So, for example, one of my favorite graphic novels by famous comic book writer Grant Morrison
Which is basically about DC's famous Justice League (Superman, Batman, Wonder Woman) meeting their evil parallel counterparts!!

Batman vs. Owlman! Superman vs. Ultraman! Wonder Woman vs. Power Woman! As the image above suggests, each are parallel versions of each other!

Each is a parallel version of the respective character. In other words, you kinda recognize the ROLE of the character but the character itself is fundamentally different and seems out of place, compared to what you are familiar with...

And I realized..this is kind of a cool way to think about the parallel niches that you can see in starfish that live a WHOLE world away from one another. VERY similar. But VERY different!

EXPLAIN!

So, let's take the example of the New Zealand Stichaster australis...

(image from Seafriends.org!)

Stichaster australis is a shallw-water species that occurs along the rocky intertidal throughout the temperate regions in New Zealand.

Their main food is the abundant New Zealand green mussel, Perna canaliculus.

Apparently, S. australis is one of the mussel's primary predators in the rocky intertidal! Hmmm...where have we heard that before??

(Image from FAO Fisheries!)

A 1971 paper (click here to see ref) that was studying the ecology of Stichaster australis by the famous ecologist R.T. Paine showed that this was similar to the North Pacific Pisaster ochraceus, in that S. australis played an important role in regulating mussel populations!
Paine experimentally removed S. australis from a rocky intertidal region for 9 months, resulting in the mussel expanding its vertical distribution by 40% of the space it had to grow into!!

Species richness of the area where Perna had "overgrown" decreased from 20 to 14 species

And when they removed Stichaster in conjunction with the other dominant organism-the kelp Durvillea antarctica, they found that the mussels had occupied 68 to 78% of the available space in the area!!

(Image from the NZ Plant Conservation Network)

So, without the "control", of the predator or the space occupied by the kelp, the mussels would just spread out into the area and go as FAR as they could take it.

Okay.. So what?
I've mentioned the ecological role of the North Pacific Pisaster ochraceus as the typical example of what's called a keystone species aka a species whose presence has a "disproportionate effect on its environment"
Basically, removing Pisaster results in a similar cascade of mussels going amok! Diversity takes a nose dive, etc.

And what I've just described for Stichaster australis is pretty much the same thing! Except that they live at opposite ends of the Earth!

The intertidal habitat in New Zealand has a lot of close parallels to those in California or anywhere on the west coast of North America...

You've got mussels..except that in North America they are Mytilus spp.
and in New Zealand (Southern Hemisphere)..you instead have Perna canaliculus

(Image from FAO Fisheries!)

In the North American system you have Macrocystis pyrifera
but in the southern hemisphere you have Durvillea antarctica

(Image from the NZ Plant Conservation Network)

and finally, you have the asteriid Pisaster ochraceus in the Northern Hemisphere... with its distinctive intertidal skeletal morphology
And here's the stichasterid, Stichaster australis in the Southern Hemisphere.. with a curiously, SIMILAR appearance!
Here we have Stichaster in a big cluster. Various hypotheses on why they do this I've read have focused mainly on how these big cluster maximize reproduction...
...and here we have the rocky-intertidal Pisaster in a curiously similar cluster. Reasons I've heard offered why-have extended to resisting dessication...but I suppose maximizing reproduction is also possible....Thus, the ecological roles these two species appear the SAME but the players are different.

This actually extends beyond being different in a lot of cases. Several of these taxa are not even closely related!

Stichaster australis is found ONLY in New Zealand and Pisaster ochraceus is found ONLY on the west coast of North America, and belong to two completely separate families!
(although admittedly, the families are related).

and yet, they have a similar convergent appearance, live in the same settting, feed on the same kind of food, and maybe even practice similar kinds of behaviors.
There are MANY more intertidal invertebrate species, including various snails and barnacles, that have similar parallels...

and course, New Zealand has MANY more starfish species intertidally-and relative to say, those in North America, their ecological roles are pretty poorly understood..

but in this instance, same role-but different players!! Not parallel earth-but parallel starfish!!